Millimeter-Wavelength Characterization of the CO Emission of Comets 174P/Echeclus, 29P/Schwassmann-Wachmann, and C/ 2016 R2 (Panstarrs)

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Graduate Theses and Dissertations Graduate School

November 2019

Millimeter-wavelength characterization of the CO emission of comets 174P/Echeclus, 29P/Schwassmann-Wachmann, and C/ 2016 R2 (PanSTARRS)

Kacper Wierzchos University of South Florida

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Scholar Commons Citation Wierzchos, Kacper, "Millimeter-wavelength characterization of the CO emission of comets 174P/Echeclus, 29P/Schwassmann-Wachmann, and C/2016 R2 (PanSTARRS)" (2019). Graduate Theses and Dissertations. https://scholarcommons.usf.edu/etd/8695

This Dissertation is brought to you for free and open access by the Graduate School at Scholar Commons. It has been accepted for inclusion in Graduate Theses and Dissertations by an authorized administrator of Scholar Commons. For more information, please contact [email protected]. Millimeter-wavelength characterization of the CO emission of comets 174P/Echeclus,

29P/Schwassmann-Wachmann, and C/2016 R2 (PanSTARRS)

Kacper Wierzchos

A dissertation submitted in partial fulﬁllment of the requirements for the degree of Doctor of Philosophy Department of Physics College of Art and Sciences University of South Florida

Co-Major Professor: Maria Womack, Ph.D. Co-Major Professor: Andreas Muller, Ph.D. Myung K. Kim, Ph.D. Matthew Pasek, Ph.D. Zhimin Shi, Ph.D.

Date of Approval: November 7, 2019

Keywords: Comets, Rotational spectroscopy, Radio astronomy

Dedicated to my parents, who ever since an early age, encouraged my curiosity and made of me the scientist that I am today. Acknowledgements

There are many factors involved in the successful completion of a Ph.D program. I firmly believe that the most important is your Major Professor. This work could not have been possible without the guidance of Dr. Maria Womack who was the most knowledgeable and understanding advisor and Major Professor that I could have ever wished for. Thank you. As reflected in the dedication of this work, I’m infinitely thankful to my parents. They have both been amazing life coaches, always stimulating my curiosity and rational thinking. It goes without saying that I’m greatly indebted to my wife Sheila, who has been very sup- portive and accommodating throughout the course of my graduate studies and who always encourages me to follow my passion and my dreams. Babe, you know this could not have been possible without you. On a technical note, the help of the staff of the Arizona Radio Observatory 10m Sub- milimiter Telescope and the IRAM 30m antenna is greatly appreciated as well as the help provided by Yamil Nieves with the LaTex formatting of the current dissertation. I would also like to acknowledge the support from the NSF grants AST-1615917 and AST-1945950 and the University of South Florida Duckwall Summer Research Fellowship. Table of Contents

List of Figures...... iii

List of Tables ...... iv

List of Abbreviations ...... v

Abstract ...... v

Chapter 1: Introduction ...... 1 1.1 Investigating cometary volatiles: Radio Spectroscopy ...... 4 1.2 Excitation processes of linear molecules ...... 7 1.3 Calculation of column densities and production rates ...... 8

Chapter 2: Carbon monoxide in the distantly active Centaur (60558) 174P/Echeclus at 6 au...... 16 2.1 Introduction ...... 16 2.2 Observations and Results ...... 20 2.2.1 ARO SMT 10-m ...... 21 2.2.2 IRAM 30-m ...... 23 2.3 Analysis and Discussion ...... 24 2.3.1 Spectral line proﬁle ...... 24 2.3.2 Excitation and production rates ...... 25 2.3.3 CO production in Centaurs ...... 26 2.3.4 Outbursts, outgassing and fragmentation ...... 30 2.4 Conclusions ...... 31

Chapter 3: CO gas and dust outbursts of Centaur 29P/Schwassmann- Wachmann ...... 33 3.1 Introduction ...... 33 3.2 Observations ...... 37 3.2.1 Millimeter-wavelength spectra ...... 37 3.2.2 Visible magnitudes ...... 38 3.3 Results ...... 40 3.4 Discussion ...... 42 3.4.1 Quiescent activity of gas and dust ...... 42

i 3.4.2 CO outbursts ...... 44 3.4.3 Dust outbursts ...... 46 3.4.4 Causes of outbursts ...... 47 3.5 Summary and Conclusions ...... 52

Chapter 4: C/2016 R2 (PANSTARRS): A comet rich in CO and depleted in HCN...... 53 4.1 Introduction ...... 53 4.2 Observations and Results ...... 54 4.3 Analysis and Discussion ...... 56 4.3.1 CO outgassing velocities and spatial extent ...... 56 4.3.2 Production rates of CO and HCN ...... 59 4.3.3 High N2 Production Rates ...... 63 4.4 Conclusions ...... 64

Chapter 5: Case study: Evolution of the CO outgassing and observations of 13CO and HCN in C/2016 R2 (PanSTARRS) ...... 66 5.1 Introduction ...... 66 5.2 Observations and Results ...... 68 5.3 Analysis and Discussion ...... 72 5.3.1 CO heliocentric evolution ...... 72 5.3.2 13CO J=2-1 emission ...... 78 5.3.3 Depleted HCN emission ...... 80 5.4 Conclusions ...... 81

Chapter 6: Summary of Results ...... 82

References...... 87

Appendix A: Proof of copyright permissions ...... 107

Appendix B: Author’s Work ...... 108 B.1 Refereed Publications ...... 108 B.2 Selected non-refereed publications ...... 110 B.3 Conference Presentations ...... 111

About the Author ...... End Page

ii List of Figures

Figure 1: Solar system diagram...... 2

Figure 2: CO(2-1) spectrum of comet C/2016 R2 (PanSTARRS)...... 3

Figure 3: Arizona Radio Observatory 10m Submillimiter Telescope...... 5

Figure 4: CO(2-1) emission in comet 29P/Schwassmann-Wachmann...... 6

Figure 5: Spectrum of the CO(2-1) emission of 174P/Echeclus...... 22

Figure 6: IRAM spectrum of 174P/Echeclus...... 24

Figure 7: Speciﬁc production rates for distant comets and Centaurs...... 28

Figure 8: Spectrum of 29P/Schwassmann-Wachmann...... 38

Figure 9: Lightcurves of 29P with CO gas curve...... 41

Figure 10: First detection of CO(2-1) in comet C/2016 R2...... 56

Figure 11: Map of the CO(2-1) emission in comet C/2016 R2...... 57

Figure 12: Last detection of CO(2-1) in C/2016 R2...... 70

Figure 13: CO(1-0) spectrum of C/2016 R2...... 72

Figure 14: CO(6-5) spectrum of C/2016 R2...... 73

Figure 15: 13CO spectrum of C/2016 R2...... 74

Figure 16: HCN(3-2) spectrum of C/2016 R2...... 75

Figure 17: Evolution of Q(CO) with heliocentric distance of C/2016 R2...... 76

iii List of Tables

Table 1: Parameters needed to model production rates...... 9

Table 2: Values of hν/kTrot for the application of the Rayleigh-Jeans correction.. 14

Table 3: Constants used for the derivation of production rates...... 15

Table 4: Observations of CO J=2-1 line in 174P/Echeclus...... 21

Table 5: Typical visual magnitude observations of 29P/SW1...... 35

Table 6: Measured and derived properties of dust and gas in 29P/SW1 coma... 36

Table 7: CO(J=2-1) observations of 29P/SW1 using the ARO SMT 10-m telescope 39

Table 8: Observations of R2 with the ARO 10m SMT...... 55

Table 9: Compiled Q(CO)/Q(HCN) ratios in CO-rich and other comets...... 62

Table 10: Observations of Comet C/2016 R2...... 71

iv List of Abbreviations

IRAM...... 30m Telescope of the Institut de Radio Astronomie Millimetrique

SMT...... Arizona Radio Observatory 10m Submillimiter Telescope au...... Astronomical unit

ηc...... Beam eﬃciency Q(CO)...... Carbon monoxide production rate

FWHM...... Full Width Half Maximum

∆...... Geocentric distance r...... Heliocentric distance

Q(HCN)...... Hydrogen cyanide production rate

JD...... Julian date

JFC...... Jupiter-family comet

KBO...... Kuiper Belt object

∗ TAdv...... Line area δv...... Line-shift mol s−1...... Molecules second −1

v Abstract

Comets are fascinating minor solar system bodies. They contain some of the most pristine and unprocessed material found in the solar system. As a comet approaches the Sun it displays the characteristic cometary coma and tail. This is due to the release of volatile species through a variety of processes. In the present work I studied the carbon monoxide emission of three very unique comets; 174P/Echeclus, 29P/Schwassmann-Wachmann and C/2016 R2 (PanSTARRS) with diﬀerent radio telescopes at millimeter wavelengths. After molecular hydrogen, carbon monoxide is the second most abundant molecule in the universe, and is also the most volatile of the species found in most comets. The study of its release mechanisms, content and distribution within the coma of a comet can constrain current models of cometary and solar system formation. Some of the most relevant results of this work include: Detection of carbon monoxide in comet/Centaur 174P/Echeclus -only the third Centaur in which CO has been detected-, evidence for non-correlation between the CO emission and dust production in comet 29P/Schwassmann-Wachmann, and the ﬁrst detection of 13CO in a comet. The results obtained through the realization of this work have been published or submitted for publication in peer-refereed journals and have contributed to our present understanding of solar system chemistry.

vi Chapter 1 Introduction

Comets are minor bodies in the solar system that contain well-preserved samples of dust grains and gas from the protosolar nebula cloud of which the solar system formed [1]. These objects are some of the least processed bodies of the solar system, and their scientific value lies in their potential role in clarifying and explaining the processes that gave birth to the solar system. It is thought that comets originally formed in the outer regions of the protoplanetary disk [2], where ices are able to condense and form primitive agglomerates of icy material. Due to the super volatile nature of some ices like CO and HCN found in cometary nuclei, comets undergoing different thermal and processing histories have different volatile abundances [3]. As a comet approaches the inner solar system it develops a surrounding cloud of diffuse gaseous and dusty material, called a coma. This cloud of material will grow in brightness and size as the comet approaches the Sun. From a dynamical point of view, there are two distinct comet populations. Short period comets (orbital period < 200 years), and long period comets (orbital period < 200 years). It is hypothesized that short period comets come from the Kuiper Belt, whereas long-period comets are thought to come from the Oort Cloud (Fig.1). Short-period comets usually have orbits that are confined to the ecliptic plane and their aphelion is rarely situated beyond Uranus. These comets are more devolatilized compared to long-period comets due to their multiple perihelion passages. This devolatilization translates into smaller and fainter comae and tails. Long-period comets are not confined to the ecliptic, i.e. can have large inclinations. Normally they are brighter, have larger comae and longer

1 Figure 1: Orbital diagram of the Kuiper Belt (hypothesized origin of short-period comets) and the Oort Cloud (origin of long-period comets) tails than short-period comets. The most famous recent example is comet C/1995 O1 (Hale- Bopp), that reached naked eye visibility from even urban light polluted areas. Spacecraft missions and high resolution radar imaging shows that the shape and size of cometary nuclei varies greatly. The largest known comet is 95P/Chiron, with an estimated nucleus diameter of ∼ 230 km [4]. Most comets are under 10 km in diameter and can have very irregular shapes. For example, the nucleus of comet 1P/Halley measured 16 x 7 x 7 km [5] whereas comet 81P/Wild is a 2 km spherically shaped object [6]. The bulk of the cometary nuclei is thought to remain very unprocessed except for the most volatile of the species. This is mainly due to the very low temperatures at which comets are exposed through their orbit. Thus the observation of comets provides us with a window to the past of the solar system. Cometary nuclei are primarily composed by a mixture of water ice, carbon monoxide and carbon dioxide ice, methane, ammonia and silicates (sand and rocks). Large comets like 95P/Chiron are thought to have a mainly rocky core whereas small and volatile rich comets like C/2012 S1 (ISON) are more rich in ices.

2 In the years 1995 - 1997 two comets provided us with a great deal of knowledge on cometary compositions and many new identifications of cometary molecules were made. These comets were C/1995 O1 (Hale-Bopp) and C/1996 B2 (Hyakutake). As a comet approaches the Sun, the solar flux heats its nucleus and volatiles are released in higher quantities peaking at perihelion. The released volatiles will usually be accompanied by the release of dust, which is entrained with the ices that form the nucleus. A parameter that serves as a proxy for the dust emission of a comet is the so-called Afρ, which was first introduced by A’Hearn 1984 [7]. Afρ is calculated with the assumed dust albedo A, a dust filling factor f, and the radius of the coma at the comet distance ρ.

4∆2r2 F Afρ = c (1.1) ρ F

Where ∆ is the geocentric distance, ρ is the radius of the coma in the same units as ∆, r is the heliocentric distance in au, F is th ﬂux density of the Sun at 1 au and Fc is the ﬂux density of the comet within the aperture in the same units as F .

CO(2-1) Spectrum of C/2016 R2 PANSTARRS

0.3

-1

0.33 km s / channel

2017 Dec 22.21 UT

r = 2.98 au

0.2 (K)

0.1 T*

0.0

-20 -15 -10 -5 0 5 10 15 20

-1

Offset Velocity (km s )

Figure 2: CO line in comet C/2016 R2 PANSTARRS. The cometocentric velocity is denoted by the dashed line at the zero km s−1 position. Note how the CO emission is not signiﬁcantly shifted from the zero position, suggesting a low gas expansion velocity.

3 Out of the more than 50 molecules detected in comets, only a handful are frequently abundant. By far, the most abundant cometary ice is H2O, which is also responsible for the activity at small heliocentric distances. Other usual cometary species include CO, CO2,CH3OH and HCN. H2O, CO and CO2 are the most interesting since they are directly related to the activity of the comet through sublimation [8]. No two comets have the same chemical composition or abundances, and constraining the compositional heterogeneity of cometary nuclei is vital for interpreting cometary composition in terms of the physical conditions operating in the protosolar disk at the time of planet formation. Willing to contribute to the current body of knowledge about the solar system formation and chemistriy, in the present dissertation I characterized the CO emission of comets 174P/Echeclus, 29P/Schwassmann-Wachmann and C/2016 R2 (PanSTARRS) at millimeter wavelengths.

1.1 Investigating cometary volatiles: Radio Spectroscopy

Linear molecules with electric dipole moments greater than zero produce rotational transitions. These transitions occur in the radio portion of the spectrum and are studied with radio telescopes. These devices collect and amplify the incoming radio waves emitted by celestial bodies. In general, radio telescopes are formed by one or more antennas (parabolic dishes) that bring the radio waves into focus Fig.3. At the focus we usually find the receiver and amplifier which boosts the weak signal to a level that can be measured and interpreted. Electronics in radio telescopes are kept at very low temperatures to minimize the thermal or electronic noise. The incoming signal is recorded on a disk and processed and analyzed with specialized software. With radio spectroscopy not only it is possible to detect emission lines and thus the molecules that originated them in comets, but measuring the line area contained under an emission line like the one showed in Fig.2 lets us calculate the column densities of a molecule and translate it into the production rate of that species in molecules per second. This determination of molecular abundances in comets is very useful for comparing different

4 Figure 3: The Arizona Radio Observatory 10m Submillimiter Telescope in Mt. Graham, AZ. Most of the results reported in this dissertation were obtained with this instrument. Photograph taken by the author during one of his observing runs in March 2018. comet populations. Moreover, radio spectroscopy is a specially useful technique for the study of molecules in cold environments due to the low temperatures at which rotational transitions occur. This technique has provided many ﬁrst detections of cometary molecules and is more useful than IR or UV spectroscopy when observing comets at large heliocentric distances. Radio Spectroscopy also allows us to accurately determine physical characteristics of the coma like gas expansion velocity, temperature, the physical extent of a certain molecule within the coma or even the rotation period of a comet [10]. There are several physical parameters that can be inferred by the line width alone of an emission line like the one shown in Fig.2. For example, the line width will be mainly aﬀected by Doppler broadening and broadening caused by an expanding source. In thermodynamic equilibrium, the motion of the molecules of a species will result in the broadening of a line. When this happens, the random and turbulent motion of the molecules will follow a Maxwellian distribution of

5 Figure 4: CO emission in comet 29P/Schwassmann-Wachmann. Note the typical double- peaked structure of the line. Figure reproduced from Womack, Sarid & Wierzchos 2017 [9].

velocities. This distribution can be combined with the Doppler formula that relates frequency shift with radial velocity to derive an expression for the full width half maximum FWHM [11]: 2ν 2kT 1/2 ∆ν = ln 2 k + V 2 (1.2) D c M t

Where ν is the frequency of the transition, M is the atomic molecular mass,Vt is the most probable turbulent velocity (which is assumed to have a Maxwellian distribution), and Tk is the kinetic temperature of the moving molecules. Thus measurements of the FWHM of a line, can be used to estimate the kinetic temperature Tk of the molecules within a comaetary coma. Also, an expanding source (like the outgassing shell of a comet) will produce a line broadening. A radiating shell of expanding gas will give rise to a double peaked line proﬁle with a blue-shifted peak (corresponding to the radial component of the gas expansion velocity that points towards Earth) and a red-shifted peak (corresponding to the radial component of the gas that points outwards of the Earth direction) Fig.4. This characteristic double-

6 peaked line proﬁle is not observed on Fig.2 due to the relatively low spectral resolution of the telescope used. Because of this double-peaked nature of the line, it is possible to estimate the gas expansion velocity Vexp by measuring the blue-shifted component of a line or by considering the blue-shifted region of a line at the FWHM Fig.4.

1.2 Excitation processes of linear molecules

A description of the mechanisms governing the excitation of the molecules within the coma is needed in order to interpret the spectra and derive a production rate. The two main mechanisms capable of producing a significant excitation of cometary volatiles at radio wavelengths are collisional excitation and fluorescence radiative processes. The dominant excitation mechanism in the inner coma and close to the comet’s nucleus will be collisional excitation, since that is where the density of the gas is higher [12]. The species that is most abundant in comets is water, thus the collisions of a studied molecule X with water molecules should be taken into account. Collisions involving ions are generally neglected [2] due to the low abundances of ionic species compared to water. The rotational population of the ground state of a linear molecule X is thermalized by the collisions with water molecules. According to Bockelee-Morvan et al. 2004 [2] the collision rate C of a gas with water molecules is defined as:

C = σn(rc)V (1.3)

Where σ is the collisional cross-section, n(rc) the column density of water as a function of the cometocentric distance rC , and V is the mean velocity between the water molecules and the molecules of the species X for a given temperature (See Equation 14 ). The value n(r) can be derived from:

QH2O n(rc) = 2 (1.4) 4πrc Vexp

7 Where QH2O is the water production rate of comet C/1995 O1 (Hale-Bopp) at that heliocentric distance as empirically determined by Biver et al. 2002 [13], and Vexp is the gas expansion velocity of the species X. The value of QH2O for comet C/1995 O1 (Hale-Bopp) is usually used in calculations of other long-period comets because this comet is the only comet with this rate accurately determined throughout its perihelion passage and beyond.

The collisional cross-section, σ, for common neutral species found in comets (like H2 and CO) is poorly studied [2] and most cometary excitation models assume values of σ ∼ 1x10−14 to 5x10−14 cm2 [14–16]. In the outer coma, where the gas densities are lower the most important excitation mechanism will be fluorescence [17] with an excitation dominated by the balance between spontaneous decay and solar-pumping. This condition is called fluorescence equilibrium and happens when the populations of the levels absorbing and emitting photons are steady and controlled by fluoresence, i.e., the absorption and subsequent emission of a photon with less energy by an atom or molecule. The key excitational parameter at fluorescence equilibrium is the rotational population distribution ffluo which is dependant on the Einstein value for spontaneous decay Aul and the excitation rate of the transition gr which in turn depends on the heliocentric distance r [12].

1.3 Calculation of column densities and production rates

In order to calculate the production rate of a linear molecule X, ﬁrst we have to ﬁnd the number of molecules per unit area along a line of sight. This is what is called the column density. Once the column density has been calculated, and having the expansion velocity of the gas within the coma and the scale-length of the molecules, we can use the Haser model [18] to derive the production rate of a molecule [2, 12, 17, 19]. The main idea is to measure the area under an emission line like the one shown in Fig.2 and translate it into column density, then use the Haser model to convert the column density into a molecular production rate. The general form of column density can be expressed as the number of molecules in an

8 Table 1: Parameters needed to model production rates

Observational parameters Heliocentric distance (au) r Geocentric distance (au) ∆ Half-power beam width (arc sec) θ Telescope beam eﬃciency η −1 Emission line area (K km s ) T*adv Excitational parameters Frequency of the emission (Hz) ν Upper total rotational angular momentum Ju Energy above ground state (K) ∆Eu Population distribution at ﬂuo. eq. ffluo −1 Fluo. excitation rate of the transition (s ) gr −1 Einstein value for spont. emission (s )Aul Collisional parameters Collisional cross section (cm2) σ Transition distance (cm) rtran Mass of a molecule (amu)X mx Mass of a water molecule (amu) mH2O 2 −1 Water production rate (mol s )QH2O

energy level u integrated over a pathlength dρ:

Z Ntot = nudρ (1.5)

The column density of a species in a comet depends on a variety of observational, excitation and collisional parameters listed in Table1.

