Molecular Production Rates and Abundances in ER61

Melissa Huber University of Missouri – St. Louis

Advisor: Dr. Erika Gibb

Abstract C/2015 ER61 is a relatively recently discovered Oort Cloud comet. We observed four parent volatile compounds using the ISHELL at IRTF on Mauna Kea. We report the volatile compound abundances of C2H6, CH3OH, and OH on April 16 and April 17 relative to water in the comet. Relative to other comets, ER61 appears to be “organically enriched” with respect to its abundances. This classification system helps us determine the role that comets played in shaping life on Earth, as these comets may have brought water and other molecules to Earth that helped develop the evolution of organic matter.

Introduction The Oort Cloud is the name given to the shell of icy objects that lies in the far reaches of our solar system. This cloud is more or less spherical and is famously thought to be the point where most of the long-period comets that have been observed originated from. The prevailing theory from astronomers is that our solar system began as a protoplanetary disk that spent epochs forming into our sun and planets. The comets and objects that now make up the Oort Cloud are believed to have formed closer to the Sun, but upon the formation of the larger planets, Jupiter in particular, these objects were forced out due to gravitational scattering (1). These objects scattered before stabilizing in their current location, forming the Oort Cloud. While the Oort Cloud itself is very far away from the Sun, the comets and objects within can be disturbed by passing stars and other celestial objects, or by actions within the disk of the Milky Way itself. These disturbances can cause comets to be knocked loose from the Oort Cloud, whereupon they proceed to speed themselves back towards the Sun (3). One of the key features of interest in comets is that they have remained relatively unchanged from their initial formation within our solar system some 4.6 billion years ago. By studying these comets and their makeup, we can achieve a better understanding of the formation of our solar system and the forces that helped shape the planets from our original protoplanetary disk. One of the other questions that we seek to resolve in our studies is regarding the origin of the Oort Cloud comets. Some astronomers have suggested that some, but not all, of the comets located within the Oort Cloud may have been picked up from another star within our Sun’s original cluster. These comets may have become dislodged from the orbit of the other star and fell into the gravitational pull of our own Sun, thus becoming part of the Oort Cloud (1). However, comets have not remained completely untouched since their inception in the solar system. The comets of the Oort cloud have had their outer layers affected by radiation from stars like our Sun, which has penetrated the comets as they travel through space. This radiation bombardment causes a buildup of a dark refractory organic layer on the comet that keeps the inner layers preserved, up until the comet makes its approach towards the Sun. Comets are also affected by a process known as space weathering, which is the result of passage through space over billions of years. This process is a series of interactions with other objects in space such as micrometeorites or any number of high energy photons that impact the comet (2). These interactions tend to have a minimal effect on the comets themselves due to the dark refractory layer that is built up by solar radiation. When a comet approaches close enough to the Sun, the heat given off can cause the comet to crack as a result of the volatile gases contained within heating up and expanding. Once these volatile gases have begun escaping from the comet’s inner layers, they interact with high energy photons and become excited to a point where we are able to observe the emission effects. These effects are best observed within the coma trailing behind the comet’s nucleus, from which we can determine abundances that make up the parent volatiles. For our studies, we examine these parent volatiles of the comet in order to learn more about the physical makeup and origins of the comet itself. We target emissions from comet wavelengths containing H2O, NH3, CH3OH, CH4, HCN, OCS, C2H2, C2H6, CO, and H2CO. The abundances of these volatile compounds is what we look for when examining comets, as they can be radically different from comet to comet. From these abundances, we are able to create a taxonomy that compares the comet’s organic makeup compared to H2O. This way we are able to say whether a comet is “enriched,” “normal,” or “depleted” in organic compounds when compared to H2O. In section II, we discuss the observations and data analysis. Our results are discussed in section III.

Observations and Data Reduction The Oort Cloud comet C/2015 ER61 was observed while it was fading from an outburst on April 16-17, 2017. This comet was first discovered on March 15, 2015 by Pan-STARRS (The Panoramic Survey Telescope and Rapid Response System) located at Haleakala Observatory in Hawaii. On its first observation, ER61 was 8.44 AU from the Sun and had a magnitude of 21.5. On our observed pass in April 2017, it approached with an Earth Minimum Orbit Intersection Distance of 0.101 AU and a perihelion of 1.042 AU with a total magnitude of 10 mag (4). All data were acquired via the ISHELL spectrograph from the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. Observations were performed with a 6-pixel wide slit, which allowed us to acquire high spectral resolution data. We used the standard ABBA nod pattern, with 7.5” beam separation along the 15” long slit. This means that the spectra were added and subtracted in an A-B-B+A sequence. Using this method, we were able to cancel out telluric emissions that would hinder our observations and thus get clearer data. This data were processed using custom algorithms in order to filter out further instabilities such as high dark current pixels and any hits from cosmic rays (5). Using telluric models, we were able to assign wavelength scales to the spectra and determined column burdens for the absorbing species in Earth’s atmosphere. This atmospheric model was then altered to fit the resolution of the comet spectrum and scaled to match the comet’s continuum level. We then subtracted the atmospheric model from the processed cometary spectra row by row, and thus were able to obtain residual cometary line fluxes, which are shown in Figures 1 and 2. We took the residual fluxes of the cometary emission lines and corrected them to account for the monochromatic transmittance at the Doppler shifted position of each line.

Analysis

The following volatile compounds were detected: OH, H2O, C2H6, and CH3OH. In order to determine the presence of these molecules, we compared the observed spectra to models of the molecules at a range of temperatures. These models were also shifted in wavenumber to match the Doppler Shift of the observed emission of the molecules coming from the coma of ER61. We then took whichever model best matched the emission spectra with its corresponding temperature and used that to determine the rotational temperature of that molecule. The rotational temperature of ER61 was best constrained by H2O on April 17 with an average rotational temperature of 68K. This is because the H2O setting contains multiple lines that are sensitive to temperature changes, unlike the other volatiles that we observed in this comet.