In radio astronomy, the first step in deriving the molecular column density will be to correct for the beam efficiency of the radio telescope used. Radio telescopes can be treated and modeled as antennas and it is desirable to account for the the efficiency η of the antenna integrated only over the main beam, neglecting the side lobes and the back lobe of the antenna. This parameter is specific for every radio telescope [20].

9 Thus we deﬁne the line area corrected for beam eﬃciency as:

∗ Tadv = Ta dv/η (1.6)

The beam of the telescope, θ, is expressed in angular units (usually arc seconds) and same as with the resolution of an optical telescope, the beam width of a radio telescope depends on the diameter of the reﬂector and the observed wavelength:

λ θ = 251643.3 (1.7) D

Where θ is in arc seconds, λ, is the wavelength of the observed emission and D is the diameter of the antenna. This observational parameter has to be converted to units of distance on the plane of the sky at the comet’s distance, which is refereed to as the projected radius of the beam ρ and it is usually expressed in cm.

∆θπ · 1.49 × 1013 ρ = , (1.8) 7200 · 180

where 1.49 x 1013 is the value of an astronomical unit in cm.

At this point we should decide the excitational temperature, Tex, of the studied molecule X. It is possible to measure the temperature within a comet’s coma via in situ experiments with spacecraft missions [21], with radio observations of multiple rotational transitions of the same molecule [22], or it can also be theoretically derived using the Plank’s law of black body radiation [19]. Biver et al. 2002 [13] derived an empirical expression for Tex using radio observations of diﬀerent rotational transitions of CO, CH3OH and H2S in comet C/1995 O1 (Hale-Bopp). This expression is widely used in cometary excitation modeling and it directly

relates Tex with the heliocentric distance r.

−1.24 Tex = 116 · r , (1.9)

10 where Tex is expressed in K, and r in astronomical units. For pure rotational transitions, this excitation temperature at millimeter wavelengths is often called the rotation temperature,

Trot.

Let us now introduce the rotational partition function for linear molecules Zrot, deﬁned as the statistical sum over all the molecular rotational energy levels according to Equation 49 of Mangum & Shirley 2015 [19]:

∞ X −(EJ /kTrot) Zrot = (2J + 1)e ; (1.10) J=0 where k is the Boltzman’s constant in the CGS system with a value of 1.38 x 10−16 cm2g

−2 −1 s K and, EJ , are the rotational energy levels for a linear molecule X. Those energy levels

EJ can be written as a multiterm expansion as function of J(J+1) [23] but using the rigid rotor approximation, it can be simpliﬁed and written as:

EJ = hB0J(J + 1) (1.11)

Where B0 is the rigid rotor rotation constant. Now that we have the rotational partition function, and assuming that we are in local thermodynamical equilibrium LTE we can write the fraction of molecules in the upper level J corresponding to the observed rotational transition with energy ∆Eu as:

(2J + 1)e−(∆Eu/Trot) fuLT E = (1.12) Zrot

We can now obtain the total population distribution of the transition by subtracting the fraction of molecules in the J level from the rotational population distribution at ﬂuorescence equilibrium, ffluo:

ftot = ffluo − fuLT E (1.13)

11 Where ffluo depends on the excitation rate of the transition, gr, and the Einstein coeﬃ-

cient of spontaneous decay Aul. The value of ffluo as a function of gr and Aul is tabulated in Table A1 of Bockelee-Morvan & Crovisier 1985 [24]. At heliocentric distances within 3 AU water is the most abundant species that outgaesses by sublimation, and many water molecules will collide with the molecule X that we intend to study introducing a collisional excitation [12]. The mean velocity between water molecules

and molecules of the species X for a given temperature Trot can be written as:

8kT 1 1 1/2 V = rot + (1.14) π mH2O mx

The point within the coma (in cometocentric coordinates) at which the radiative excitation rate of the transition gr is equal to the collisional excitation rate, C, is deﬁned as the transition distance rtran:

1/2 1/2 σQH2OV rtran = (C/gr) = 2 , (1.15) gr4πrc Vexp

where Vexp is deﬁned as the gas expansion velocity in the comet’s coma. As discussed by

Biver et al. 2002 [13], Vexp can be estimated in a spectrum by taking the half width of the

−1 −1 FWHM of the emission line. Typical values of Vexp vary between 0.1 km s and 0.9 km s depending on the heliocentric distance of the comet r.

Having already deﬁned the transition distance, rtran, and taking into consideration the

fraction of molecules in the upper level fuLT E, and the total population distribution of the

transition ftot, we can now write the population of molecules in the upper state of the observed transition when taking into account collisional and ﬂuorescence excitation as:

f fu = f + uLT E (1.16) tot rtran 2 1 + ( ρ )

12 Let us now deﬁne the column density for the upper state of the transition at the Rayleigh

- Jeans limit (hν/kTrot<<1 ) which is the case for low frequencies observed in comets that are closer than approximatively 1 au from the Sun. According to eq.10 from Bockelee-Morvan et al. 2004 [2]: 2 rj Tadv8πkν Nu = (1.17) hcAul

Where c is the speed of light and Aul the Einstein spontaneous emission coefficient for the specific transition u → l. At the wavelengths of the rotational transitions, the pure rotational excitation by solar flux is negligible due the weakness of the solar radiation at these wavelengths. However, at r > 3 au, rotational excitation by the cosmic background radiation must be taken into account [14]. To do so, we introduce a cosmic background correction defined as:

−1 JTbg Cbcorr = 1 − (1.18) JTrot

where JTbg and JTrot are the Plank’s equations for a temperature equivalent to the cosmic

background Tbg = 2.735 K and Trot respectively:

−1 (hν/kTbg) JTbg = e − 1 (1.19)

−1 (hν/kTrot) JTrot = e − 1 (1.20)

Now we can introduce a Rayleigh-Jeans correction such as:

hν 1 Rjcorr = (hν/kT ) (1.21) kTrot (1 − e rot )

Whether to apply the Rayleigh-Jeans correction or not will depend on ν and Trot (which

in turn depends on the heliocentric distance r). Values of hν/kTrot for some of the most

13 Table 2: Values of hν/kTrot for the application of the Rayleigh-Jeans correction.

CO(2-1) (ν = 230 GHz) hν/kTrot T=5K 2.21 T=40K 0.277 T=70K 0.158

CO(6-5) (ν = 691 GHz) hν/kTrot T=5K 6.64 T=40K 0.830 T=70K 0.474

HCN(3-2) (ν = 265 GHz) hν/kTrot T=5K 2.55 T=40K 0.319 T=70K 0.182

frequently observed molecules and transitions at the typical Trot at which comets are exposed are listed in Table2. By multiplying equations (1.17),(1.18) and (1.21) we ﬁnd an expression for the column density taking into account the Rayleigh-Jeans correction and the cosmic background correction:

0 rj Nu = Nu CbcorrRjcorr (1.22)

And combining this expression with (15) we get to the ﬁnal column density that takes into account collisional and ﬂuorescence excitation mechanisms:

0 Ntot = Nu /fu (1.23)

This column density will be next used to the derive the ﬁnal molecular production rate. The estimation of cometary molecular abundances requires the column densities that have been measured from the area under the emission line of the spectrum to be converted into molecular production rates in molecules s−1. In cometary science the standard model used to convert column density into a production rate i.e. into an outgasing rate is the Haser model [18]. This model is based on the assumptions that the outgassing is isotropic with

14 Table 3: Constants used for the calculation of production rates in the forthcoming chapters

−1 2 Molecule ν (GHz) ∆Eu (K) Aul gr (s ) σ (cm ) CO(1-0) 115.268 5.53 7.7×10−8 2.6×10−4 2×10−14 CO(2-1) 230.537 16.59 6.9×10−7 2.6×10−4 2×10−14 CO(6-5) 691.473 116.15 2.3×10−5 2.6×10−4 2×10−14 13CO(2-1) 220.398 15.86 6.5×10−7 2.6×10−4 2×10−14 HCN(3-2) 265.886 25.52 8.4×10−4 - 5×10−14 a molecule sublimating directly from the nuclear surface at a constant rate and expanding at a constant velocity outward in a radial manner. With these assumptions, the production rate of a species X is related to the number of particles Ntot in the coma as:

βρ Q = 2NtotVexpρe (1.24)

Where β is deﬁned as the reciprocal scale-length in cm of the molecule X (inverse of velocity × lifetime). Values of the scale-length for the most common volatiles found in comets were determined by Cochran 1985, Fink et al. 1991 and A’Hearn et al. 1995 [25–27]. In the present dissertation production rates of several molecules have been calculated for comets 174P/Echeclus, 29P/Schwassmann-Wachmann and C/2016 R2 (PanSTARRS). The constants needed for the derivation of these production rates are listed in Table3 and can be found in NRAO’s Splatalogue [28], Chandra et al. 1996 [29], Biver et al. 1999 [14], Crovisier & Encrenaz 1983 [30], and Lis et al. 1997 [31].

15 Chapter 2 Carbon monoxide in the distantly active Centaur (60558) 174P/Echeclus at 6 au

Note: Most of the work in this chapter was published in The Astronomical Journal, 2017, 153, 5. Wierzchos, K., Womack, M., and Sarid, G. Carbon monoxide in the distantly active Centaur (60558) 174P/Echeclus at 6 AU https://doi.org/10.3847/1538-3881/aa689c

2.1 Introduction

Minor body (60558) 174P/Echeclus is enigmatic: a rare dual comet-asteroid object that is sporadically active throughout its orbit. It has peculiar behavior that may provide important tests to Solar System models. When originally discovered by Spacewatch in March 2000, it was at r = 15.4 au from the Sun and had an asteroidal appearance and orbit. Thus, it received the designation 2000 EC98 (60558). Five years later, it underwent a major outburst and produced a sizable dust coma, and so it also received the short-period comet designation 174P/Echeclus [32]. It is mostly dormant, with four known instances of activity: 1) a dust coma was seen in 2000 January (r = 15.4 au) in pre-discovery Spacewatch images, 2) a massive outburst in 2005 December (r = 13.1 au)[33], 3) a moderate outburst in 2011 May (r = 8.5 au)[34], and 4) another moderate outburst in August 2016 (r = 6.2 au) [35]. Echeclus belongs to a population of Solar System minor bodies known as Centaurs, whose orbits range between Jupiter and Neptune (5 – 30 au), have small inclinations, and may be in transition between the inner and outer Solar System [36–39]. Echeclus is one of the few

16 Centaurs to have a relatively high eccentricity (0.44), and it is sometimes called a “Jupiter- coupled” object [40]. Its last perihelion in 2015 April brought it relatively near the Sun: q = 5.8 au. Most of the known Centaurs are asteroids, but 10% to 15% of them exhibit some kind of cometary activity [41]. The evolution of KBOs into Centaurs and then Centaurs into JFCs can be understood as a diffusion from the Kuiper Belt into the inner Solar System after being sequentially exposed to the gravitational influence of the giant planets [37, 42]. Thus, their measured properties are valuable for constraining Solar System formation models [43, 44]. Echeclus exhibited unusual behavior during its second outburst, which increased in total visual magnitude from 21 to 14 and produced a large dust coma. Interestingly, optical CCD images showed that most of the coma was separated from the central nuclear condensation, and appeared more diffuse than most cometary comae [41, 45]. The detached coma’s distance changed over time (from 1 – 9") and even appeared to backtrack at one point, reminiscent of a satellite in a hyperbolic orbit [33, 46]. The overall angular extent of the detached coma was ∼ 2’ (projected to be 1,040,000 km across at the comet’s distance) surrounding a significantly brighter area 12" in diameter [47]. Further analysis concluded that the detached coma may have originated from a fragment a few kilometers or less in diameter, possibly ejected by impact or explosive outgassing from the much larger primary nucleus [45, 48]. The fragment’s true nature is still unknown and it has not been seen since late 2006, nor was it seen in pre-discovery images. It may have been an isolated event that subsequently disintegrated into smaller pieces or sub-fragments [48]. After the second outburst dissipated, Echeclus showed no signs of coma for many years [45, 49, 50], until the third outburst in 2011, when it increased in total visual magnitude from ∼ 19 to 14 [34, 51]. In this instance, the coma was not detached, and reached a diameter of ∼ 35", which corresponds to a projected distance of 180,000 km at Echeclus. The fourth outburst recently occurred on 2016 August 28, when Echeclus was r = 6.2

17 au from the Sun, post-perihelion, with an r’ magnitude of = 15.21, and an asymmetric coma 14" across [35]. The coma was observed on multiple occasions, reaching a maximum size of ∼ 72", or 280,000 km, on 2016 September 12 (Soulier, J.F., personal communication). The coma persisted until the beginning of October, and to date, no secondary component is reported from this outburst. Optical and infrared measurements provided additional important information about Echeclus. Its diameter is large for a comet, but typical of Centaurs. The best value of its

diameter is from Duffard et al. 2014 [52], who derive DEcheclus = 64.6 ± 1.6 km using data from Spitzer and Herschel space observatories. Dust production rates were measured to be ∼ 700 kg s−1 during the second outburst, and only 10 to 20 kg s−1 for the third outburst [51]. Spectrophotometric measurements of the surface during a dormant stage provided colors of V-R = 0.47 ± 0.06, and a R-I of 0.50 ± 0.06, which are consistent with the dynamical model of Centaurs originating from the Kuiper Belt [53]. The dust coma colors obtained two months after the massive second outburst were V-R = 0.50 ± 0.06 and R-I of 0.58 ± 0.05, which is slightly redder than the reflected nucleus colors. This was interpreted as evidence for the presence of large grains of up to ∼ 3mm across, [53–55], and inconsistent with an impact origin for the outburst [41]. Developing comae at such large distances is unusual. Most comets are not active at heliocentric distances beyond ∼ 3 au, where the temperature is too low for the water ice to sublimate (e.g., [56]). Nonetheless, many comets are observed to have outbursts, or other lower-level outgassing, and other volatiles have been suggested as drivers of distant comet activity. Crystallization of amorphous water ice near the surface and subsequent release of trapped highly volatile molecules is considered to be an important contributor to activity for some comets within 5 – 7 au [57, 58], see recent review by Womack et al. 2017 [9]. If gaseous emission is identified in Echeclus, this can provide important clues to the nucleus’ chemical composition. Some leading contenders are CO, CO2, and CH4, which are cosmogonically

1r’ Sloan is a variant of the photometric R ﬁlter.

18 abundant with suﬃcient equilibrium vapor pressures to vigorously sublimate at very large (>35 au) heliocentric distances. Other well-studied Centaurs include 29P/Schwassmann-Wachmann 1 (hereafter 29P), 2060 Chiron, 10199 Chariklo, 5154 Pholus, 165P/Linear, 39P/Oterma, 165P/Linear, 8405 Asbolus, and C/2001 M10 (NEAT), some of which also exhibit activity at large heliocentric distances [52, 59, 60]. Four Centaurs have additional unusual features: Chariklo and Chiron appear to have ring-like structures [61, 62], and there are reports of both 29P and Echeclus having a secondary source of dust emission (and a second small CO outgassing region for 29P), possibly emitting from a chunk or fragment in the coma [63, 64]. In order to test physical and chemical models of Centaurs (and hence by extension models of KBOs and JFCs), it is important to determine whether gaseous activity is common among Centaurs.

Spacecraft-based studies show that CO2 is the dominant volatile for most distant comets [65–67], and many models incorporate it as a likely driver of distant activity [68, 69]. Unfor-

tunately, CO2 is diﬃcult to observe from the ground, and we focused on carbon monoxide emission, which is readily observable in cold cometary gas through its rotational transitions [17]. Observations of two other well known Centaurs, 29P and Chiron, have activity that can be explained by CO outgassing, possibly involving the crystallization of amorphous water ice [70, 71]. It is diﬃcult, however, to draw general conclusions about Centaurs based on such limited data. Previous attempts to detect CO in Echeclus with the JCMT 15-m telescope yielded an upper limit of Q(CO) < 3.6×1027 mol s−1 near aphelion at r = 14.8 au [72, 73]. This limit was interpreted as being incompatible with there being much solid CO ice on the surface, but does not constrain models any further. Obtaining a new, sensitive, measurement of CO production rate in Echeclus at r ∼ 6 au and within the predicted temperature zone where amorphous water ice readily crystallizes, would provide an important observational constraint to models.

19 Echeclus passed through its perihelion on Apr 2015 at r=5.8 au and this provided an excellent opportunity to search for CO emission. In late spring 2016 it was r ∼ 6 au, which is ∼ 2.5 times closer to the Sun and Earth than the previous best measurement of CO. Thus, the nucleus would be signiﬁcantly warmer, and any gas coma would appear larger in the sky, both of which should be more favorable for detecting CO outgassing. Observing at this distance provides a good test to nucleus models, which predict that outgassing should occur if CO ice, or trapped CO gas, is relatively near the surface. Echeclus is also within the ∼ 5-8 au zone where insolation can trigger amorphous water-ice to undergo crystallization, which also might release trapped CO gases [68]. CO emission has been observed in other comets at this distance, most notably 29P and Hale-Bopp [13, 70, 74, 75]. Thus, we performed the most sensitive search to date for CO emission in a Centaur, Echeclus. Here we present a 3.6-σ detection of CO emission, derive CO production rates, and discuss implications.

2.2 Observations and Results

We searched for CO emission in Echeclus via its J=2-1 rotational transition at 230.538 GHz with two instruments: the Arizona Radio Observatory (ARO) Sub-millimeter 10-meter telescope (SMT) and the Institut de Radio Astronomie Millimetrique (IRAM) 30-m facility. Access to both telescopes was granted via director’s discretionary time. The SMT was used to collect spectra from 2016 May-June, which led to a CO detection. During the SMT time, there were no reports of optical observations by the amateur community, thus its activity status is unknown. On 2016 28 Aug, the comet reportedly underwent its fourth observed outburst, and produced a dust coma. Its visible brightness increased from ∼ 18 mag to 15, which it maintained for several days. A dust coma appeared and remained until at least mid-October. After the outburst, we immediately requested target-of-opportunity time on the IRAM 30-m telescope to search for residual CO emission that might be associated with the outburst. Observing time was granted, but not until 24 days after outburst. At this time Echeclus still had a dust coma, but its visible magnitude had dropped to ∼ 17 (MPS 730323).

20 Thus, we assume that the IRAM data were obtained when Echeclus was not outbursting, but possibly still outgassing at a lower level. For both telescopes, comet positions and velocities were obtained from the Jet Propulsion Laboratory (JPL)/HORIZONS ephemeris system. Observations and results for both observing runs are summarized in Table Tab.4 and explained below.

Table 4: Observations of CO J=2-1 line in 174P/Echeclus

UT Date Telescope r ∆ Tadv Q(CO) (2016) (AU) (AU) (mK km s−1)(×1026 mol s−1) May 28 – Jun 09 SMT 10m 6.1 6.6 4.8 ± 2.0 7.7 ± 3.3 Sep 21 IRAM 30m 6.3 5.3 < 28 < 12.2

2.2.1 ARO SMT 10-m

Observations using the SMT were carried out toward Echeclus between 2016 May 28 and June 13 when the comet was an average of r = 6.13 au from the Sun and ∆ = 6.64 au from the Earth. The dual polarization 1.3 mm receiver was used, with ALMA Band 6 sideband-separating mixers. All of the data were obtained in beam-switching mode with a reference position of +2’ in right ascension, and an integration time of 3 minutes on the source and 3 minutes on the sky reference position. The temperature scale for all SMT

∗ ∗ receiver systems,TA, was determined by the chopper wheel method, where TR=TA/ηb, and

ηb = 0.74 at 230 GHz. The backends were a 2048 channel 1 MHz filter bank used in parallel (2 x 1024) mode and a 250 kHz filter also in parallel (2 x 250). The SMT has a half power beamwidth of θ = 32" at 230 GHz, which corresponds to an average projected radius of 78,740 kilometers at the comet’s distance. During this time, the comet’s heliocentric, r, and geocentric, ∆, distances ranged from r = 6.12 – 6.14 au and ∆ = 6.72 – 6.58 au. The spectra provided velocity resolutions of 0.325 km s−1 and 1.301 km s−1 for the 250 kHz and 1 MHz resolutions, respectively. The 250 kHz filter was the highest spectral

21 Figure 5: Spectrum of the CO J=2-1 rotational transition obtained from the Arizona Radio Observatory Submillimeter 10-m telescope between 2016 May 28 and June 09 (upper panel) and rebinned by 2 channels (lower panel). The line has a FWHM line width of ∼ 0.53 km s−1 and is shifted from the predicted ephemeris velocity (denoted by dotted vertical line) by -0.55 km s−1. This line profile is consistent with emission of a cold gas from beneath the nuclear surface on the sunward side of the comet. resolution available, but the line still may not be resolved, since CO emission in distant comets is known to be very narrow. The 1 MHz line was significantly broader than the expected CO linewidth and thus was not used in the analysis. Focus and pointing accuracy were checked periodically on nearby planets, and the pointing consistently had an uncertainty of < 1" RMS. During June 10-13, the observing run suffered bad weather conditions and instrumental problems, and thus data from this time period were excluded. Otherwise, the weather was mostly very good and there were very few technical issues. Only scans with a telescope system temperature, Tsys, under 500K were used, which excluded 15 scans, resulting in a total of 23 hours of scans accumulated on the source. Individual scans were checked to rule out possible bad channels at the position of

22 the observed feature. During the 2016 May-June observations with the SMT, we detected the CO J=2-1 line in Echeclus, as shown in Fig.5. The feature was present in both spectral polarizations, and was seen in the data throughout the observing period. This feature has a cumulative

signal-to-noise ratio of 3.6-σ. Taking into account the SMT beam eﬃciency of ηb = 0.74, the observed line has an intensity of TA = 8.53 ± 2.71 mK and line area of TAdv= 4.8 ± 2.0 mK km s−1 if a gaussian ﬁt is assumed.

2.2.2 IRAM 30-m

The IRAM observations were conducted during six hours on 2016 September 21, over a single transit, when Echelus was at r = 6.31 au and ∆ = 5.38 au. The measurements were made using the wobbling secondary mirror with a 2’ sky reference position, with the EMIR 230 GHz receiver, and high resolution VESPA autocorrelator at 80 KHz/channel resolution as backend. The spectral resolution (0.104 km s−1 when converted into Doppler velocity) is high enough to resolve most spectra from cold gas in distant comets. At this frequency, the IRAM half power beamwidth is θ = 10.7", which corresponds to 41,000 km at the comet’s

geocentric distance on September 21. The beam eﬃciency is ηc = 0.59, and then the main

∗ beam temperature is Tmb=TA/ηc. A total of 43 scans were obtained on source of 6 minutes each. Focus and positional accuracy were checked periodically on Uranus. The pointing consistently had an uncertainty of < 1" RMS. No line was detected with IRAM data (Fig. 6), and the noise RMS uncorrected for beam eﬃciency, within ± 20 km s−1 of the tuned frequency is 10.49 mK.

23 Figure 6: Spectrum of the CO J=2-1 rotational transition obtained from the IRAM 30m telescope on 2016 September 21.

2.3 Analysis and Discussion

2.3.1 Spectral line proﬁle

Spectral line proﬁle characteristics, such as linewidths and velocity shifts, provide important information about the gas emission release. A gaussian ﬁt to the feature in the

−1 SMT Echeclus spectrum yields a narrow line with ∆VFWHM ∼ 0.53 ± 0.23 km s that is blue-shifted by a small amount from the expected comet ephemeris geocentric velocity, -0.55 ± 0.10 km s−1, see Fig.5. The feature is not spectrally resolved, so the true linewidth could be narrower. The blue-shifted, narrow line proﬁle is very similar to what was observed at 6 au for 29P and Hale-Bopp, which gives additional credence to the 3.6-σ detection. It also suggests a common outgassing mechanism for CO at this heliocentric distance. The CO line in Chiron was also narrow, but not blue-shifted from the ephemeris velocity. The blue-shifted,

24 narrow line proﬁle is very similar to what was observed for CO emission at 6 au for 29P and Hale-Bopp, and comparable to what was measured for Chiron at 8.5 au [13, 64, 70, 71, 74–76]. The line proﬁle characteristics in Fig.5 are consistent with emission of a cold gas originating beneath the nucleus surface on the sunward side, and is also explained by hydrodynamical models which predict very low temperatures for cometary volatiles in the absence of photolytic heating [39, 77]. With respect to the observing geometry at this time, Echeclus’ phase angle was very low, β ∼ 8 degrees, which means that the sunward side was very similar to the Earthward side.

2.3.2 Excitation and production rates

Using the ARO SMT spectrum, we derive a total column density of N(CO) = (4.9± 2.0)12 cm−2 using both collisional and fluorescence excitation in a manner similar to Crovisier & Le Bourlot 1983 and Biver 1997 [17, 78]. We assumed an optically thin gas and rotational and excitation temperatures of 10K and a gas expansion velocity of 0.2 km s−1, consistent with values used by Biver et al. 2002, Gunnarsson et al. 2008 and Drahus et al. 2017 [13, 64, 73] for CO emission in comets and Centaurs at large heliocentric distance. The low temperature and expansion velocity values are also consistent with the spectral line profile parameters described in the previous section. A production rate of Q(CO) = (7.7 ± 3.3)×1026 mol s−1 was calculated for May-June when Echeclus was r = 6.1 au from the Sun, assuming isotropic outgassing of CO from the nucleus and a photodissociation decay model [18]. Applying a narrow cone ejection model, which may be justified by the blue-shifted spectrum consistent with sunward-side emission, reduces the production rate by 40%. Our calculations assume that the CO emission fills the telescope beam (∼ 150,000 km at the comet’s distance). A detailed study of CO and CO2 showed that substantial amounts could be produced by extended sources in inner Solar System comets, within ∼ 2-3 au from the Sun [79]; however, because we are concerned with an object much farther out, we assume that all CO is from the nucleus.

25 We did not detect a line with the IRAM 30-m data, and derive a 3-σ upper limit by

∗ multiplying the RMS TA=10.49 mK, correcting it for beam eﬃciency (ηc = 0.59) by 3 and using a spectral window of the same size as the observed FWHM (0.53 km s−1) in the SMT observations. We calculate an upper limit of Q(CO) < 12.2×1026 mol s−1 for September 21, when the comet was 6.3 au from the Sun. This limit is higher than the value we derived a couple months earlier with the SMT.

2.3.3 CO production in Centaurs

Carbon monoxide emission is now reported for three Centaurs. Since its ﬁrst cometary detection in 1994, CO is strongly and repeatedly seen in 29P, an unusual object with a nearly circular orbit, ranging from r = 5.72 to 6.25 au. CO emission is reported once for Chiron [71] near perihelion, and now in the present work for Echeclus shortly after perihelion. Since subsequent searches did not detect CO in Chiron a second time, questions were raised about whether the Chiron CO detection was real [59]. An independent re-analysis of the original data maintained that the CO line was formally detected [72]. We assume that the CO detection in Chiron was likely from gaseous emission during a relatively brief (few days) outburst. Our derived CO production rate for Echeclus of Q(CO) ∼ 7.7×1026 mol s−1 is the lowest measured Q(CO) in any Centaur, and is ∼ 5 times lower than the upper limit of Q(CO) < 3.6×1027 mol s−1 derived by [73]. This value is also ∼ 10-50 times lower than what was reported for 29P and Hale-Bopp at r = 6 au. Although Hale-Bopp is not a Centaur, we include it for its value as a relatively unprocessed comet (it is a long-period comet and thus spends most of its time at very large heliocentric distances), and because CO was observed throughout its orbit [13]. The signiﬁcantly lower CO output of Echeclus compared to 29P and Hale-Bopp is note- worthy, since all three have similar sizes and the measurements are all at ∼ 6 au from the Sun.

Echeclus is approximately the same diameter (DEcheclus ∼ 65 km) as Hale-Bopp (DHB ∼ 60

26 km, [80]) and 29P (D29P ∼ 60 km, [81]). This fortuitous alignment of size and heliocentric distance allows us to eliminate some key parameters when comparing CO production rates. Thus, Echeclus’ low CO output clearly indicates that it is highly CO-poor compared to both 29P and Hale-Bopp. We cannot tell whether this is due to a difference in composition from different formation environments, or whether Echeclus originally had more CO, and is now devolatilized. Since Echeclus and 29P are both considered to be Centaurs, this difference in CO output is especially striking. Further discussion about the similarities of Q(CO) in 29P and Hale-Bopp is found in [9]. Typically, CO production rates are measured in Centaurs and comets that are not at the same heliocentric distance, making it difficult to compare CO output. One recent study used upper limits of Q(CO) and an energy balance model with a high albedo, which ruled out substantial amounts of CO ice being present on Centaur surfaces [73]. To explore Centaur outgassing further, we take a different approach. If we assume that Centaurs and comets have similar chemical and physical compositions, then their CO production rates should scale commensurate with surface area and distance from the Sun. Thus, we calculated their specific production rates, Q(CO)/D2, and plotted the results in Fig.7, following the method we introduced in [9]. We also calculated upper limits to specific production rates for other Centaurs using measurements from Drahus et al. 2017 [73], which are included in the figure. Following this line of reasoning, many Centaurs appear CO-poor when compared to comet Hale-Bopp. For example, Echeclus and Chiron both appear to produce 10-50× less CO/area than Hale-Bopp, and may be even more CO-poor when compared with 29P. We think that the Echeclus and Chiron detections are credible; however, even if one instead conservatively considers these CO observations as upper limits, then the conclusions still stand. In fact, if these were only upper limits, then Echeclus and Chiron would produce even less CO than inferred from the detections. When we include upper limits derived from Drahus et al. 2017 [73], even more Centaurs appear deficient or depleted in CO and may, in fact, be inactive.

27 Figure 7: Specific production rates, Q(CO)/D2, for distant comets and Centaurs. The solid line is a fit to data for Hale-Bopp, assuming Q(CO)=3.529×r−2, and diameter DHale−Bopp=60 km; values for Chiron and 29P are plotted assuming production rates from [9] and DChiron=218 km and D29P =60 km. This plot shows that, when adjusted for surface area and heliocentric distance, Echeclus and Chiron produce ∼ 10-50 times less CO than Hale-Bopp, a proxy for a relatively unprocessed comet. Upper limits to specific production rates for other Centaurs were calculated from Drahus et al. 2017 [73] and show that 29P is the only CO-rich Centaur found to date. All Centaurs with values below the Hale-Bopp line are considered to be deficient or depleted in CO.

As the figure shows, other Centaurs, such as Chariklo and 342842, are each CO-poor by ∼ 10× compared to Hale-Bopp at the same heliocentric distance. Also, the upper limits for 8405 Asbolus and 95626 are consistent with objects that produce at least 5× less CO/area than Hale-Bopp. Note that all the values from Drahus et al. 2017 [73] are upper limits, so the true CO output from these objects may be far lower, or even totally inactive. The “non- detection-upper-limit” can be difficult to interpret, especially if the objects are not known to be active, such as Chariklo and Pholus. Due to the different sublimation curves of different volatile species and the way they are maintained inside the nucleus, the outer solar system

28 can host objects that may seem depleted, but are actually quenched. This quenching can be due to a leg deposit forming, desiccation of upper layers or change of orbit (e.g., [68]). Interestingly, with the notable exception of 29P, no Centaurs produce the same amount of CO per surface area as Hale-Bopp. Thus, we conclude that there may be a significant difference in composition and/or bulk properties between most Centaurs and Hale-Bopp. We note that most of the Centaurs below the line in Fig.7 have the largest nuclei, which is important because specific production rates depend on the square of nucleus diameter. As discussed in Womack et al. 2017 [9], CO devolatilization may be expected in large Centaurs due to increased radiogenic heating. It is important to keep in mind, however, that this is partly a selection effect, because only significant limits have been made to date only for Centaurs with diameters greater than 65 km. We point out that 29P, which is the most CO prolific Centaur, is on the small size of Centaurs with a diameter of ∼ 60 km. It will be very useful to obtain more constraining Q(CO) measurements on smaller Centaurs. It may also be helpful to extend this analysis to other comets; however, we were unable to find any other distant comet Q(CO) measurements beyond 4 au which had independently determined Q(CO) and diameters. Even though Centaurs are thought to share a common origin in the Kuiper Belt, the well- known different outgassing behavior in Chiron and 29P has been attributed to the differences in composition and evolution history [82, 83]. The CO deficiency apparent in some Centaurs (consistent with Fig.7) may be explained by the devolatilization of small KBOs, in which only large KBOs are able to retain volatiles since the rate of volatile loss is controlled by the gravity and surface temperature [84, 85]. We bring up another potentially important characteristic: activity rate. Both Chiron and Echeclus (CO-deficient) outburst only rarely, sometimes with years between outbursts; whereas, 29P (CO-rich) has multiple outbursts per year. Since Echeclus and Chiron are only occasionally active, one possible cause is that emitted gas (possibly CO) originates from sub-surface ice patches, or possibly as trapped volatiles that are freed by the crystallization

29 of amorphous water ice. Echeclus and Chiron may have had more CO in the past and substantially devolatilized after moving into their Centaur orbits. In contrast, we note that 29P undergoes an outburst every ∼ 50 days on average. 29P looks like an outlier Centaur, by producing so much CO. Perhaps it is a more recent entrant to its current orbit than recently thought, or perhaps it has a different origin from most Centaurs and had more CO incorporated during its formation. Whether CO or another volatile is dominant in Echeclus or Chiron, we cannot tell, but we note that the measured CO production rates are high enough to explain measured dust production rates. Other highly volatile and abundant gases, such as CO2, CH4 and N2 may contribute to distant activity, but they are difficult to observe and none have been detected in any Centaur to date. Their sublimation temperatures are low enough that they may also be expected to be sublimating if any patches are sufficiently close to the surface, so they cannot be ruled out.

2.3.4 Outbursts, outgassing and fragmentation

We brieﬂy turn our attention to four Centaurs known to have additional unusual features. It is interesting that fragments, rings and ring arcs have been reported very near, or even in orbit around these Centaurs: Chariklo, Chiron, 29P and Echeclus. The origins of these features are still unknown. It is further interesting to postulate a correlation between the CO-activity level and the mass and distribution of these secondary components. One possible explanation is that perhaps these Centaurs’ nuclei are more inhomogenous in their highly volatile ices, namely CO, and thus they might be more prone to eject chunks of various sizes, as a strong outburst can release large fragments, whereas weaker outgassing can expel smaller particles [86]. Then, due to their mass and gravitational pull, the lofted material can remain in semi-bound or bound orbits around the primary. Subsequent evolution into the features we observe today (fragments, rings, arcs) should be studied more carefully, as well as its correlation with actual observed activity. We leave this for future work.

30 If CO is present in Echeclus and other Centaurs, and is released when amorphous water ice becomes crystallized, then amorphous water-ice should be present in their KBOs progenitors and in their possible evolution, the JFCs. However, thus far, the ice found in several KBOs, like Charon or Quaoar, is crystalline and not amorphous [72]. These studies cannot rule out the existence of some patches of amorphous water ice on the surface, or just below the surface. Such patches might reach the critical temperature once the object comes close enough to the Sun. The behavior of Centaurs displaying volatile outbursts when pulled into the inner Solar System from the Kuiper Belt region was found to be consistent with a devolatilized KBO progenitor with a compositionally layered structure [83]. Alternatively, the low CO outgassing observed in Echeclus may be due the presence of a crust on their surface, making it diﬃcult for the volatile molecules to ﬂow through the dust external layers [82]. This last scenario is considered unlikely for Echeclus, due to the fact that after the second outburst and fragmentation, virtually all the activity was from the detached coma, with very little, if any activity coming from the newly exposed inner regions [45]. Further observations and modeling of other Centaurs, especially active ones, could provide important clues to the bulk properties of their nuclei and place powerful constraints to Solar System models. In particular, further observations of Echeclus are needed as it moves away from perihelion over the next few years.

2.4 Conclusions

We report a 3.6-σ detection of the CO J=2-1 line in Centaur 174P/Echeclus during 2016 May-June with the SMT 10-m telescope, which yields to a Q(CO) production rate of (7.7 ± 3.3)×1026 mol s−1, when the object was at r = 6.1 au and ∆ = 6.6 au. No such line was detected 24 days after the 2016 August 28 outburst using the IRAM 30-m, but a 3-sigma upper limit of Q(CO) < 12.2×1026 mol s−1 was inferred at r = 6.3 au, which is consistent with our SMT measurement. Our data are consistent with Echeclus being a CO-deﬁcient

31 body, when comet Hale-Bopp’s data at ∼ 6 au is used as a proxy for a relatively unprocessed nucleus. When scaled by surface area and compared with CO measurements from other Centaurs, we see that specific production rates, Q(CO)/D2, from Echeclus and Chiron are ∼ 10-50 times below that of Hale-Bopp, and 29P, which are known to be CO-rich. The lower CO output of Echeclus and Chiron may mean that they incorporated less CO into their nuclei than many other comets, or they may have lost a significant amount due to devolatilization while in their relatively close-to-the-Sun Centaur orbits. It is puzzling why 29P, another Centaur, is evidently CO-rich. Although no other CO detections exist for other Centaurs, stringent upper limits to their specific production rates show that several other Centaurs are also notably absent of CO outgassing, including Chariklo, 8405 Asbolus, 34842 and 95626. Further measurements of CO production rates and fragmentation in Echeclus and other Centaurs are needed to constrain models of the bulk structure and strength of Centaurs, their KBO predecessors and JFC successors.

32 Chapter 3 CO gas and dust outbursts of Centaur 29P/Schwassmann-Wachmann

Note: This chapter is currently under review with The Astronomical Journal 2019 Wierzchos, K. and Womack, M., CO gas and dust outbursts of Centaur 29P/Schwassmann- Wachmann

3.1 Introduction

Comets are icy bodies that contain well-preserved and unprocessed material from the early stages of the solar system formation. They formed at large heliocentric distances where temperatures were low enough for many ices to condense. Their nuclei contain trapped volatiles, refractory grains and rock. When they get close enough to the Sun, ices sublimate and release gas and dust grains that form envelopes known as comae. Some of these grains have frozen ices on them, which may then sublimate once immersed in the solar radiation ﬁeld. Measuring the composition and structure of cometary objects, and how they become active, provides important observational constraints to models of the early solar system [1, 87, 88]. Comets are often classiﬁed into two broad categories based on their orbital periods; long- period (Porb > 200 years) and short-period comets. Long-period comets are distributed with random inclinations and large distances out to the Oort Cloud, whereas short-period comets tend to be found closer in and near the ecliptic, due to originating from the Trans-Neptunian region of the Kuiper Belt. Many short-period comets undergo a transitional Centaur phase and eventually become Jupiter Family comets [37, 89, 90].

33 There is no other object like 29P/SW1. It occupies a nearly circular orbit, slightly larger than Jupiter’s, with an eccentricity e = 0.045, a semi-major axis a = 5.99 au, and inclination i =9.39 degrees according to JPL Horizons. Thermal modeling of Spitzer-IRAC photometry suggest that the nucleus of 29P/SW1 is much larger than a typical comet, with a radius of 32

± 2 km [91]. Originally classified as a short-period comet (Porbital = 14.6 years), dynamical studies show that 29P/SW1 is a Centaur that probably only recently transferred to its current location, a newly discovered “Gateway" transitional orbit to the JFC population, and may eventually become the brightest comet seen by humankind [90]. 29P/SW1 is one of ∼ 30 Centaurs known to display comet-like activity. The vast majority of its activity is documented with observations at visible wavelengths. Almost all the light from 29P/SW1’s visible coma is due to the reflection and scattering of sunlight off the dust particles in the coma with small contributions from some ions and radicals [92]. Thus, visible lightcurves are used to analyze changes of the bulk dust production. One of the most puzzling properties of 29P/SW1 is the fact that it is the only Centaur or comet that exhibits a continuous “quiescent” stage of activity [93]. Moreover, this permanent coma is frequently boosted by dramatic transient episodes or outbursts (Table5). The orbital distribution of outbursts appears to occur preferentially after perihelion and before aphelion [94–96] and is reported to average about seven outbursts per year in the most recent orbit [97, 98]. Here we define an dust outburst as a brightening increase of at least 1 magnitude of the nuclear magnitude (typically the central 5-10" part of the coma) within a few hours to one day, which is consistent with other publications [91, 97–99]. The dust coma can also present morphological changes as the outburst progresses [100]. 29P/SW1’s nucleus is probably larger than all JFCs [91] and in a 1999 survey, 29P/SW1’s absolute nuclear magnitude was found to be brighter than all other Jupiter Family comets measured to date [101]. It is intriguing to consider how dramatically its behavior may change in a few thousand years from now if its orbit takes it significantly closer to the Sun to become a JFC [90].