Figure 1 – Comet residual spectrum (black) and best-fit models shifted vertically for clarity, for C2H6 (red), CH3OH (blue), and OH (orange) for comet C/2015 ER61 on 2017 April 16. The 1σ error envelope (green) and the telluric model plus the best-fit comet emission models (purple) are over-plotted on the residual.

Figure 2 – Comet residual spectrum (black) and best-fit models shifted vertically for clarity, for OH (blue) and H2O (red) for comet C/2015 ER61 on 2017 April 17. The 1σ error envelope (green) and the telluric model plus the best-fit comet emission models (purple) are overplotted on the residual. Table 1 contains the best-fit temperatures, the corresponding production rates of each molecule, and the abundances of each molecule within the coma. The data suggest that C/2015 ER61 was more productive on April 17 compared to April 16, based on the production rate of H2O. Table 1 Molecular Production Rates and Abundances in ER61

2017 UT Setting/Order Species Gamma Qtote 1026 T_rot(K) Mean-Q[s-1] Error-Std Qncc 1026 Date mol s−1 mol s−1

April 16 Lp1/154 C2H6 9.15E-18 2.31±0.109 56 9.2240e+25 7.6522e+24 3.99 CH3OH 2.36±0.216 90 4.8601e+26 1.0607e+26 2.06 Lp1/155 C2H6 8.74E-18 2.54±0.052 90 2.0061e+26 1.3409e+25 0.791 CH3OH 3.38±0.074 30 8.5896e+26 1.7140e+26 2.54 OH 20 4.2066e+28 1.1967e+28 April 17 Lcustom/175 H2O 3.12E-16 2.83 ±0.037 100 2.6428e+30 8.6548e+29 0.933 OH 2.63±0.162 40 2.0691e+30 3.3632e+29 0.787 Lcustom/176 H2O 3.06E-16 2.90±0.026 81 2.1752e+30 5.6638e+28 0.750 OH 3.90±0.035 60 2.1692e+30 3.0898e+29 0.556 Lcustom/177 H2O 3.05E-16 6.072±0.040 25 6.9059e+29 3.1436e+29 2.730 OH 2.53±0.097 70 1.9606e+30 3.1742e+29 0.775 Lcustom/178 H2O 2.98E-16 3.93±0.0473 65 1.2507e+30 7.5119e+29 0.318 OH 3.77±0.036 65 3.4787e+30 -NaN 0.922 Lcustom/179 H2O 2.93E-16 4.00± 0.044 69 1.8098e+30 1.0772e+29 0.453 OH 6.39± 0.034 20 3.9686e+30 -NaN 0.621

Table 2 contains the observations made on each date along with the telescope setting, the change in velocity, and the angle of the slit used for observations on April 16 and 17. Table 2 Log of IR Spectral Observations of Comet C/2015 ER61

c −1 e f 2017 April UT Setting Δν (cm ) UT Start-End Molecule(s) (orders) Tint Slit PA (deg) Date (s) April 16 Lp1 2915.40-3126.16 3:24-17:44, 18:37- C2H6, CH3OH, OH 474 0.75 slit 253 angle 20:12 (154,155) April 17 Lcustom 3317.47-3565.81 15:01-15:40 H2O, OH (175-179) 474 0.75 lit 252 angle

Discussion The above figures come from the emission spectra of the coma of ER61 on April 16 and April 17. C/2015 ER61 seems to fit the current taxonomy of comets as “organically enriched”, due to the values of molecules OH, C2H6, and CH3OH appearing extremely close to the values of H2O observed within the comet. These values all appear in great abundance, particularly OH which has nearly the same values as H2O observed on April 17. These abundances of trace volatiles are used to create ratios and are expressed alongside the simultaneously measured H2O within the comet (5). Of all the observed volatiles, OH is the most abundant after H2O, nearly reaching the same values in all observed instances on April 17. Of the remaining two volatile compounds that were observed on April 16, the next most abundant was CH3OH, followed by C2H6. Notably, in two of the cases, OH actually surpassed H2O in measurement on the Lcustom setting 176 and 179. Even though the water production rate was higher on the second of these settings, OH surpassed H2O by a much higher margin.

Future Work The Oort Cloud comet repository has 1012 comets with only 29 of these having been analyzed using the method described in this paper. More comets need to be processed in order to understand more about the overall composition of this comet cluster. In particular, NH3 has only been detected in 14 comets thus far and needs further analysis in order to provide an adequate average value measurement. Once enough comets have been sampled, the current taxonomy of the comet composition can be reexamined and we can achieve a better understanding of the role of comets within the early solar system.

Acknowledgements All the data in this paper were collected using ISHELL at IRTF on Mauna Kea, operated by the University of Hawaii’s Institute for Astronomy and funded by NASA and the NSF. Melissa A. Huber wishes to recognize and acknowledge the significant cultural role and veneration that the summit of Mauna Kea has always had within the indigenous Hawaiian community. Melissa A. Huber was funded by the NASA Missouri Space Grant Consortium. Melissa A. Huber also wishes to acknowledge the help of Professor Dr. Erika L. Gibb, graduate student Nathan X. Roth, and the Department of Physics and Astronomy at the University of Missouri – Saint Louis.

Biography Melissa A. Huber is an undergraduate student pursuing a Bachelor’s Degree in Physics along with a minor degree in Mathematics at the University of Missouri – Saint Louis. After completing her degrees she intends to go to graduate school for Astrophysics to do further research in this field.

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