34 Table 5: Typical visual magnitude observations of 29P/SW1

Quantity Quiescent Outbursting mv ∼ 17 16-10 2 mhelio ∼ 13 12-6

The Centaur has continually exhibited a dust coma since its discovery almost a century ago [93, 102–105]. This is remarkable, because at ∼ 6 au from the Sun, its nucleus is not warm enough to support much water-ice sublimation, the main source of activity for most comets, which become active when within ∼ 3 au of the Sun. Thus, 29P/SW1’s coma must be generated by a different mechanism, as with other distantly active comets [9]. Many alternatives have been proposed, including sublimation of a cosmogonically abundant low-condensation temperature ice, an amorphous-to-crystalline phase change of water-ice, HCN polymerization, cryovolcanism or meteoroid impacts [70, 106–109]. A strong candidate is CO outgassing in some physical process. Millimeter-wavelength CO emission was first detected in 29P/SW1 over 25 years ago and it has been measured on numerous occasions at millimeter- and infrared-wavelengths with high enough production rates to support lift off of the observed dust coma [64, 70, 77, 110]. CO has the highest production rate, by far, of all molecules detected in 29P/SW1 thus

+ far (Table6). Other species present in the coma are H 2O[66], CN, [92], CO [119], and

+ N2 [117, 120]. Water vapor was detected with low production rates [66], and preliminary

reports of high-resolution spectra of H2O state that the water emission is consistent with sublimation from icy grains in the coma and not from the nucleus [121]. CN is a short-lived radical daughter species, probably created from photo-destruction of HCN in the coma. HCN emission was claimed to be detected in 29P/SW1 with relative abundances similar to that of Hale-Bopp at 6 au, and also with spectral line proﬁle consistent with sublimating from icy grains in the coma; however, no quantitative values have been published yet [121]. Thus, the HCN values in Table6 were derived from CN measurements (see Womack et al. 2017

2 mhelio is the apparent magnitude, mv, corrected for geocentric distance with equation 1. In this work we refer to this quantity as m(1, r, θ).

35 Table 6: Measured and derived properties of dust and gas in 29P/SW1 coma

Material Production rate Mass loss rate3 Expansion speed Reference 1027 mol/s (kg/s) (km/s) Dust - 430 – 4700 0.05 – 0.15 [91, 111–116] CO 10 - 70 460 – 3200 0.20 – 0.50 [9, 14] H2O 6.3 188 – [66] + 4 N2 (N2 ) 0.39 17 – [9, 117] HCN (CN)5 0.008 0.3 – [92] H2CO 0.1 <5 – [118] CS <0.21 <15 – [118] CO2 <0.35 <25 – [66] C2H6 <0.57 <28 – [110] CH3OH <0.55 <29 – [118] CH4 <1.3 <34 – [110] H2S <1 <56 – [118] HC3N <0.95 <81 – [118] C2H2 <2.7 <117 – [110] NH3 <10.9 <309 – [110]

[9] for more details). The amount of CO+, detected during outbursting [119] and quiescent stages [122], is

+ consistent with forming from CO and not CO2 [123]. N2 is detected in very small amounts,

suggesting that N2 is not signiﬁcantly present in the coma or nucleus [9]. Signiﬁcant upper

limits were obtained for the highly volatile CO2 [66], and C2H6, CH4,C2H2, NH3, and

CH3OH [110], which indicate that all of these make much less of a contribution to the gas coma than CO (even when combined, see Table6). With a perihelion distance of only q = 5.72 au, the continuous presence of both dust and CO makes this object unique among the comet and Centaur populations. It may be typical of the beginning state of cometary activity for Centaurs progressing toward a Jupiter Family comet orbit [90]. Thus far, very few works present contemporaneous measurements of the dust and gas, and they present mixed results about the possible correlation between gas and

3Gaseous mass loss rates were calculated using measured production rates and appropriate atomic mass units. 4 + N2 production rate is inferred from N2 measurements. 5HCN production rate is inferred from Q(CN).

36 dust correlation in 29P/SW1. [105, 124]. An important observational constraint is to quantify whether and if so, how much, CO production is connected to dust outbursts. In order to assess this, we used secular lightcurves derived from visual brightness measurements obtained nearly at the same time as our CO millimeter-wavelength spectra.

3.2 Observations

3.2.1 Millimeter-wavelength spectra

We monitored emission from the CO J=2-1 transition at 230.53799 GHz toward 29P/SW1 with the Arizona Radio Observatory 10-m Submillimeter Telescope (SMT) during 2016 February – May and 2018 November – 2019 January (see Table7). The times listed in the table are at the end of the observing period for each day. In both observing campaigns, the data were taken with the SMT 1.3 mm receiver with ALMA Band 6 sideband-separating mixers in dual polarization. The backend used for the observations consisted of a 500 channel (2 x 250) ﬁlterbank with a resolution of 250 kHz/channel in parallel mode. This frequency resolution corresponds to 0.325 km s−1 per channel at the

CO (2-1) frequency. The SMT beam diameter at this frequency was θB = 32". All of the scans were taken in beam-switching position mode with a throw of +2’ in azimuth, and the system temperatures remained between 200-400K. All the scans were obtained by integrating three minutes on the source and three minutes on the sky for subtraction. The position of the comet was checked periodically against the ephemeris position provided by JPL Horizons. Tracking was found to be within < 1" RMS during both of the observing epochs. Additionally, pointing and focus was updated on planets and bright radio- source every six to ten scans.

37 Figure 8: A typical CO spectrum of comet 29P/SW1 obtained with the ARO 10-m SMT.

3.2.2 Visible magnitudes

We used measurements of 29P/SW1’s apparent visual magnitude, mv, to construct a secular lightcurve spanning the two time periods in which we monitored Q(CO). We used magnitudes from experienced observers who were well- versed in how to obtain, reduce and report CCD magnitudes of comets in order to minimize observer diﬀerences [125] and Womack, M. et al. in prep. In both epochs, all of the lightcurve data was obtained from the Lesia database of cometary observations6 and the Minor Planet Center Observation Database7 recorded with unﬁltered CCDs. The magnitudes from Lesia were obtained with telescopes ranging between 0.2-m and 0.4-m in aperture diameter by the observers J.-F. Soulier, F. Kugel, J.-G.Bosch, J. Nicolas, T. Noel, and P. Ditz. The observations from the MPC were obtained by TRAPPIST (0.6-m telescope in La Silla, Chile) and by J. Drummond (0.35-m telescope at Possum Observatory in Gisborne, NZ).

6http://lesia.obspm.fr/comets/index.php 7https://minorplanetcenter.net/db_search

38 Table 7: CO(J=2-1) observations of 29P/SW1 using the ARO SMT 10-m telescope

∗ 28 UT Date r ∆ TAdv ∆vFWHM δv Q x10 au au mK km s−1 km s−1 km s−1 mol s−1 2016 Feb 25.7 5.96 6.59 48±3 0.72 -0.32 3.00±0.17 2016 Feb 26.7 5.96 6.57 59±5 0.89 -0.39 3.67±0.28 2016 Feb 28.6 5.96 6.55 97±8 0.74 -0.35 5.93±0.47 2016 Feb 29.7 5.96 6.54 55±5 0.74 -0.48 3.38±0.28 2016 Mar 01.8 5.96 6.52 49±3 0.83 -0.36 3.05±0.20 2016 Mar 03.8 5.96 6.50 48±3 0.92 -0.36 3.01±0.20 2016 Mar 21.7 5.95 6.24 71±3 0.75 -0.26 4.36±0.22 2016 Mar 28.6 5.95 6.14 60±7 0.79 -0.33 3.70±0.36 2016 Mar 31.6 5.95 6.09 59±5 0.68 -0.38 3.62±0.28 2016 Apr 09.6 5.95 5.94 65±3 0.80 -0.33 4.01±0.16 2016 Apr 15.6 5.95 5.85 42±3 0.70 -0.29 2.62±0.18 2016 Apr 24.6 5.94 5.70 54±3 1.06 -0.25 3.34±0.15 2016 May 29.5 5.93 5.20 59±4 0.74 -0.34 3.63±0.18 2018 Nov 02.2 5.76 5.17 89±9 0.99 -0.14 4.16±0.43 2018 Nov 10.2 5.76 5.28 86±6 1.13 -0.30 4.06±0.29 2018 Nov 17.1 5.76 5.38 61±3 0.85 -0.26 2.93±0.30 2018 Nov 24.2 5.76 5.48 69±4 0.94 -0.18 3.44±0.18 2018 Dec 12.2 5.76 5.75 78±4 0.87 -0.30 4.06±0.20 2018 Dec 21.1 5.76 5.92 42±2 0.52 -0.43 2.23±0.13 2019 Jan 08.1 5.76 6.19 79±3 0.85 -0.34 4.40±0.17 2019 Jan 15.0 5.76 6.29 57±4 0.56 -0.36 3.26±0.21

We chose nuclear magnitudes measured with photometric apertures of 5 - 7" centered on the nucleus, in contrast to much brighter “total” magnitudes, which generally encompass most of the visible coma. Coma contamination typically occurs close to the nucleus; however, nuclear magnitudes are more likely than total magnitudes to pick up short-term changes in cometary activity.

We corrected the apparent visual magnitudes, mv, for the Centaur’s geocentric distance according to

m(1, r, θ) = mv − 5log10(∆) (3.1)

39 Where ∆ is the comet’s geocentric distance and ranges from 5.2 - 6.6 au for our observing periods. We also applied a phase correction to account for scattered light using the phase function φ[θ] normalized to 0◦ with

m(1, r, 0) = m(1, r, θ) + 2.5log(φ[θ]), (3.2)

following the method of Schleicher & Blair 2011 [126]8.

3.3 Results

The CO (2-1) transition was readily detected each day after a set of three to six scans, which were co-added to produce one spectrum (see Fig.8). Using a Gaussian ﬁt the average

−1 full-width half-maximum linewidth for the CO emission was ∆VFWHM = 0.87± 0.33 km s . This linewidth is somewhat broader than what is reported for other telescopes [70, 75] that used higher spectral resolution and were able to fully resolve the line. The emission was consistently blue-shifted by an average of δv = -0.32 ± 0.16 km s−1. In order to derive CO column densities from our spectra, we used measured ﬂuxes of the 2-1 line, and an excitation model that assumes ﬂuorescence and collisional excitation following the modeling methods of Covisier & Le Bourlot 1983 [17] and Biver 1997 [118]. We assumed an optically thin gas, an excitation and rotational temperatures of 10K and a gas expansion velocity of 0.3 km s−1. CO production rates were calculated from the column densities assuming a simple isotropic and constant radial outgassing model [18]. We plotted values of Q(CO) and m(1, r, 0) versus Julian date (see Fig.9). In order to test for possible correlations between the gas and dust production, we set as a default for the average quiescent CO production rate to align with the value of the quiescent visual magnitudes. Throughout our 21 days of CO observations, 29P/SW1 always produced at least Q(CO) = 2-3 ×1028 mol s−1 with regular variations up to ∼ 4 ×1028 mol s−1 and a peak of Q(CO) ∼ 6×1028 mol s−1 (see Table7). The corrected visible magnitudes ranged

8https://asteroid.lowell.edu/comet/dustphaseHM_table.html

40 Figure 9: 29P/SW1 Lightcurves of CO production rates (blue-ﬁlled squares) and visible magnitudes corrected for heliocentric distance and phase angle (open circles) during 2016 (top) and 2018/2019 (bottom). One CO outburst was detected on 2016 February 28 and four dust outbursts were seen on 2016 March 14, 2018 November 22, 2018 December 09, and 2019 January 08 (marked as A, B, C, and D respectively). The average quiescent phase magnitude is indicated by a horizontal dashed line, and was ∼ 0.4 mag brighter in 2018/2019 than in 2016. Each panel spans 120 days. 41 from m(1, r, 0) = 9.3 to 13.7 with an average quiescent value of m(1, r, 0) ∼ 12.7.

3.4 Discussion

3.4.1 Quiescent activity of gas and dust

The behavior of cometary bodies is governed by the structure and composition of their nuclei, including how well-integrated the dust, ice and gas components are [68]. CO is proposed to be the dominant source of the activity in 29P/SW1 observed at optical and infrared wavelengths, with a linear relationship between the CO and dust production rates involved in generating the seemingly permanent dust coma [107]. Broadly speaking, the CO millimeter-wavelength spectra and visible magnitude data presented here are consistent with data reported elsewhere for the Centaur [64, 70, 75, 77, 95, 97, 99, 118, 127]. The average CO production rates during the non-outbursting state are Q(CO) = (3.1 ±0.3)×1028 mol s−1, which is comparable to what is typically measured, especially considering that not all groups use exactly the same modeling or excitation temperatures and expansion velocities (for further discussion of consistent modeling of CO emission in distant cometary objects, see Womack et al. 2017 [9]). Carbon monoxide is typically assumed to be the main outgassing agent for 29P/SW1, because water-ice sublimation is very inefficient at ∼ 6 au, and CO is detected with high production rates that match and sometimes exceed the reported dust mass loss rates (see Table6). Further support for this model is the fact that, when observed at high enough spectral resolution the CO line profile has a strong narrow blue-shifted velocity component, a weaker red-shifted line, and a skirted feature that is well-explained by a mix of sunward and nightside emission, along with some CO possibly produced from an extended icy grains coma [64, 77, 114]. The source of sunward emission is thought to be fairly close to the nucleus surface near the subsolar point to provide the continuous blue-shifted outgassing, and supports an updated model of outbursts [91]. Although our spectra are relatively low resolution, the line profile is consistent with arising from a cold, slightly blue-shifted and

42 asymmetric gas (see Fig.8). This narrow line profile of a cold gas may be a common feature in CO emission from many distantly active Centaurs and comets [9]. In addition to Centaur 29P/SW1 (∼ 6 au), this characteristic was documented in Centaurs 174P/Echeclus at ∼ 6 au [128] and 95P/Chiron at 8.5 au [71], and comets C/1995 O1 (Hale-Bopp) beyond 4 au [74, 76], C/2016 R2 (PanSTARRS) at ∼ 3 au [22, 129], and C/1997 J2 (Meunier-Dupouy) at 6.3 au [124]. Given the similarity of CO emission profiles in the Centaurs and distantly active comets observed so far, future observations, especially with simultaneous measurements of the dust coma, will be invaluable for constraining models of the volatile and refractory structure of the nuclei, and ultimately to their formation environment. Interestingly, the Centaur’s quiescent stage measurably brightened at visible wavelengths between 2016 and 2018/2019. This difference of ∼ 0.4 magnitudes in the visible lightcurve can be seen by eye in Fig.9, and is the difference between the average quiescent magnitude of m(1, r, 0) = 12.89±0.21 in 2016 (r = 5.94 au) and ∼ 12.52 ±0.19 in 2018/2019 (r = 5.76 au). The Centaur was only about 3% farther from the Sun in 2016, which is not a large enough change to generate a ∼ 45% change in brightness due to insolation change, which should vary as r−2. Additional measurements are needed to confirm whether this is a longterm brightening trend or just a variable nature of the dust production’s quiescent state. The quiescent CO production rate may be ∼ 10-15% greater in 2018/2019 than 2016, but clearly did not increase by 45%, which if true, would have placed it at Q(CO) ∼ 4×1028 mol s−1 in early 2018-2019 (lower panel of Fig.9). A previous study claimed a lack of correlation between the gas and dust production in 29P/SW1 evident in a study of Q(CO) and visible magnitude over 20 years ago [124], but we think these results should be revisited. This conclusion for 29P/SW1 is in contrast to a strong correlation that the data showed for long-period comets C/1995 O1 (Hale-Bopp) and C/1997 J2 (Meunier-Dupouy). However, it appears that the work included all Q(CO) and magnitude values, regardless of whether they were obtained during the quiescent or

43 outbursting phases (or mixed). If the Centaur were having either a CO or dust outburst during the measurements described by Biver 2001 [124], then this would signiﬁcantly obscure any possible correlation. Comets Hale-Bopp and Meunier-Dupouy had far fewer outbursts, and so their data were highly likely obtained during when the comets were in a quiescent state. Also, the Q(CO) and magnitude pairs used in [124] were not obtained simultaneous, but were often separated by 2-3 days. Thus, although useful for showing that 29P/SW1 is a dust-poor comet compared to Hale-Bopp and Meunier-Dupouy, the results from Biver et al. 2001 [124] may not be appropriate to test models about whether CO outgassing is correlated for quiescent activity. To address this, the Q(CO) and magnitude values should be simultaneous and when 29P/SW1 was in a quiescent phase for both the gas and dust.

3.4.2 CO outbursts

A dust outburst frequently brightens 29P/SW1’s visual magnitude by 1-7 magnitudes over its average quiescent value. CO outgassing is often assumed to play an important role in the dust outbursts, largely due to the adequacy of models to explain the quiescent activity, even in diﬀerent ways [107, 130]. We constructed the Q(CO) and visible magnitude lightcurves to test the hypothesis that CO production is connected to dust outbursts, and if so, by how much. Our analysis shows that the timing of the CO- and dust-outbursts are not always correlated, raising doubt for how the outbursts are created. Our observations recorded the development of one CO outburst over ﬁve days. Starting at 2016 February 25.7 the CO production rate doubled within 70 hours and then returned to the original quiescent production rate three days later. As Table7 shows, there was no measurable change in either the Doppler shift or FWHM linewidth during this time, which implies that the doubling of CO production did not arise outgassing mechanism (described in section 3.4.1). We point out that Biver 1997 [118] also reported a factor of ∼2 increase in Q(CO) in 29P/SW1 that decayed in 2 | 3 days after the peak. These data were obtained during 1995 November 15-19 with the IRAM 30-m telescope. Thus, there are two documented

44 cases of development of a CO outburst, and both lasted ∼ 4-5 days. The 2-3 day return to quiescent level production rates after the CO outburst peaks is consistent with most of the molecules traveling out of the radio telescope beam after a few days. For example, consider that the SMT FWHM beamwidth at 230 GHz is 32", which corresponds to a projected radius of 63,500 km at the comet’s distance on 2018 November 24 (∆ = 5.4 au). If we assume a CO expansion velocity of 0.3 km s−1 (consistent with all modeling of CO mm-wavelength spectra), then we estimate that molecules traveling parallel to the plane of the sky (zero blueshift to the geocentric velocity) would “leave the beam” in about 2.4 days (t=x/v=6.35 x 104 km / 0.3 km s−1 = 2.4 days), and more quickly with the smaller IRAM beam. Molecules traveling out of the plane of the sky would remain in the beam a little longer since their tangential velocity is reduced. Thus, the total ∼ 4-5 day duration of the CO outbursts seen with the ARO SMT in 2016 (this paper) and with IRAM 30-m in 1995 [118] are consistent with a very short-term release of CO molecules from the nucleus and then most of the molecules moving out of the telescope beam. Additional observations of CO outbursts in 29P/SW1 with higher temporal, spectral and spatial resolution would significantly constrain models of the processes affecting CO at large distances from the Sun, but further analysis is beyond the scope of this paper. Analysis of optical spectra of 29P/SW1 in 1990 proposed that the CO+ and dust production were not strongly coupled. In particular, the column density of the (3-0) CO+ band at ∼ 4000 increased by a factor of 2.5, while the continuum magnitude (dust) measured simultaneously at ∼ 4450 was unchanged. One possible interpretation was that this was evidence that the gas and dust was not entrained [105], but this depended on an interpretation that CO+ was a good tracer of the CO production rate. In comets, CO+ can be produced by the photoionization of CO, but this process is slow and scales as r−2, and is therefore ineffective at this large a distance from the Sun and is unlikely to be the source of most of the observed CO+ [92]. Instead, CO+ column density and variability can be better explained by solar wind proton impact onto cometary CO. This process is strongly dependent on solar

45 wind particle velocities [105, 123, 131]. Thus it appears that for 29P/SW1, CO+ is not a straightforward proxy for CO, and is a better tracer of solar wind particle velocities at 29P/SW1’s location for a given date. Thus, the reported variations of CO+/dust continuum are likely not indicative of real changes of the gas/dust ratio without taking into account contributions from the solar wind.

3.4.3 Dust outbursts

There are four short-lived increases of ∆m > 1 mag in the visible lightcurve. Two of the dust outbursts appeared to occur without a substantial increase in CO production at approximately the same time (labeled B and C in Fig.9), and the data are inconclusive about the other two (A and D). Here we describe all four outbursts in detail. On 2016 March 14 29P/SW1 underwent a ∼ 3 magnitude dust outburst (labeled A in Fig.9), which corresponds to a ∼ 16-fold increase in brightness. Unfortunately, the nearest in time Q(CO) measurement was 6 days after the peak of the dust outburst. As we discuss in section 3.4.2, there was a CO outburst that occurred ten days before this dust outburst. After peak brightness, the Centaur steadily decreased in brightness over weeks to its quiescent value. This is longer than the typical dust outburst; in fact, A took longer to decay than a similarly large outburst of ∼ 3.3 magnitudes on 2018 November 22 (B) and longer than many other outbursts of this magnitude change in other surveys [97, 99]. Instead of one longer than average outburst, we consider that the lightcurve pattern for A may be due to two or more outbursts occurring within 3 - 5 days of each other. The second large outburst, B, increased in brightness by ∼ 3.3 magnitudes, which it appeared to maintain for a few days before decaying. As Fig.9 shows, 52 hours after the start of this outburst, CO was detected with a typical quiescent outgassing value. When the CO emission was detected, the dust outburst was still at peak value. Therefore, outburst B appears to have been triggered, and even maintained over a ∼ 2 days, without an increase in the CO production rate.

46 Outburst C began on 2018 December 09 and coincided with a slightly elevated CO production rate. If the dust outburst mechanism for B and C were tied to a significant release of CO, then such a CO outburst must have resolved itself very quickly, within 2-3 days. This is further indication of either no CO involvement with a dust outburst, or perhaps a scenario where two other short-lived CO outbursts occurred when we were not collecting data with the SMT. Regardless, whatever the mechanism was behind the B and C outbursts, they did not lead to a long-lived increase of the CO emission. The dust outburst labeled D which peaked on 2019 January 08 coincides with an increase in Q(CO) over its quiescent value. Unfortunately, there are not enough measurements of the CO emission to further explore the gas relationship to the dust for this outburst. Lastly, the dust outbursts in the lightcurve demonstrate an important point about the sporadic nature of the outbursts. 29P/SW1’s rotation period is in great disagreement between different authors with values ranging between 14 hours and 50 days [93, 98, 104, 132, 133]. This is a substantial problem, because an accurate estimate of 29P/SW1’s rotation period is needed to model its activity in more detail. Trigo et al. 2010 [98] partly favor a rotation period of ∼ 50 days because of the stated annual average of ∼ 7 dust outbursts/year, which corresponds to a rate of one outburst every ∼ 50 days. However, this average time between outbursts carries a high dispersion: many > 1 magnitude outbursts occur only 1-2 weeks apart as the data in Fig.9 show. Even the quiescent state is highly variable over days to weeks, which makes it difficult to establish the beginning and ending times of dust outbursts, and furthermore, some dust outbursts may overlap in time, which is likely the case for our outburst A. Thus, as Trigo et al. 2010 [98] point out, this potential agreement with ∼ 50 days obtained from an average of 7 outbursts/year may just be serendipitous.

3.4.4 Causes of outbursts

Despite its nearly circular path, the observing geometry (speciﬁcally the geocentric distance and phase angle) of 29P/SW1 can change noticeably over its orbit. However, the

47 maximum relative contribution to brightness that these changes can make is ∼1.5 magnitudes, and it takes several months, not days, to achieve such an effect. The telescope pointing errors were less than 2" during the CO observations, and this is not large enough to significantly affect the measured fluxes. Thus, the dust outbursts for which 29P/SW1 is so well known, and the two CO outbursts (one presented here and one from [118], require physical causes to explain their characteristics. The nuclei of comets and Centaurs are assumed to be porous media containing a mixture dust particles and volatile ices entrapped in adsorption sites [134]. Some models (eg. [64, 135, 136]) assume the nucleus is composed substantially of amorphous water-ice containing

trapped highly volatile gases, like CO or CO2, whose release is triggered by phase change of crystallization of the amorphous water-ice. At 29P/SW1’s distance from the Sun, the effective blackbody surface temperature is ∼ 120 K, which meets the energy threshold for the amorphous-crystalline change. Once released, due to different volatilities, gases may travel different paths out of the nucleus, some perhaps starting with sublimation from below the surface. In addition, other surface processes which trigger erosion and mass loss, and may reveal fresh surface, leading to new release of gas without simultaneous release of dust (and vice versa) [68, 107, 137, 138]. The 2016 CO outburst provided us with an opportunity to test whether the rapid increase of CO output is accompanied by a significant dust increase. Intriguingly, the timing of the 2016 CO outburst does not coincide with a dust outburst. The dust coma maintained a nearly constant quiescent stage magnitude of m(1, r, 0) = 12.9 ± 0.1 for at least the next ten days (Fig.9). Thus, either the CO outburst led to little-to-no increase in dust production, or it created a dust outburst after a ∼ 10 day delay. Regarding the 1995 CO outburst, simultaneous measurements of the visible magnitudes would be valuable, but thus, far none are reported [118], and thus, this 2016 event is the only known dataset of CO outburst in 29P/SW1 with quasi-simultaneous information about the dust. The CO gas outburst patterns are similar to some of the visible outburst patterns in

48 that they have a rapid rise with a slow decline. The CO gas outbursts last approximately the same time, or perhaps are a little shorter, than the dust outbursts. When documented with lightcurves, the decay times of dust outbursts can be complicated by the close timing and overlapping of multiple outbursts, which can obscure an individual profile. However, most singleton dust outbursts appear to return to quiescent values in 3-5 days, sometimes longer [97–99]. The longer decay time for dust could be explained by the dust having a lower expansion velocity than the gas [91]. It is not clear what causes the dust outbursts. If CO is involved, it must be a very small amount, on par with the typical quiescent Q(CO) values, and it will be difficult to use existing telescope instrumentation to document any increases in Q(CO) that lead to dust outbursts. The data presented in this paper show that while one of the dust outbursts B may have been associated with an increased CO production rate, the CO outburst of early 2016 did not lead to an increased dust production for at least 10 days. Thus, CO outbursts do not always lead to dust outbursts. Moreover, dust outbursts do not always require increased CO production, as B and C occurred while CO showed little to no increase over quiescent activity. It is remarkable that after a significant CO outburst occurs in 2016, there is no change in the visible lightcurve for at least ten days. Clearly, a doubling of CO production does not always lead to an increase of dust production. Furthermore, two of the dust outbursts (B and C) do not coincide with a substantial increase in CO production. The other two dust outbursts (A and D) may have CO involvement, but there are not enough CO measurements to be definite on this point. The lack of correlation of gas and dust behavior during the time of the CO outburst shows that the CO and dust production are not always significantly entrained in 29P/SW1. The lack of timing coordination of the gas and dust behavior in early 2016 may provide important observational constraints for the models. The paucity of CO measurements around the start of the dust outburst A makes drawing conclusions difficult, but there is some evidence for CO involvement, as the Q(CO) and

49 visible magnitude declines at similar rates during the weeks after the outburst, particularly on March 21 | 31. This similar apparent rate of decline could be also consistent with an extended source of CO from the coma grains at this time (e.g., Gunnarsson et al. 2003 [75]). There was no such evidence for an extended source in the CO outburst that occurred a few weeks earlier. We brieﬂy consider other volatile candidates that might be involved in triggering the dust outbursts. A common candidate proposed for activity in many distantly active comets

is CO2 [9, 66, 139]. In particular, Gunnarsson er al. 2008 [64] suggest that CO2 (with its higher atomic weight) might be mass-loading with CO in 29P/SW1 and slowing down its exit into the coma. However, using simultaneous spectroscopy from the Akari infrared

26 −1 telescope, CO2 has a very low measured upper limit of Q(CO2) < 3.5 ×10 mol s in 29P/SW1, corresponding to a mass loss rate of dM/dt <25 kg s−1 and a mixing ratio of

Q(CO2)/Q(CO) < 0.01 [66]. Measurements of the aggregate emission from CO2+CO at

4.5 µm were made with photometry that are also consistent with very little CO2 in this

Centaur (Harrington Pinto, O. et al. in prep.) This is well below the predicted value CO2 in Gunnarsson et al. 2008 [64] and thus the data are not consistent with signiﬁcant mass-

loading of CO2 with CO. Thus, CO2 outgassing is unlikely to be involved in signiﬁcant mass loading with CO, or with the dust outbursts, unless only a very minimal amount (< 25 kg s−1) is needed.

O2 is another possibly abundant molecule with high volatility (Tsub ∼ 24K, [140]). Due to

its symmetric nature, O2 has no dipole moment, and hence no rotational transitions. Other

O2 transitions are diﬃcult to observe in comets at any wavelengths [141, 142]. However,

using mass spectroscopy techniques, O2 was detected in the comae of 67P with Rosetta and Halley with Giotto, with unexpectedly high amounts of ∼ 4 % relative to water [143, 144].

No measurements, including upper limits, exist for O2 in 29P/SW1; however, one should not rule it out as a substantial contributor to activity at ∼ 6 au. If it is released at the same percentages as 67P and Halley, then it could be present in 29P/SW1’s coma at the rate of

50 2.4 ×1026 mol s−1 (13 kg s−1), if one scales by the water production rate measured by Akari spacecraft [66]. However, it is important to keep in mind that 67P and Halley were closer to the Sun when the measurements were made and will have a very diﬀerent outgassing proﬁle than does 29P/SW1, which orbits much farther out with very little water-ice sublimation.

Given the very low sublimation temperature, however, if O2 is present in similar amounts in 29P/SW1, it may play a signiﬁcant role in the activity at 6 au.

As for other possible candidates for outbursting activity, C2H2 and NH3 also have relatively low sublimation temperatures (57K and 78K respectively [140]) and are detected in other comets with low abundances. They have not been identiﬁed in 29P/SW1, but their

−1 −1 upper limits are not very constraining: Q(C2H2) < 117 kg s and Q(NH3) < 309 kg s (Table6), so efforts to observe these two molecules may be fruitful. Other cosmogonically abundant species with low sublimation temperatures already have very low upper limits placed on them, and are unlikely to compete with CO in the coma (see Table6). In order to constrain the mechanisms driving the activity of this enigmatic Centaur, further observations of 29P/SW1 at all wavelengths with myriad techniques are strongly encouraged | especially simultaneously. 29P/SW1 recently went through perihelion in 2019 March, and it has now entered the post-perihelion epoch when its activity typically increases [95, 96]. Furthermore, 29P/SW1’s activity level increased significantly after its orbit changed due to a close interaction with Jupiter in 1975. In 2038, 29P/SW1 will have another important conjunction with Jupiter that is predicted to double its eccentricity [90]. 29P/SW1’s activity level may very well be affected by this new orbit change and is another motivation for closely monitoring the Centaur in the next few decades. A world-wide observing campaign for 29P/SW1 was initiated in 2018 by M. Womack, G. Sarid and T. Farnham. Participation in the campaign9 is strongly encouraged as it can help different teams coordinate observing runs to increase the chances of obtaining simultaneous measurements and maximizing scientific return. 9http://wirtanen.astro.umd.edu/29P/29P_obs.shtml

51 3.5 Summary and Conclusions

For years it has been hypothesized that CO outgassing stimulates the release of dust outbursts in 29P/SW1, as well as the quiescent dust coma. Comparison of the Q(CO) and visible lightcurve data, an effective tracer of dust production in 29P/SW1, show that the relationship between outbursts of dust and gas is not always this straightforward. The data do not support the commonly held assumption about 29P/SW1 that increased CO outgassing must always be involved with the dust outbursts. Between 2016 February 20 and March 14, 29P/SW1’s dust coma was in a quiescent stage with an average magnitude of m(1, r, 0) ∼12.85 ±0.17). During this time, the CO production rate doubled quickly and then decayed in 2-3 days. The CO outburst had no effect on the dust production as indicated by the visible lightcurve. Another indication that the CO and dust are not strongly correlated for the outbursts is seen in the second observing campaign. On 2018 November 22, 29P/SW1 underwent a significant dust outburst (B) which stayed elevated for ∼ 2 days. Then, ∼52 hours after the start of the dust outburst, the CO production rate was found to be at the typical quiescent value. Thus, CO was at the typical quiescent value when the dust outburst was still going on. This suggests that the CO outgassing is uncoupled to the dust outburst. Also possibly useful for constraining models of the Centaur’s activity is that 29P/SW1’s quiescent (non-outbursting) magnitude increased in brightness by ∼ 45% over two years despite only being 3% closer to the Sun. Over this time, the CO production rate showed evidence for possibly a 10-15% increase.

52 Chapter 4 C/2016 R2 (PANSTARRS): A comet rich in CO and depleted in HCN

Note: The bulk of the work in this chapter was published in The Astronomical Journal, 2018, 156, 1. Wierzchos, K. and Womack, M. C/2016 R2 (PANSTARRS): A Comet Rich in CO and Depleted in HCN https://doi.org/10.3847/1538-3881/aac6bc

4.1 Introduction

Comet C/2016 R2 (PANSTARRS) -hereafter R2- was discovered at r = 6.3 au from the Sun on 2016 September 7 when it exhibited a 20" wide coma at 19.1 visible magnitude [145]. It has an estimated orbital period of 20,000 years, a highly eccentric orbit tilted at an angle of 58 deg to the ecliptic, and a semi-major axis of a ∼ 740 au. These orbital characteristics identify the object as an Oort Cloud comet, but not dynamically new, since it has presumably already had many journeys through the inner solar system [146]. Upon discovery, this comet exhibited a coma at a distance where most comets appear inactive. Water is the dominant ice in all comets and is not heated enough by the Sun to sublimate eﬃciently until much closer, typically r = 2 − 3 au. Thus, comet R2 also receives the “distantly active comet" classiﬁcation. Instead of water-ice sublimation, the observed comae of distant comets are generally considered to be due to release of cosmogonically abundant hypervolatile species, such as CO and/or CO2 [9, 65–67, 128]. By late 2017, optical images of R2 revealed a deep-blue-colored coma and ion-tail with an absence of dust. The blue color in the coma is largely due to emission from CO+ (a

53 + photoionization production of CO) and to some extent, N2 , which were both observed to be strong, with a ratio of N2/CO = 0.06, one of the highest ever reported for a comet [147,

+ + 148]. The strong presence of CO , and the lack of CO2 emission in these optical spectra indicate that the comet’s activity is probably dominated by the outgassing properties of

+ CO and not CO2. The comet’s high N2 abundance is very important, because its likely parent, N2 is typically depleted in comets, and like CO, N2 sublimates at extremely low temperatures [9, 149]. The abundance of N2 is high enough that it may play a substantial role in R2’s distant outgassing behavior. Furthermore, N2 is also an important molecule for astrochemical models of the solar system and other planetary systems [150–152].

4.2 Observations and Results

We used the Arizona Radio Observatory 10-m Submillimeter Telescope in order to search for CO J=2-1 (at 230.53799 GHz) and HCN J=3-2 (at 265.88643 GHz) emission in comet C/2016 R2 during 2017 December - 2018 January when its heliocentric and geocentric distances were r ∼ 2.9 au and ∆ ∼ 2.1 au, respectively (Table8). We used the dual polarization 1.3 mm receiver with ALMA Band 6 sideband-separating mixers for all observations. The mode of data acquisition was beam-switching mode with a reference position of +2’ in azimuth. An integration time of 3 minutes on the source and 3 minutes on the sky reference position for each scan was used. System temperatures were typically in the low 300 K for all

∗ the data. The temperature scale for all SMT receiver systems, TA, was determined by the

∗ chopper wheel method, with TR=TA/ηb, where TR is the temperature corrected for beam efficiency and ηb is the main beam efficiency of the SMT with a value of ηb = 0.74 for both the CO and HCN frequencies. The backends consisted of a 2048 channel 1 MHz filterbank used in parallel (2 x 1024) mode and a 250 kHz/channel filterbank also in parallel (2 x 250). The 250 kHz/channel filterbanks provided the equivalent velocity resolutions of 0.325 km s−1 for CO J=2-1 and 0.282 km s−1 for HCN J=3-2. The 1 MHz resolution filterbanks were significantly broader than the expected CO and HCN linewidths and thus were not used in

54 Table 8: Observations of R2 with the ARO 10m SMT

∗ −1 28 −1 Molecule UT Date r (au) ∆(au) TAdv (K km s )Q(×10 mol s ) CO(2-1) 2017-12-22.21 2.98 2.05 0.26±0.01 4.4±0.2 2017-12-23.09 2.97 2.05 0.28±0.02 4.6±0.4 2017-12-30.14 2.94 2.06 0.26±0.02 4.6±0.4 2017-12-31.13 2.93 2.06 0.27±0.02 4.6±0.4 2018-01-16.03 2.86 2.15 0.27±0.02 4.6±0.4 HCN(3-2) 12-23.19 & 01-16.15 2.92 2.10 <0.030 < 0.0008

the analysis. The ARO SMT beam size is θB = 32" at the CO J=2-1 frequency and 27" at the HCN J=3-2 frequency.

After every six scans on the comet, we updated the pointing and focus on Uranus and on the strong radio-source Orion-A. The pointing and tracking were showing an accuracy of < 1" RMS throughout the observing epochs. The comet’s phase angle ranged from 7 degrees (on 2017 Dec 22) to 15 degrees (on 2018 Jan 16). CO emission was detected and remained relatively constant during this time (Table8). The CO J=2-1 line was detected in R2 during a single six minute scan on the UT 2017 Dec 22 observations, and the total for the first day is shown in Fig. 10. The spectrum in Fig. 10 is the first detection of CO emission in this comet, which we announced to the astronomical community in a preliminary report [153]. Both polarizations showed the line, and little change was observed in the line intensity, shape or area throughout the observing period. The ∼ 3 channel wide (FWHM) line profile yielded a ∆VFWHM ∼ 0.79 ± 0.33 km s−1 if a Gaussian fit is assumed. The line is typically blue-shifted by a small amount ranging from δv = -0.08 to -0.18 km s−1. We also mapped the CO emission on UT 2018 Jan 16.1 to assess its spatial extent in the inner coma. The map was constructed with a 9-point grid technique centered on the nucleus position. The map had 16" spacings and integrations of 6 minutes for each position. The pointing separations for the map are equal to the half-power beamwidth (HPBW) of the SMT 10-m dish at this frequency (see Fig. 11). The map was aligned along the RA and

55 CO(2-1) Spectrum of C/2016 R2 PANSTARRS

0.3

-1

0.33 km s / channel

2017 Dec 22.21 UT

r = 2.98 au

0.2 (K)

0.1 T*

0.0

-20 -15 -10 -5 0 5 10 15 20

-1

Offset Velocity (km s )

Figure 10: The first detection of CO emission in C/2016 R2 on UT 2017 Dec 22, when the comet was at r = 2.9 and ∆ = 2.1 au. The CO J=2-1 rotational line was bright with an intensity of TA* = 0.3 K easily detected in single scans with the Arizona Radio Observatory Submillimeter 10-m telescope. The spectrum was obtained with 250 kHz/channel spectral resolution with a Gaussian measured linewidth of FWHM = 0.79 km s−1 and was not significantly shifted from the comet ephemeris velocity, which is indicated by a vertical dashed line at zero offset velocity.

Dec axis and the direction to the Sun is indicated in the ﬁgure. We searched for the HCN J=3-2 transition on 2017 Dec 23 and 2018 Jan 16 and did not

detect a line down to a cumulative 1-sigma level of TA* = 0.010 K in the 250 kHz/channel

ﬁlterbanks (Table8). We also searched for, and did not ﬁnd, CH 3OH (251 GHz), H2CO (218

+ + GHz), N2H (279 GHz), HCO (276 GHz) and CS (244 GHz). The signiﬁcance of those non-detections and limits will be discussed in a later paper.

4.3 Analysis and Discussion

4.3.1 CO outgassing velocities and spatial extent

Important modeling parameters, such as expansion velocity, vexp, and outgassing patterns, can be extracted from spectral line proﬁles and maps that have suﬃciently high

56 .Teln sbten23canl ie(orsodn oFH f06 0.99 - 0.66 of FWHM to (corresponding wide channels 2-3 between s is km line The 10). by velocity Fig. ephemeris comet’s the from blue-shifted with slightly aligns data CO R2 the how address briefly we Here models. the from coma. one the and in in greatest, CO is source heating for isotropic solar sources an where point different subsolar two the from consider emanating typically one comets, models activity Comet [ 13]. resolution tailward the (center to compared nucleus when the zero side of at sunward lines location the on dashed ephemeris increased vertical the slightly indicated side. with at be as indicated intensity may right is in and the toward position) peaked speed sky is the emission ephemeris on Sun the CO comet’s 64") the with x The velocity. to (64" 16 direction km Figure. January 96,000 The the 2018 x distance. km in on 96,000 projected constructed to comet’s corresponds R2 the map C/2016 the at from of size emission The CO SMT. ARO of Map 11: Figure is,w xmn h Oln rfie hc a igevlct opnn n is and component velocity single a has which profile, line CO the examine we First, −

1 TA*(K) ,adb tigaGusa,w eie WMlnwdhof linewidth FWHM a derived we Gaussian, a ﬁtting by and ), Offset velocity(kms 57 -1 ) δv -0.12 = ± ∆ .0k s km 0.20 v HM W F 0.85 = − 1 (see ± 0.33 km s−1. Thus, the data are consistent with having a FWHM linewidth of ∼ 0.8 km s−1. The small velocity shift we observe of ∼ -0.1 km s−1 for the comet with a low phase angle is consistent with at least some of the outgassing takes place on the Earth-facing side. The half-width half-maximum (HWHM) linewidth measured on the blue-ward wing is proportional to the outﬂow velocity of the gas (see Biver et al. 1999 [14]), and thus, we

−1 estimate the gas expansion velocity to be vexp = 0.50 ± 0.15 km s . This is comparable to what was measured in other comets at this same heliocentric distance, such as C/1995 O1 (Hale-Bopp) and C/2006 W3 (Christensen) [75, 154]. [155] also report seeing CO emission from this comet approximately three weeks after our ﬁrst detection, but with a velocity redshift of ∼ +1 km s−1 and a somewhat broader linewidth of 1.0-1.3 km s−1. We also observed the comet during this time and do not conﬁrm their redshifted velocity component. The nearly-centered and narrow CO spectrum of R2 is consistent with a simple model of

−1 cool gas expanding isotropically with vexp ∼ 0.50 km s , which is what we used to calculate production rates. The data are also consistent with a more detailed scenario if one looks more carefully at the spectral line profile. First, we revisit Hale-Bopp and Christensen, which also produced substantial CO at r = 3 – 4 au. The CO emission in these comets were also slightly offset from the ephemeris velocity by δv ∼ -0.1 km s−1 [74, 78, 154]. For Hale-Bopp, the CO line profiles were fit by a detailed two-component model comprised of isotropic outgassing of cold CO gas, combined with a blue-shifted velocity component associated with a sunward side active area [75]. This model produces two peaked lines in comets beyond 4 or 5 au and single peaked lines for comets within 4 au. Therefore, the CO emission is also consistent with CO arising from a mix of a sunward-side active area and a symmetric source either in the coma or from the nucleus. We also examined the maps for clues about the CO outgassing. Fig. 11 shows that the emission peaks at the nucleus position provided by the ephemeris. Furthermore, CO emission was readily detected at all positions out to at least 45" with a decrease in intensity by 20-40% relative to the line at the center position, consistent with isotropic outflow of

58 CO. There is evidence for a slight sunward enhancement of emission on the sunward side compared to the tailward side. This gives further support for contribution from an active area releasing CO on the sunward side, as is also indicated from the spectral line proﬁle.

+ It is not clear what could generate isotropic CO emission in the coma. Given that CO2 was not detected in the optical spectrum of R2 [147] and we did not see CH3OH or H2CO down to significant limits (Wierzchos, in prep.), it is not likely that CO was produced in significant amounts by photodissociation of these species, which are plausible secondary sources for CO in other comets. Also, this comet has an almost nonexistent dust coma, and thus CO is not likely to come from refractory cometary grains in the coma. Perhaps additional CO is released by sublimating water ice grains in the coma that were ejected from the nuclear sunward-side facing CO source. Measurements of OH or H2O emission would be useful constraints to the CO production model in R2. Much higher resolution spectra, ≤ 0.1 km s−1, would also be valuable for testing models of CO production. In principle, more detailed modeling of the spectral line profile and mapping data could significantly constrain models of the release mechanisms for CO and physical conditions in the coma, but this requires higher spectral and spatial resolution data, and is beyond the scope of this paper.

4.3.2 Production rates of CO and HCN

We calculated column densities assuming the excitation was dominated by collisional and fluorescence contributions, following the modeling described in Crovisier & Le Bourlot 1983, Bockelee-Morovan & Crovisier 1985 and Biver 1997 [17, 24, 118]. We assumed a rotational and excitation temperature of 25K, which is consistent with the empirical fit to Hale-Bopp CO data described in [13]. The column density for CO was fairly constant, with an average value of N(CO) = (1.89 ± 0.14)x1014 cm−2. In order to calculate production rates, we assumed a gas expansion velocity of 0.50 km s−1, which is consistent with the CO spectral line profile and values

59 from other comets at this distance, as described in Section 4.3.1. Using a photodissociation decay model [18] and assuming isotropic outgassing of CO we find an average production rate of Q(CO) = (4.6 ± 0.4)×1028 mol s−1 between 2017 December 22 and 2018 January 16 (see Table8). [155] report higher CO production rates, despite reporting similar line intensities. We think the different production rates are the result of using different modeling parameters, such as expansion velocity. There is insufficient detail about the modeling in their preliminary announcement to warrant further comments.

As discussed in section 4.3.1, CO2 is not likely to be a signiﬁcant parent of CO in this

comet. Infrared observations of CO2 would be very useful in quantifying any contributions from CO2 to the coma and/or CO emission. Also, to date, no searches for OH or H2O emission have succeeded, which implies that water-ice sublimation is probably not responsible for most of R2’s activity. Thus, the major driver at this distance is probably CO outgassing. The CO production rate of R2 is very high and approximately half that of C/1995 O1 Hale-Bopp at this same distance from the Sun. Based on the high Q(CO) values, we consider

R2 to be “CO-rich." Other CO-rich comets, typically have CO/H2O > 8% [3] and the ratio for R2 may be substantially higher than 8%, since water has yet to be detected. We point out that these observations were obtained when the comet was at ∼ 3 au from the Sun, which is too far for water-ice to sublimate eﬃciently. Thus, the relative CO/H2O content in the nucleus may be much higher than we can determine at this heliocentric distance.

The nucleus’ radius, RR2, can be estimated based on the assumption that Q(CO) is proportional to the nucleus surface area and the insolation received, and then compared to comets at the same heliocentric distance for which both Q(CO) and radius are independently

29 −1 known. For example, if we use RHB = 30 km [156] and Q(CO) = 1.8×10 mol s for Hale- Bopp (at 3 au) [13], then the average Q(CO) = 4.6 ×1028 mol s−1 at ∼ 3 au corresponds to a radius of R ∼ 15 km. Similarly, from a comparison with comet Christensen’s radius upper

28 −1 limit of RCh < 13 km [157] and Q(CO) = 3.9×10 mol s [154] at 3 au, we derived that R < 14 km. CO activity may not, in fact, scale directly with surface area for this comet,

60 but if it does, then we find that the nucleus radius need not exceed RR2 ∼ 15 km in order to explain the measured CO production rate. We derived a 3-sigma upper limit of Q(HCN) < 8.0x1024 molecules s−1 from all the HCN data. For comparison, this is ∼ 100 times lower than observed for Hale-Bopp at the same distance [13]. Our non-detection of HCN emission is consistent with the absence of the CN band at 3880 Angstroms, as reported by [147], assuming that CN is caused by photolysis of HCN. We briefly compare the CO and HCN production rates with other comets, since emission from these two species are commonly detected and their ratio may provide insights to the chemical composition of the nucleus and/or coma (Table9). The comets identified by name are those reported to be CO-rich. Also listed in the table are average values for larger groups of comets, such as Oort Cloud, Jupiter Family, and finally an “all comets" average value. As the table shows, the relative production rate value derived for R2 is extraordinarily high: Q(CO)/Q(HCN) > 5000 at r ∼ 3 au. The average value for all comets measured is Q(CO)/Q(HCN) ∼ 25 and this ratio varies by less than a factor of three between Jupiter Family Comets (JFCs) and Oort Cloud Comets (OCCs). The only group where it noticeably departs from the average value is for CO-rich comets, which are listed individually in the top panel of the table. It is perhaps not surprising that the comets designated as CO-rich also have elevated Q(CO)/Q(HCN) values, but even among these CO-rich comets the limit derived for R2 is the highest values to date for any comet. The very high Q(CO) and very low Q(HCN) in R2 is difficult to understand in terms of typical comet compositions. At 3 au, R2’s comet nucleus has not received much solar heating and so it will preferentially release CO over HCN, due to its higher volatility. This behavior was seen in the abundance ratio of CO/HCN in Hale-Bopp, which decreased as the comet got closer to the Sun and more HCN was released (see Biver et al. 2002 [13] and Table9). There are not many measurements of both CO and HCN in comets at ∼ 3 au, but R2’s value is substantially higher than those measured for the CO-rich comets Hale-Bopp, Christensen or C/2010 G2 Hill in the

61 Table 9: Compiled Q(CO)/Q(HCN) ratios in CO-rich and other comets

Comet Q(CO)/Q(HCN) r10 (au) Reference C/2016 R2 (PANSTARRS) >5000 2.9 This work 29P/Schwassmann-Wachmann 1 330011 5.8 [9, 158] C/2006 W3 (Christensen) 243 3.2 [154] C/1995 O1 (Hale-Bopp) 125-650 3 [13] 52-91 0.9 [13, 159, 160] C/2010 G2 (Hill) 70 2.5 [161] C/1996 B2 (Hyakutake) 96 0.6, 0.7 [162] C/1999 T1 (McNaught-Hartley) 46 1.3 [163, 164] C/2001 Q4 (NEAT) 31 1.0 [165] C/2009 P1 (Garrad) 36 1.6, 2.1 [166–169] C/2013 R1 (Lovejoy) 34 1.3 [170] Oort Cloud Comets 28 - [3] Jupiter Family Comets 9 - [3] All comets 25 - [3]

range of 2.5 - 3.0 au, suggesting that R2’s high CO/HCN ratio cannot be explained solely due to volatility diﬀerences between the two molecules. Interestingly, the highest CO/HCN values were obtained in comets known to be both distantly active and CO-rich (R2, 29P, Christensen and Hale-Bopp). This is worth looking into further, but the data are sparse. Even for a comet at 3 au, the HCN upper limit that we derived is extraordinarily low. Another possible clue is that R2’s coma is largely gaseous with very little dust [147], and this may be related to the signiﬁcantly decreased amounts of HCN and other volatiles. The chemical composition of R2’s coma is noticeably atypical when compared to other comets.

10r is the heliocentric distances at which production rates were measured. Two values of r are listed when CO and HCN measurements were not simultaneous. 11We assumed Q(HCN) ∼ Q(CN) for 29P, see Womack et al. 2017 [9].

62 4.3.3 High N2 Production Rates

Searching for additional clues to the unusual chemical composition of this comet, we now turn our attention to molecular nitrogen. Measuring cometary N2 is of considerable importance for many reasons, including testing models of the condensation and incorporation of ices in the protosolar nebula, and calculating the N2/NH3 abundance ratio, which is a key diagnostic of primordial physical and chemical conditions [150, 171]. N2 is also a highly volatile molecule and it can contribute to comet activity if signiﬁcantly incorporated in the nucleus. Furthermore, N2 is trapped and released in a manner similar to Argon, and thus,

+ detecting N2 emission in any coma suggests that Ar may also be present in high amounts

[172]. Despite its importance, it has been diﬃcult to measure the N2 abundances for all but

+ a few comets [173–177]. Strikingly, N2 optical emission is clearly detected in R2, with a measured abundance ratio of N(N2)/N(CO)=0.06 [147, 148].

The N2/CO ratio is an important observational constraint for testing the formation environment for cometary ice; it is determined by the temperature of the gases when they were incorporated into the ice as well as any subsequent processing that may have preferentially aﬀected one volatile over the other. In order to place this in context, we brieﬂy review two general scenarios for comet formation: one where comets agglomerated from pristine amorphous water ice grains originating from the interstellar medium onto which N2 and CO condensed in the protosolar nebula [178]. This model proposes that the N2/CO ratio in ices strongly depends on the temperature of the materials at the time the volatiles condensed

+ + or were trapped. The ratio derived from R2’s optical spectra of N2 and CO agrees very well with the predicted value of N2/CO = 0.06 for icy planetesimals forming in the solar nebula at about 50 K [149, 172]. Alternately, comets may have agglomerated from crystalline water-ice grain clathrates that trap N2 and CO. Due to its relatively small size, N2 is not readily trapped by clathrates, which leads to a lower predicted ratio of N2/CO ranging from

∼ 0.002 to 0.02 [179]. Thus, perhaps the measured value of N2/CO = 0.06 in R2 is not consistent with a clathrate model.

63 In addition to being relatively rich in N2, we note that this comet is severely depleted in HCN, as discussed in the previous section. Physicochemical models of nitrogen chemistry in protostellar disks show that photodissociation of N2 leads to production of HCN [180]. It is interesting to consider that comet R2 may have formed in the protosolar nebula disk where

there was signiﬁcant N2 shielding that led to the high N2 and decreased HCN abundances.

N2 cannot undergo rotational transitions due to lack of a permanent dipole moment, and

thus emits no radiation at millimeter-wavelengths. However, because the N2 abundance can

put such strong constraints on comet formation models, we derived an N2 column density

and production rate using the N2/CO abundance ratio calculated from optical spectra and our CO results. We chose the CO data from 2017 Dec 22, because it is closest in time to

the [148]N2/CO value on 2017 Dec 8-10. We derived an N2 column density of N(N2) =

13 −2 27 −1 (1.1 ± 0.2)x10 cm and a production rate of Q(N2) = (2.8 ± 0.4)x10 molecules s .

−1 This production rate corresponds to a mass loss rate of 130 kg s for N2. Determining the

N2 production rate in this manner can be useful in comparison with those of water and/or

NH3, if these volatiles’ abundances are established through direct observation or via their daughter products [181, 182].

4.4 Conclusions

We report the first detection of neutral CO emission, and an upper limit of HCN, in comet C/2016 R2 (PANSTARRS) at r ∼ 3 au. The CO line profile shape is characteristic of CO emission seen in other comets at this distance, with a linewidth of ∼ 0.8 km s−1 and is slightly blue-shifted from the ephemeris velocity. A 64" x 64" map (∼ 96,000 km x 96,000 km at the comet’s projected distance) shows that the CO emission peaks in intensity at the ephemeris position and decreases by 20-40% at the off-centered positions. The spectra and map are consistent with CO arising from a combination of an isotropic source and an active area on the sunward side. If comet R2’s CO output is proportional to surface area of the nucleus, then we find that

64 the radius need not exceed RR2 ∼ 15 km in order to explain the measured CO production rate. Thus, R2 may be larger-than-average in size, but need not be a giant comet in order to explain the measured CO production rates. The very large amount of CO, and the apparent absence, or very low outgassing rate, of HCN, leads to a CO/HCN production rate ratio over 5000. This is remarkably high compared to other comets at r = 3 au, even when including other comets known to be CO-rich. The high CO/HCN ratio cannot be explained solely due to volatility diﬀerences between the two, and may represent a compositional diﬀerence between R2 and most other comets. When considered along with the high outgassing rate of N2, it is interesting to consider whether comet R2 formed in a region of the protosolar nebula with substantial N2 shielding, which could have led to higher N2 and decreased HCN abundances.

N2 production rates were derived from the N2/CO ratio [148] and our CO production

27 −1 rates, and were calculated to be Q(N2) = (2.8 ± 0.4)x10 molecules s at 3 au. N2 production rates will be valuable for comparison with those of water and/or NH3, if detected in this comet. R2’s coma composition is clearly very diﬀerent from other comets observed thus far, both in the high N2 abundance, and signiﬁcant decrease in other typically abundant molecules, such as HCN. Further observations of this comet along all spectral ranges are highly encouraged, especially those of NH2 in the optical, and NH3, CO2 and CH4 in the infrared in order to measure the key diagnostic ratios N2/NH3, CO/CO2 and CO/CH4, which will provide observational tests for formation models of the environment of this comet.

65 Chapter 5 Case study: Evolution of the CO outgassing and observations of 13CO and HCN in C/2016 R2 (PanSTARRS)

5.1 Introduction

Comet C/2016 R2 (PANSTARRS) -hereafter R2- was discovered in 2016 September 7 by the Panoramic Survey Telescope and Rapid Response System. Upon discovery the comet was displaying strong cometary activity at a large heliocentric distance of 6.3 au, a fact that is unusual, and which suggested that sublimation of water ice was not the main driver of cometary activity at this heliocentric distance [183]. R2 is a long-period Oort Cloud comet whose nucleus’s diameter is in the range of 3-30 km [22, 129], and according to JPL Horizons it has an estimated orbital period of P = 19,100 years, eccentricity of e = 0.99 and inclination of i = 58.2◦. It reached perihelion on 2018 May 09 at a heliocentric distance of 2.6 au. Ever

+ + since the detections of strong CO and N2 and lack of other typical cometary volatiles in the Fall of 2017 [147], the comet has been the subject of numerous observing campaigns at various wavelengths [22, 129, 184, 185]. In 2017 and 2018, we observed R2 with the Arizona Radio Observatory 10-m Submillime- ter Telescope [129] and found that in addition to the CO+ reported earlier, the comet was very rich in CO with a mass loss of 5,000 kg s−1 and signiﬁcantly depleted in HCN. The high abundance of CO and N2 suggests that R2 not only belongs to the rare group of CO-rich comets, but it also belongs to the even rarer group of comets whose spectra is dominated

+ + by CO and N2 emissions, of which only a handful are known: C/1961 R1 (Humason) and C/1908 R1 (Morehouse) [186, 187] and possibly 29P/Schwassmann-Wachman and C/2002

66 VQ94 (LINEAR) [22, 184]. The very unusual chemical composition of R2 when compared to other comets draws into question under what conditions it had to have formed. Further analysis showed that the very high N2 and very low HCN abundance in the coma is probably indicative of the comet nucleus composition and is consistent with the comet forming in the outer reaches of the solar nebula where conditions were very cold. The high N2/HCN ratio in R2 may be due to the primordial N2 being shielded from solar radiation, thus inhibiting the reactions that readily produce HCN [129]. At the very far distances from the Sun, the volatiles would be very cold and a sizable amount of N2 could stay in the icy form, and thus be less available for gas-phase or photochemical reactions. Results from the multi-wavelength observing campaign undertaken by Biver et al. 2018 [22] conﬁrmed the large CO abundance and the depleted HCN amounts that we measured. They also established that R2 is surprisingly dust-poor when compared to other comets and is also depleted in the typically abundant volatiles H2O,CH3OH, H2S, and H2CO. As for its origin, Biver et al. 2018 [22] postulated an alternate scenario where R2’s unusual composition might be due to the nucleus originating as a collisional fragment from a Kuiper belt object (KBO). This hypothesis is supported by the fact that several KBOs have surfaces rich in N2.

+ The strong CO and N2 emission and water depletion was once again confirmed by Opitom et al. 2018 [184] using the UVES spectrograph on the ESO VLT optical telescope. Additionally, they detected CH at 431.5 nm, and for the first time in any comet, they detected the [NI] forbidden lines at 519.79 and 520.02 nm, which they attributed to the high abundance of N2 [184]. Another multi-wavelength ground- and space-based campaign [185] quantified the molecular abundances of 12 species and the first detections of the typically abundant volatiles CH4,

CO2 and H2O in this comet. In this study, the CO2 production rate was derived using Spitzer infrared space telescope photometry of combined emission from CO and CO2 molecules at

67 4µm, where the CO2 contribution was extracted using our simultaneous ground-based CO spectroscopic results obtained on 2018 February 13 during one of the follow-up observing runs at the ARO SMT addressed in this chapter. Thus, a small subset of the data in this chapter already appeared in another publication on which I am a co-author.

Interestingly, the only ratios by which R2 is comparable to other comets are CH3OH/CO2,

CH3OH/CH4 and CO2/CO. The other ratios are highly deviated from what is observed in other comets, further conﬁrming the unusual nature of R2. In this work we followed-up on the behavior of this enigmatic comet after our initial observations in late 2017 December and early 2018 January discussed in the previous chapter. Our main motivation was to conﬁrm the marginally detected HCN that was reported by another group [22] and to monitor the CO output as the comet moved through perihelion in order to study its response to the solar heating. This would allow us to improve modeling of the CO excitation and outgassing, and compare it with the also CO-rich comet C/1995 O1 (Hale-Bopp). We also completed a dedicated search for the isotopologue 13CO, which has not been yet reported in any comet12.

5.2 Observations and Results

We used the Arizona Radio Observatory (ARO) 10-m Submillimeter Telescope (SMT) to observe CO(2-1) at 230.537 GHz, CO(6-5) at 691.473 GHz, 13CO(2-1) at 220.398 GHz and HCN(3-2) at 265.886 GHz, and the ARO 12m Radio Telescope to observe the CO (1-0) transition at 115.269 GHz toward R2 on various dates between 2017 December and 2019 June (see Table 10). We observed the comet throughout its perihelion passage, ranging from heliocentric distances of r = -2.98 au to 4.73 au and geocentric distances of ∆ = -2.05 au to 4.35 au. Here, negative distance refers to the pre-perihelion portion of the orbit. For all instances in which

12We follow the convention of identifying the principal isotopic species of atoms in a molecule without giving the atomic weights explicitly.

68 the data were taken at the nucleus position the beam switching mode we used a positive 2’ throw in azimuth for sky measurement. At the 10m SMT, we used the the 1.3mm ALMA Band 6 dual polarization receiver where the chopper wheel method was used for the determination of the temperature scale

∗ ∗ TA, with TR=TA/ηb. System temperatures remained under 500K throughout the observing

period. For all three frequencies we used the recommended antenna efficiency of ηb = 0.74. Two spectral resolutions were available at the SMT with the following filterbanks: 250 kHz/channel (500 channels in parallel) and 1000 kHz/channel (2048 channels in parallel). The bandwidth was 128 and 512 MHz respectively. Only the 250 kHz/channel filterbank was used at the SMT as it provided a significantly better spectral resolution. At the frequencies of 220, 230, 265 and 691 GHz, the corresponding velocity resolutions at the 10m SMT were 0.34 km s−1, 0.32 km s−1 and 0.27 km s−1 and 0.11 km s−1 per channel

with FWHM beam-diameters of θB = 34", θB = 31", and θB = 27" and θB = 10" respectively. At the ARO 12m telescope we used the Multi-Band Receiver also in dual polarization mode. The temperature scale was also calibrated with the chopper wheel method and we used an antenna beam eﬃciency of ηb = 0.59. The backend was the ARO Wideband Spectrometer providing a spectral resolution of 0.051 km s−1 per channel. At this frequency, the FWHM beam-width of the 12m antenna was θB = 51". Pointing and focus was updated at both telescopes every six to ten scans on a planet or on a bright radio-source. The accuracy of the pointing and tracking was ∼ 1" RMS at all epochs. The CO(2-1) data were of very high quality and the CO emission was clearly detected on every night, even on individual six-minute scans. As an example, Fig. 12 shows the the composite spectrum obtained on the last date of our campaign, which is when the comet was the farthest from the Sun, and the CO emission was the weakest. This spectrum corresponds 66 scans (3.3 hours on the source) and yet the CO detection was quite strong.. Given the comet’s very eccentric orbit, this could very well be the last detection of CO in R2 as it will

69 Figure 12: CO J=2-1 spectrum of C/2016 R2 obtained on 2019 June 12, when R2 was at r = 4.73 au and ∆ = 4.35 au. The CO line area in this spectrum is the weakest that we observed (see Table 10), yet the line was still strong and clearly identiﬁed.

very soon be beyond the observing capabilities of ground- and space-based telescopes. As for the 13CO line, it was marginally detected when co-adding a total of 611 scans between 2017 December 30 and 2019 February 11 amounting to 30.5 hours of integration time on the source (Fig. 15). Starting on 2017 December 30 we took simultaneous scans centered on the CO frequency with scans at 220 GHz for 13CO at the other receiver sideband (Table 10). The heliocentric and geocentric distances presented in Table 10 are the median values of the observing epoch. The excitational parameters needed for the calculation of the production rates described in this chapter are listed in Table3 of Chapter 1.

70 Table 10: Observations of Comet C/2016 R2 between 2017 December and 2019 June

∗ 28 Molecule UT Date r ∆ TAdv δv F W HM Q x10 (au) (au) (K km s−1) (K km s−1) (K km s−1) (mol s−1) CO(2-1) 2017 Dec 22.92 2.98 2.05 0.279±0.028 -0.18 0.80 5.20±0.50 2017 Dec 30.14 2.94 2.06 0.290±0.014 -0.14 0.76 5.26±0.25 2017 Dec 31.24 2.94 2.06 0.298±0.018 -0.13 0.75 5.42±0.32 2018 Jan 16.05 2.86 2.15 0.261±0.039 0.04 0.80 4.97±0.75 2018 Feb 13.27 2.75 2.43 0.251±0.041 -0.25 0.75 5.50±0.89 2018 Mar 02.17 2.70 2.63 0.264±0.051 -0.32 0.84 6.27±1.20 2018 Mar 05.11 2.69 2.66 0.242±0.024 -0.27 0.75 5.80±0.59 2018 Mar 08.10 2.69 2.70 0.244±0.017 -0.27 0.79 5.95±0.41 2018 Apr 03.20 2.63 2.96 0.229±0.024 -0.29 0.80 6.12±0.63 2018 Apr 04.09 2.63 2.96 0.246±0.016 -0.27 0.82 6.57±0.43 2018 Oct 28.97 3.15 3.57 0.124±0.021 -0.32 0.72 3.88±0.65 2018 Oct 29.68 3.15 3.57 0.114±0.015 -0.35 0.69 3.57±0.46 2018 Nov 17.95 3.27 3.59 0.102±0.017 -0.56 0.71 3.20±0.52 2018 Dec 14.92 3.44 3.61 0.093±0.010 -0.31 0.74 2.93±0.31 2019 Jan 14.79 3.66 3.62 0.073±0.010 -0.26 0.85 2.04±0.31 2019 Jan 15.47 3.66 3.62 0.081±0.029 -0.22 0.79 2.53±0.91 2019 Feb 11.82 3.84 3.62 0.061±0.012 -0.26 0.81 1.95±0.33 2019 Feb 12.83 3.84 3.62 0.070±0.007 -0.18 0.86 2.23±0.23 2019 Feb 21.51 3.91 3.63 0.062±0.016 -0.25 0.79 1.99±0.51 2019 Mar 08.72 4.20 3.64 0.060±0.016 -0.31 0.90 1.92±0.51 2019 Mar 09.71 4.03 3.64 0.061±0.008 -0.27 0.86 1.96±0.26 2019 Mar 28.64 4.17 3.69 0.060±0.014 -0.29 0.88 2.00±0.46 2019 Mar 29.66 4.17 3.69 0.075±0.005 -0.28 0.88 2.48±0.15 2019 Apr 11.62 4.27 3.75 0.048±0.007 -0.23 0.82 1.66±0.26 2019 Apr 12.47 4.28 3.75 0.042±0.014 -0.24 0.85 1.44±0.48 2019 Jun 12.45 4.73 4.35 0.034±0.008 -0.29 0.70 1.18±0.35 13CO(2-1) 2017 Dec-2019 Feb 2.71 3.50 0.003±0.001 -0.26 0.34 0.12±0.04 CO(1-0) 2018 May 04.20 2.60 3.24 0.052±0.012 -0.33 0.64 10.1 ± 2.3 2018 May 05.21 2.60 3.24 0.044±0.014 -0.25 0.63 8.55 ± 2.73 CO(6-5) 2018 Mar 02.12 2.70 2.63 0.247±0.091 -0.22 0.52 7.50 ±5.10 HCN(3-2) 2017 Dec 23.26 2.97 2.05 <0.0116 - - < 0.00063 2018 Jan 16.41 2.86 2.16 <0.0141 - - < 0.00081 2018 Mar 03.34 2.70 2.64 0.006±0.005 -0.07 0.37 0.00048 ±0.0034

71 Figure 13: CO(1-0) spectrum obtained with the ARO 12m Radio Telescope. The line is slightly blue-shifted from the ephemeris velocity, as with all the other CO spectra. The spectrum is the result of co-adding all the scans obtained on 2018 May 04 and 05. The larger feature to the right is from a galactic molecular cloud in the background that is separated in velocity space by its diﬀerent Doppler velocity.

5.3 Analysis and Discussion

5.3.1 CO heliocentric evolution

In order to calculate CO and 13CO and HCN molecular column densities we assumed that excitation conditions were dominated by ﬂuorescence and collisions. Our model is largely based on the models of Crovisier & Le Bourlot 1983 [17], Bockelee-Morvan et al. 1985 [24] and Biver 1997 [118], and it is described in depth in Chapter 1. Gas production rates were calculated using a model that assumes isotropic outgassing [18]. Uncertainties for production rates are 3-σ in Table 10 and Fig.17. They have been

72 Figure 14: CO(6-5) spectrum of R2 obtained on 2018 March 02 with the 10m SMT. Helio- centric distance was 2.70 au and ∆ = 2.63 au

∗ p ∗ calculated as δTAdv = rmschannel×3 × (FWHM×∆v), where δTAdv is the 3-sigma line area uncertainty, rmschannel is the rms per channel as measured on the spectra, FWHM is the full width at half maximum of the line and, ∆v is the velocity resolution. For instances where only upper limits have been derived (as in our 2017 December 23 and 2018 January 16 HCN data), the limits are also 3-sigma, calculated with the FWHM of the posterior HCN

−1 detection. An expansion velocity of v = 0.50 km s and a rotational temperature of Trot = 25 K was assumed in all production rate calculations. We ﬁnd that the average FWHM of the CO(2-1) line was FWHM = (0.79 ± 0.16)

−1 −1 km s with an average blue lineshift of δv = (-0.20 ± 0.16) km s . Due to the spectral resolution of the SMT at this frequency (0.325 km s−1) it was not possible to resolve the line into the blue- and red-shifted components, but the observed CO line was asymmetrical and peaking towards the blue-shift in all cases Fig. 12.

73 Figure 15: 13CO spectrum resulting from co-adding all the scans obtained between 2017 December 23 and 2019 February 11 with the ARO 10m SMT. The detection is 5.7σ in line area.

Our monitoring of comet R2 over time shows that the nucleus had a clear response to the solar heating, producing more CO at closer distances to the Sun. Our data show that

29 −1.5±0.4 this correlation can be described as Q(CO) = (2.7±0.6)×10 ×rh pre-perihelion and

29 −2.5±0.3 Q(CO) = (6.6±1.6)×10 ×rh post-perihelion. (Fig. 17). This outgassing behavior is consistent with nucleus outgassing that is dominated by insolation, and resembles that of comet C/1995 O1 (Hale-Bopp). CO production rates are very sensitive to nucleus temperature due to the high volatility of CO [183].

−1 The CO 1-0 line has a FWHM = (0.63 ± 0.01) km s and an average blue-shift of δv = (-0.29 ± 0.01) km s−1 consistent with the CO(2-1) spectra. Additionally, on 2018 March 02 we detected the CO(6-5) line at 691 GHz with the 10m SMT (Fig. 14) and on 2018 May 04 and 05 we detected the CO(1-0) line with the ARO 12m telescope (Fig. 13). The 1-0 and 6-5 spectral line proﬁles have similar linewidths and Doppler shifts as the 2-1 lines. Details on the FWHM and line-shifts of these detections are presented in Table 10.

74 Figure 16: Spectrum corresponding to all of our HCN data (6 scans in December 2017, 7 scans in 2018 January and 43 scans in 2018 March 03). This line ﬂux corresponds to a 5.4σ detection.

For comparison Biver et al. 2002 [188] reported that C/1995 O1 (Hale-Bopp) behaved

−1.9 −2.0 with slopes of rh pre-perihelion and rh post-perihelion at comparable heliocentric distances. Like Hale-Bopp, this inverse-square response for R2 is consistent with its CO outgassing predominantly caused by solar heating of the nucleus. The observed CO outgassing is consistent with CO being released from the sub-surface layers of the nucleus, possibly from an evenly distributed source. If CO sublimated directly from an icy patch, we would expect to see a variation in the CO output due to the rotation of the nucleus, but this is not present in our data, nor the data of any other groups. We also note that CO was not produced at the same rate before and after perihelion, having slightly steeper rate post-perihelion. This means that for a given heliocentric distance, the comet released less CO on the way out of the inner solar system. One possibility is that

75 Figure 17: CO production rates Q(CO) plotted against heliocentric distance (r). Note how the CO production rate increases as the comet gets closer to the sun and decreases as it moves away. This is due to the response of CO to solar heating. Perihelion is denoted by the blue vertical line at 2.63 au. Both axes are on a logarithmic scale. Pre-perihelion −1.5±0.4 values are negative and to the left. The CO(2-1) emission followed a power law of rh −2.5±0.3 pre-perihelion and rh post-perihelion.There is no evidence for a post-perihelion boost to the production rate.

this can be explained by the depletion of the CO icy patch reserve as R2 approached the Sun, resulting in less CO after perihelion. A small deviation from the Q(CO) trend post-perihelion resembling a CO outburst was observed on the data collected on 2019 March 28 and 29 at r = 4.3 au (Fig. 17). The Lesia database of cometary observations13 contains CCD photometry of R2 obtained on 2018 March 25 and 2018 March 31 with a 0.2m telescope by P.Ditz and no increase in nuclear magnitude is observed on these two dates. R2 is a dust-poor comet and it has not yet been established what percentage of the visible spectrum is attributed to dust and gas, but we see no correlation between the CO and visible magnitudes during this increase of CO

76 outgassing. Thus we point out that the factor of two increase in CO on 2018 Mar 31 did not correspond with any change in the visible brightness. This may be relevant for models of CO+ emission, which rely on CO production. When comparing the dust output of different comets it is useful to derive the so-called Afρ parameter, which was first introduced by A’Hearn 1984 [7]. This parameter quantifies the solar radiation reflected by the dust grains in a coma and is widely used as a proxy for dust production rate in cometary science (see. Equation 1.1 in Introduction). In R2, Biver et al. 2018 [22] suggested that the ratio between Afρ and the total gas production for R2 is one order of magnitude lower than what is observed in most comets at similar heliocentric distances, attributing this to the low dust production of R2. On the other hand, Opticom et al. 2018 [184] derived a ratio between Afρ and Q(CN) and found the value to be high compared to other comets, which they attribute to either a high dust output (which would contradict Biver et al. 2018 [22]) or a low CN production rate. As discussed before, R2 is highly enriched in CO and depleted in HCN (and CN), thus the results of Biver et al. 2018 [22] and Opitom et al. 2018 [184] can be reconciled if R2 is indeed a dust-poor comet with very low Afρ values. Using the mean Afρ value of 670 cm reported by Biver et al. 2018 [22] between 2018 February 8 and 2018 March 8, and our average Q(CO) value obtained between 2018 February 13 and March 2 (Table 10), we calculate a Afρ/Q(CO) ratio of ∼ 0.2 cm kg−1. To illustrate how dust-poor R2 is, let us compare the Afρ/Q(CO) ratio with the also CO-rich comet C/1995 O1 (Hale-Bopp) at the pre-perihelion heliocentric distance at which we calculated it for R2 (r=2.73 au). Using the Afρ values of C/1995 O1 (Hale-Bopp) from Weiler et al. 2003 [189] and the Q(CO) production rate of C/1995 O1 (Hale-Bopp) at r ∼ 2.7au from Biver et al. 2002 [188], we derive Afρ/Q(CO) ∼ 14 cm kg−1 for comet Hale-Bopp. Thus, we see that at ∼ 2.7 au pre-perihelion the Afρ/Q(CO) ratio of Hale-Bopp is ∼ 70 times larger than that of R2. 13http://lesia.obspm.fr/comets/index.php

77 This suggests that when water-ice sublimation is not a signiﬁcant process, a high variation exists between the entrainment of dust and CO between two diﬀerent CO-rich comets. Additionally, using the Afρ value from Biver et al. 2018 [22] and our Q(HCN), we derive a Afρ/Q(HCN) ratio of ∼ 3100 cm kg−1 for R2 at 2.7 au pre-perihelion. A comparison with Hale-Bopp is possible once again if we use the Afρ value reported by Weiler et al. 2003 [189] for comet Hale-Bopp at ∼ 2.7 au pre-perihelion and the Q(HCN) reported by Biver et al. 2002 [188] at the same heliocentric distance. For Hale-Bopp we derive Afρ/Q(HCN) ∼ 1600. The fact that the Afρ/Q(HCN) ratio in Hale-Bopp is half than the ratio derived for R2 at the same heliocentric distance pre-perihelion is an indicator of the extreme HCN depletion of R2.

5.3.2 13CO J=2-1 emission

We report a 5.7σ detection of 13CO(2-1), Fig. 15, which is the ﬁrst detection of this isotopologue in a comet. We derive a production rate for this species of Q(13CO) = (1.2 ± 0.4) ×1027 mol s−1. Interestingly, if we ﬁt a Gaussian to the the 13CO line, this also results in a thinner line than what is observed for CO, yielding FWHM = (0.340 ± 0.17) km s−1

−1 13 and a blue-shift of δv = (-0.26 ± 0.17) km s . A close inspection of the CO line suggests than this FWHM is an underestimation as the line is clearly 2 channels wide, each channel being 0.34 km s−1, suggesting that this line is not the best candidate for Gaussian analysis. Due to the limited available spectral resolution, drawing conclusions about the difference between the CO, 13CO and the HCN lineshifts is difficult, and we note that in most cases these differences fall within the uncertainties. The telescope was tuned to ν = 220.398681 GHz. With the 250 kHz spectral resolution of the receiver at this frequency (0.34 km s−1 / channel), the hyperfine structure splitting of the 13CO line could not have been resolved, as the splitting of the hyperfine lines occurs in a window of 0.13 km s−1 wide. As reported by Roueff & Lique 2013 [190], it is reasonable to assume that the excitation

78 of CO and 13CO is assumed to be the same, hence we used the same value of ﬂuorescence eﬃciency as for of CO in our the 13CO line production rate derivation (g = 2.6 ×10−4). The calculation of this production rate was done using the same modeling formalism as for the previous molecules (CO and HCN) and is described in the introduction of the present

13 dissertation. For CO, we used the Einstein coeﬃcient of spontaneous emission (A21 = 6.5

−7 −1 13 ×10 cm ) and energy above the ground state for CO of ∆E2 = 15.86 K, which are tabulated in Chandra et al. 1996 [29] and NRAO’s Splatalogue [28] respectively. The determination of the 12C/13C ratio in astrochemistry and in comets in particular can be useful in constraining models of cometary formation and evolution, as this ratio is thought to change very little over timescales as along as 106 years [191]. The 12C/13C ratio has been determined from various molecules for the local interstellar medium and found to be 12C/13C ∼ 68 ± 15 [192]. Solar system values tend to be a little higher, with an average ratio of 12C/13C ∼ 89.8 for the rocky planets, 12C/13C ∼ 85.3 for the giant gas planets and 12C/13C = 98 ± 2 for the Sun [191, 193–195]. This ratio has also been determined for several comets; C/1995 O1 (Hale-Bopp) (12C/13C ∼ 94 ± 8), 17P/Holmes (12C/13C ∼ 114 ± 26) [196] and 67P/Churyumov–Gerasimenko (12C/13C ∼ 84 ± 4) [191] among others, making its determination in a particular comet relevant for comparison with other comets. For a complete list of 12C/13C cometary ratios see Table 3 of Hässig et al. 2017 [191], which have been determined from a variety of molecules, including CN, HCN but not CO. Determination of the With our marginal detection of 13CO, we can calculate the 12CO/13CO ratio by extrap- olating Q(CO) for the equivalent heliocentric distance at which 13CO was calculated using

29 −2.5±0.3 the post-perihelion relationship previously derived Q(CO) = (6.6±1.6)×10 ×rh . By

12 13 +72 13 doing so, we obtain a C/ C ratio of 45−16. Unfortunately the faintness of the CO line and the uncertainties in the relationship of Q(CO) and heliocentric distance post-perihelion yield a large uncertainty in the 12C/13C ratio. Yet another reason to be cautious when interpreting this 12C/13C ratio derived from CO measurements is the preferential photodis-

79 sociation of 13CO vs 12CO due to the self-shielding of 12CO [197], a phenomenon reported observationally by Sheﬀer et al. 1992 in molecular clouds [198]. Despite this, we note that the 12C/13C ratio here reported is consistent with the lower limit of this ratio reported by Biver et al. 2018 [22], who gives a value of 12C/13C > 54.

5.3.3 Depleted HCN emission

One of the peculiarities of R2 is its HCN depletion. During our previous observing campaign, we derived an upper limit of Q(HCN) > 8 x 1024 mol s−1, based on only ∼ 40 minutes of integration time on the HCN J=3-2 transition. Nevertheless, that yielded the highest Q(CO)/Q(HCN) ratio ever recorded in a comet: Q(CO)/Q(HCN) > 5000. In late January 2018, the same HCN transition was marginally detected with another group using IRAM, consistent with Q(HCN) ∼ 4×1024 mol s−1 by Biver et al. 2018 [22]. On 2018 March 03 we revisited the search for HCN emission in R2 and established a firm detection of HCN. We derive Q(HCN) = (4.8 ± 0.3) x 1024 mol s−1, in good agreement with the values derived by Biver et al. 2018 [22]. When we co-added all the HCN data between 2017 December 23 and 2018 March 03 -60 scans-, the line is slightly stronger, giving further validity to the long-lasting presence of HCN emission over several months (Fig. 16). Using our HCN detection of 2018 March 03 and the CO derived from the 2018 March 02 observations (Table 10) we get a CO/HCN ratio of ∼ 13,000 at 2.70 au pre-perihelion, which is exceedingly lower than what has been observed in other comets, even at ∼ 3 au where the comet has a distant activity designation. The HCN detection has a very a narrow profile FWHM = (0.37 ± 0.14)km s−1 consistent with emission coming from a cold gas. The HCN line also presents a negligible blue-shift of δv = (-0.07 ± 0.14) km s−1, which may indicate that it arises from a mostly symmetrically distributed source. We note that part of the apparent asymmetry present in the line (Fig.16) is due to hyperfine splitting of the transition. As R2 is a dust-poor comet [22], the origin of this symmetric distributed source of HCN emission remains unknown, but given the

80 uncertainties of the measurement, it is not possible to constrain the models of emission mechanisms.

5.4 Conclusions

During our observing campaign towards comet C/2016 R2 (PANSTARRS) between 2017 December 22 and 2019 June 12 we documented the CO production rates pre- and

−1.5±0.4 post-perihelion. The CO(2-1) emission followed a power law of rh pre-perihelion and −2.5±0.3 rh post-perihelion in a manner similar to comet C/1999 O2 (Hale-Bopp), increasing pre-perihelion and decreasing post-perihelion. We also report the detection of the CO(1-0) and CO(6-5) rotational transitions, which together with the CO(2-1) line, are used to estimate the rotational temperature to be Trot = 23±2K. Given that these three transitions were not obtained simultaneously, this derivation is reasonable only if we assume that the CO production rate did not change much over this month. Another important result of our campaign is the marginal detection of the elusive 13CO isotopologue at 220 GHz for the ﬁrst time in any comet. We derive a a production rate of

13 27 −1 12 13 +72 Q( CO) = (1.2 ± 0.4) x 10 mol s which yields a ratio of C/ C = 45−16. We also report HCN emission in R2 between 2017 December 23 and 2018 March 03, with a production rate of Q(HCN) = (4.8 ± 0.3) x 1024 mol s−1. This value is lower than our previous upper limit [129] and is in agreement with the marginal detection of Biver et al. 2018 [22]. This results suggests that R2 formed in N2-protected environment [129], as

HCN is the photodissociative daughter species of N2 in proto-stellar disks [180]. With the Afρ reported by Biver et al. 2018 [22], and our Q(CO) and Q(HCN) values, we derive the following ratios for R2: Afρ/Q(CO) ∼ 0.2 cm kg−1 at 2.7 au, and Afρ/Q(HCN)∼ 3100 cm kg−1 at 2.7 au, and Q(CO)/Q(HCN) ∼ 13,000 also at 2.70 au.

81 Chapter 6 Summary of Results

“Comets are like cats; they both have tails, and they both do precisely what they want to do.” This sentence by comet hunter David Levy pretty much summarizes the seemingly unpredictable nature of comets. And it is this unpredictability that drew me to them in the first place when I was an amateur astronomer as a teenager. Doing research in the field of cometary science is quite different from other disciplines. Cometary science has an element of surprise not found in other areas of astronomy and astrophysics. As a comet approaches the Sun, the volatile ices contained in its nucleus start sublimating giving rise to a plethora of phenomena. In this dissertation I had the opportunity to study three different comets using state- of-the-art millimeter-wavelength telescopes spread over two continents. The comets that I studied, though different, have one thing in common: they are all distantly active. Cometary activity at heliocentric distances greater than 4 au is not common if we take into account the bulk of the comet population, and it is often associated with the presence of CO and/or amorphous water-ice crystallization. Two of the comets that I studied (174P/Echeclus and 29P/Schwassmann-Wachmann) are short-period comets, with orbital periods of 35 and 15 years respectively, while C/2016 R2 (PanSTARRS) is a very different type of comet with an orbital period of ∼20,000 years. 174P/Echeclus and 29P/Schwassmann-Wachmann are also classified as Centaurs, that is objects that often display characteristics of both asteroids and comets, which also are in transitional orbits controlled by the gas giant planets. Moreover, the transitional nature of some Centaur orbits is exemplified by 29P/Schwassmann-Wachmann

82 and its current residence in the newly discovered Gateway zone. This object will become a Jupiter Family Comet and very likely a very bright object. When I started doing research in this field under the guidance of my advisor, Professor Maria Womack, I could not have predicted that my research would shed light on the complicated relationship between CO and dust outbursts in the most enigmatic comet known, produce the first detection of the isotopologue 13CO in a comet, or produce the detection of CO emission in only the third Centaur. These results have been proved useful in the field of cometary science and show the continued importance of CO outgassing in distantly active comets and Centaurs. It draws attention to the special region around 3-7 au where water-ice poorly sublimates and activity must be due to something else, such as CO. These observations and analyses show that CO is produced in amounts high enough to support the dust outgassing, although gas and dust outbursts may not occur at the same time. This work had additional reaches in that another collaboration used my CO production rates in comet C/2016 R2 (PanSTARRS) to extract the very difficult to measure CO2 from infrared photometry obtained with the Spitzer Space Telescope. This dissertation also shows that the outbursts for which comet 29P/Schwassmann-Wachmann is so famous may not require significant amounts of CO outgassing, and that a significant increase in CO can come out “clean” leaving very little to any dust coma behind. Additionally, this work also obtained the very first detection of 13CO in a comet, which along with my CO and HCN measurements, may be evidence that comet C/2016 R2 (PanSTARRS) formed in a cold, photon-shielded environment. In 2016, comet 174P/Echeclus had just passed perihelion. Its proximity to the Sun and its favourable placement for ground-based observations made it a perfect candidate for scrutiny. Furthermore, I knew that due to its heliocentric distance, the thermal equilibrium due to solar heating would be enough to trigger a phase change in amorphous water-ice to a crystalline state. The new state can also release trapped gases, or expose previously buried frozen CO-ice to solar heating. This object also undergoes outbursts approximately 3 times

83 per decade and has experienced episodes of fragmentation and/or release of debris. Some low sublimation temperature volatile must be driving this activity and CO outgassing was a reasonable candidate. In 2016 I observed 174P/Echeclus between May and June with the ARO 10-m SMT over nine days and detected CO emission. Before 174P/Echeclus, only two other Centaurs had reports of CO emission: 29P/Schwassmann-Wachmann and 95P/Chiron. Besides CO emission, a common characteristic of these objects is that they are active at large heliocentric distances where water-ice can not sublimate and drive their activity. The amount of CO detected in 174P/Echeclus suggests that this body is CO-deficient when compared to other relatively unprocessed comets like 1995 O1 (Hale-Bopp) or to the CO-rich Centaur 29P/Schwassmann-Wachmann. Despite being CO-deficient, CO is capable of driving the observed activity. The low amount of CO observed in 174P/Echeclus also suggests that it may have either incorporated less CO into its nucleus during its formation or that it may have become devolatilized. Yet another common characteristic between 174P/Echeclus and 29P/Schwassmann-Wachmann (and to a lesser extent 95P/Chiron) is the presence of dramatic increases of activity or outbursts. While 174P/Echeclus has only displayed a handful of outbursts since its discovery in 2000, 29P/Schwassmann-Wachmann on average undergoes more than 7 significant outbursts a year. These outbursts have puzzled astronomers since the discovery of this comet in 1927 and continue to remain a mystery. For years it has been hypothesized that the CO emission of 29P/Schwassmann-Wachmann is somehow related to the release of dust, which is the responsible for the observed increase in brightness. In early 2016 I started an observing campaign to monitor the CO output of 29P/Schwassmann- Wachmann with the Arizona Radio Observatory 10m Submillimeter Telescope. This campaign was further extended between late 2018 and early 2019, and simultaneous observations of this comet carried out in visible wavelengths by expert amateur astronomers (something very common in astronomy, but not so much in other sciences) permitted me to compare

84 the CO production of this comet with its dust activity. The results of this work suggest a very complicated relationship between CO and dust. In fact, the data does not support the year-old hypothesis that CO emission is correlated with dust. My observations reveal that 29P/Schassmann-Wachmann can undergo significant CO outbursts without experiencing an increase in dust emission and vice versa. C/2016 R2 (PanSTARRS) is a very different comet from both 174P/Echeclus and 29P/Schwassmann-Wachmann. This comet was discovered in the Fall of 2016 at a heliocentric distance of 6.3 au. In late 2017 December, I observed C/2016 R2 (PanSTARRS) also with the ARO 10m SMT, and detected neutral CO emission in this comet for the first time in very high amounts. A survey for other species commonly found in comets revealed that this object is one of the most interesting comets discovered in the recent decades. My observations revealed an extreme depletion in HCN, which is a species commonly observed in comets due to its high dipole moment and its capacity to produce bright lines in its rotational spectrum. In fact, the observed HCN depletion of C/2016 R2 (PanSTARRS) yielded the highest CO/HCN ratio observed in any comet. Other authors reported the presence of N2 in C/2016 R2 (PanSTARRS), which was also surprising as comets are usually N2-poor. The

HCN depletion and presence of N2 allowed me to hypothesize that C/2016 R2 (PanSTARRS) was formed in a N2-sheilded region of the protosolar nebula disk, as physicochemical models of protostellar disks suggest that it is the photodisocciation of N2 that leads to HCN. Also during this study, the comparison of the CO output of this comet with other CO-rich comets allowed me to put a constrain on the nucleus size of C/2016 R2 (PanSTARRS). In 2018, a world-wide observing campaign involving the Spitzer Space Telescope and several ground-based telescopes was carried out towards C/2016 R2 (PanSTARRS), and the follow-up observations of this comet that I did throughout 2018 and 2019 provided more suprising results. The evolution of the CO outgassing of C/2016 R2 (PanSTARRS) pre- and post-perihelion behaved in a manner similar to C/1995 O1 (Hale-Bopp). I was also able to conﬁrm the extreme HCN depletion by detecting HCN and deriving an even higher

85 CO/HCN ratio than the ratio derived from the December 2017 observation. But perhaps the most interesting result is the detection of the isotopologue 13CO for the first time in a comet. Several unsuccessful searches for 13CO have been carried out towards different comets since the late 90s by different authors, and the detection of 13CO in C/2016 R2 (PanSTARRS) was possible due to a combination of a very CO-rich comet and extensive observing time.

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106 Appendix A Proof of copyright permissions

Two chapters of the present dissertaton have been published in The Astronomical Journal (Chapters 1 and 3). At the time of writing, the work contained in Chapters 2 and 4 has also been been submitted for publication to The Astronomical Journal. The Astronomical Journal belongs to the American Astronomical Society (AAS) and is published by IOP Publishing. Their copiright agreement states that "AAS grants to you the non-exclusive right of republication, subject only to your giving appropriate credit to the journal in which your article is published" Their Copiright Agreement can be found at: https://journals.aas.org/article-charges-and-copyright/#reproduction_of_copyrighted_materials

107 Appendix B

Author’s Work

B.1 Refereed Publications

1. McKay, A, J., DiSanti, M. A., Kelley, M. S. P., Knight, M., Womack, M., Wierzchos, K., et al., The Peculiar Volatile Composition of CO-dominated Comet C/2016 R2 (PanSTARRS), 2019,

The Astronomical Journal, 158, 3

2. Wierzchos K., Womack M., C/2016 R2 (Pan-STARRS):C/2016 R2 (PANSTARRS): A comet rich in CO and depleted in HCN, 2018, The Astronomical Journal, 156,1

3. Pariente, E.S., Cancilla, J.C., Wierzchos, K., Torrecilla, J.S. On-site images taken and processed to classify olives according to quality – The foundation of a high-grade olive oil, 2018,

Postharvest Biology and Technology, 140, pp. 60-66.

4. Womack, M., Curtis, A., Lastra, N., Harrington Pinto, O., Rabson, D.A.,Wierzchos, K.,

Cox, T., Rivera, I., Mentzer, C., Ruﬃni, N., Jackson, C., and Micciche, A., 2018, “Comet C/1995

O1(Hale-Bopp) Visual Lightcurve V1.0, 2018, NASA Planetary Data System.

108 5. Wierzchos K., Womack M. P., Carbon Monoxide in distantly active centaur (60558)

174P/Echeclus at 6au. The Astronomical Journal, 2017, 153(5)

6. Womack M. P., Sarid G., Wierzchos K. CO in Distantly Active Comets. Publications of the Astronomical Society of the Paciﬁc, PASP, 2017, 129(973).

7. Cancilla, J.C., Aroca-Santos, R., Wierzchos, K., Torrecilla, J.S. Hazardous aromatic VOC quantiﬁcation through spectroscopic analysis and intelligent modeling to assess drinking water quality, 2016, Chemometrics and Intelligent Laboratory Systems, 156, pp. 102-107.

8. Cancilla J.C., Perez A., Wierzchos K., Torrecilla J.S. Neural Networks Applied to Deter- mine Thermophysical Properties of Amino Acid Based Ionic Liquids, Physical Chemistry Chemical

Physics, February 2016

9. Wierzchos K., Cancilla J.C., Díaz-Rodriguez P., Torrecilla J.S., Davila A.F., Ascaso C.,

Nienow J., McKay C.P., Wierzchos J. Application of artiﬁcial neural networks as a tool for moisture prediction in microbially colonized halite in the Atacama Desert. J. of Geophysical Research:

Biogeosciences. 120(6), May 2015

10. Cancilla, J.C., Díaz-Rodríguez P., Wierzchos K., Torrecilla J.S. Non-Linear Models Ap- plied to Experimental Spectroscopical Quantitative Analysis of Aqueous Ternary Mixtures of Im- idazolium and Pyridinium-Based Ionic Liquids. Sensors & Actuators: B. Chemical. Volume 206,

January 2015, 139-145

11. Wierzchos, J., Cámara, B., De los Ríos, A., Davila, A.F., Sánchez Almazo, I.M., Artieda,

O., Wierzchos, K., Gómez-silva, B., McKay, C., Ascaso, C. Microbial colonization of Ca-sulfate

109 crusts in the hyperarid core of the Atacama Desert: Implications for the search for life on Mars.

Geobiology J. 2011 January. 9(1): 44-60.

B.2 Selected non-refereed publications

1. Wierzchos, K., Discovery of three new double stars, IAU Double Stars Information Circular no.198, 2019

2. Wierzchos, K., Discovery of four new double stars, IAU Double Stars Information Circular no.194, 2018

3. Wierzchos, K., Womack, M. Comet C/2016 R2 (Panstarrs), 2017, CBET 4464

4. DiSanti, M., Bonev, B., Dello Russo, N., Vervack, R., Gibb, E., Roth, N., McKay A.,

Kawakita, H., Howell, E., Zambrano-Marin, F., Yoshida, S., Yoshimoto, K., Gonzalez, J.J., Ha- subick, W., Wierzchos, K. Comet 45P/Honda-Mrkos-Pajdusakova, 2017, CBET 4357

5. Wierzchos, K. Discovery of a new double star, IAU Double Stars Information Circular no.193, 2017

6. Wierzchos, K. Two new visual common proper motion pairs, 2017, Webb Deep-Sky society

Double Star Section Circular no.25

110 B.3 Conference Presentations

1. “Stay Active My Friend": 29P/S-W1, The Most Interesting Comet in the World Sarid, Gal;

Womack, Maria; Wierzchos, Kacper Division of Planetary Sciences” 50th Meeting. Knoxville,TN.

October 2017

2. “Systematic Characterization and Monitoring of Potentially Active Asteroids: The Case of Don Quixote Mommert, Michael; Trilling, David; Knight, Matthew M.; Hora, Joseph; Biver,

Nicolas; Womack, Maria; Wierzchos, Kacper; Polishook, David; Veres, Peter; Gustafsson, Annika;

McNeill, Andrew; Skiﬀ, Brian; Wainscoat, Richard; Kelley, Michael S.; Moskovitz, Nicholas; Har- rington, Olga, Division of Planetary Sciences” 50th Meeting. Knoxville,TN. October 2017

3. “A coordinated ground- and space-based observing campaign to measure CO2 and CO emission in C/2016 R2 (PANSTARRS) Harrington Pinto, Olga; McKay, Adam; DiSanti, Michael A.;

Kelley, Michael S.; Cochran, Anita; Dello Russo, Neil; Womack, Maria; Wierzchos, Kacper; Biver,

Nicolas; Bauer, James; Vervack, Ron J.; Bonev, Boncho; Gibb, Erika; Roth, Nathan; Kawakita,

Hideyo, Division of Planetary Sciences” 50th Meeting. Knoxville,TN. October 2017

4. “The Volatile Composition of CO-Dominated Comet C/2016 R2 (PANSTARRS) McKay,

Adam; DiSanti, Michael; Kelley, Michael S.; Cochran, Anita; Dello Russo, Neil; Villanueva, Geron- imo; Womack, Maria; Wierzchos, Kacper; Biver, Nicolas; Bauer, James; Harrington, Olga; Vervack,

Ron J.; Bonev, Boncho; Gibb, Erika; Roth, Nathan; Kawakita, Hideyo, Division of Planetary Sci- ences” 50th Meeting. Knoxville,TN. October 2017

5. “Strong emission of CO in comet C/2016 R2 (PANSTARRS) Wierzchos, Kacper; Womack,

Maria, Division of Planetary Sciences” 50th Meeting. Knoxville,TN. October 2017

111 6. “Detection of CO and HCN in the coma of Jupiter-family comet 41P/Tuttle-GiacobiniKresak”

Wierzchos, K., Womack, M. Division of Planetary Sciences” 49th Meeting. Provo,UT. October 2017

7. “New methods for deriving cometary secular light curves: C/1995 O1 (Hale-Bopp) revisited“

Womack, Maria; Lastra, Nathan; Harrington, Olga; Curtis, Anthony; Wierzchos, Kacper; Ruﬃni,

Nicholas; Charles, Mentzer; Rabson, David; Cox, Timothy; Rivera, Isabel & Micciche, Anthony.

American Astronomical Society, DPS meeting no.49. Provo, UT. 2017

8. “Carbon monoxide in the distantly active centaur 174P/Echeclus”, Wierzchos, K., Womack,

M., Sarid, G., Division of Planetary Sciences” 48th Meeting. Psadena, CA. October 2016

9. “CO and other volatiles in Distantly Active Comets“ Womack, M.; Sarid, G., Wierzchos, K.

American Astronomical Society, DPS meeting no.48, Passadena, CA. 2016

10. “Artificial neural networks in the determination of different types of Cancer” presented at the 2nd-International Caparica Conference on Profiling (ISPROF 2015)

112 11. “Feature Selection-Multilayer Perceptron (FS-MLP): an intelligent selecting and classifying tool” presented at the “Second Workshop on Lung Cancer Detection with Sensor Arrays”, Majorca,

Spain. September 2014

12. “Neural networks applied to treat data from sensors” presented at the “1st Christmas Con- ference on Sample treatments” Lisboa, Portugal December 2014

113 About the Author

Kacper Wierzchos was born in 1988 in Lublin, Poland. When he was two, he moved to Spain with his parents where he attended high-shcool. During his early education, he de- veloped a passion for astronomy when his father bought him a 90mm Maksutov-Cassegrain telescope for Christmas in 2001. Pursuing his career goal of making astronomy his profes- sion, he went on to study the Spanish Licenciatura en Ciéncias Físicas at the Universidad Complutense de Madrid. In 2008 while in college, he was one of a handful of astronomers to observe and report astrometry on the impactor 2008 TC3 before it entered Earth’s atmo- sphere. At the age of twenty-seven, he moved to the United States and started working on his Ph.D under the guidance and advise of Dr. Maria Womack at the University of South Florida in Tampa, FL. In Fall 2019, Kacper accepted a position as a Senior Research Spe- cialist at the Catalina Sky Survey in Tucson, AZ, where he conducts asteroid surveys and follow-up observations of Near Earth Asteroids. He currently lives with his wife Sheila and his cat Gordy, and despite being a professional astronomer, he is still an avid visual observer with a speciﬁc interest in lunar and planetary observation and double stars.