Active bodies in the near-Earth region: The tenuous boundary between and

Julio A. Fern´andez,Andrea Sosa Departamento de Astronom´ıa,Facultad de Ciencias, Montevideo, URUGUAY

∗ Why to focus on bodies that approach the Earth?. ∗ Comets and asteroids: Differences and gray zones. ∗ Comets in ”asteroidal” orbits: Characterization through the capture time tcap. ∗ A measure of the degree of activity of a : the equivalent fraction of active surface area f. ∗ Presentation of the results. ∗ Discussion: One or more source regions?.

IAU GA 2015 - Fern´andez& Sosa 1 The samples We consider the population of near-Earth -family comets (NEJFCs) that reach perihelion distances q < 1.3 au, and compare its dynamical and physical properties with those of near-Earth asteroids (NEAs).

Why do we only consider objects with q < 1.3 au?

There is a twofold reason:

1) Because these are the objects best sampled due to their proximity to Earth.

2) Even traces of volatile content may give rise to some level of activity upon approach to the on bodies that, otherwise, would appear as inert if observed further away.

IAU GA 2015 - Fern´andez& Sosa 2 Comets versus asteroids Our traditional view was that:

Asteroids are inactive Comets are active

∗ Near-Earth asteroids move on rather stable orbits (Tisserand parameters T > 3);

∗ Near-Earth JFCs move on unstable orbits (Tisserand parameters T < 3).

IAU GA 2015 - Fern´andez& Sosa 3 Yet, the situation has become more complex ...

∗ Some NEAs move on cometary orbits (T < 3) and have close approaches to Jupiter.

∗ Some JFCs move on ”asteroidal” orbits (Fern´andez& Sosa 2015).

∗ Some bodies show residual, and in some cases intermittent activity. Examples: 107P/Wilson-Harrington, (3532) Don Quixote (Mommert et al. 2014).

IAU GA 2015 - Fern´andez& Sosa 4 The capture time ∗ Since we are dealing with populations of objects that approach the Earth, we will focus on a parameter that measures the time that a given object has been in the Earth’s neighborhood: the capture time tcap.

We define tcap as the time it took a given comet to decrease its perihelion distance q by 1 au down to qdisc = q(tdisc), namely the observed value of q at the discovery time tdisc. This can be expressed as

q(tq+1) = q(tdisc) + 1 au

=⇒ tcap = tdisc − tq+1

IAU GA 2015 - Fern´andez& Sosa 5 Examples of JFCs in stable and unstable orbits

5 q-average of comet 4 54P/de Vico-Swift-NEAT 3 + 50 clones as a 2 function of time. average q (au) 1 Red circle: indicates when

0 -10000 -9000 -8000 -7000 -6000 -5000 -4000 -3000 -2000 -1000 0 q¯ = qdisc + 1 au time (yr)

5

4 q-average of comet 3 182P/LONEOS + 50 2 clones as a function average q (au) 1 of time.

0 -10000 -9000 -8000 -7000 -6000 -5000 -4000 -3000 -2000 -1000 0 time (yr)

(Fern´andezand Sosa 2015)

IAU GA 2015 - Fern´andez& Sosa 6 Capture times

Comet tcap (yr) 54P/de Vico-Swift-NEAT ∼ 270 182P/LONEOS > 5 × 104

Typical physical lifetime of an active JFC with q < 1.3 au : a few 103 yr (Di Sisto et al. 2009) =⇒ JFCs in stable orbits like 182P/LONEOS should have much longer physical lifetimes.

IAU GA 2015 - Fern´andez& Sosa 7 8

6

4 number

2

0 0 1000 2000 3000 4000 5000 capture time (yr)

The distribution of capture times shows that most JFCs have been in the near-Earth 3 region for no more than a few 10 yr. The spike at tcap < 100 yr suggests the presence of small comets of very short physical lifetimes.

IAU GA 2015 - Fern´andez& Sosa 8 The discovery rate of comets and asteroids in the near-Earth region

400

350

300 NEAs

250

200

150 cumulative number

100

50 JFCs

0 1960 1970 1980 1990 2000 2010 2020 discovery year

NEAs with aphelion distances Q > 4.5 au and Tisserand parameters T < 3, and near- Earth JFCs. The flattening of the discovery rate curve of JFCs suggests a much smaller population than that of NEAs.

IAU GA 2015 - Fern´andez& Sosa 9 The fraction of active surface area We can define the degree of gaseous activity displayed by a comet by the fraction of active surface area f:

Q f = H2O 4πR2Z

QH2O : production rate of gas (water) molecules, R : radius of the body, Z : water production rate per unit area of a free-sublimating water ice surface (derived theoretically from the energy balance equation).

The fraction f actually is a proxy to assess the degree of activity of a body. It cannot be taken at face value, since the activity may arise beneath the surface where the generated gases reach the surface via diffusion, or part of the sublimated gases may come from the sublimation of icy grains in the .

IAU GA 2015 - Fern´andez& Sosa 10 Computed fraction f : Active comets

Most active NEJFCs have estimated water production rates QH2O at heliocentric distances r near perihelion. Most of then also have good estimates of their nuclear radii. They move on unstable (”cometary”) orbits. Active JFCs Comet R (km) QH2O (mol/s) r (au) 1P/Halley 5.64 6.6 − 19.7 × 1029 0.87 - 0.74 6P/d’Arrest 1.66 3.02 − 3.24 × 1027 1.40 - 1.41 7P/Pons-Winnecke 2.24 3.24 × 1027 1.42 21P/Giacobini-Zinner 1.82 2.0 − 5.6 × 1028 1.08 - 1.11 26P/Grigg-Skjellerup 1.21 2.5 × 1027 1.0 45P/Honda-Mrkos-Pajdusakova 0.35 1.05 × 1027 1.15 46P/Wirtanen 0.6 9.33 × 1027 1.12 67P/Churyumov-Gerasimenko 1.72 0.47 − 1.48 × 1028 1.35-1.36 73P/Schwassmann-Wachmann 3 1.26 4.17 × 1027 1.44 103P/Hartley 2 0.65 0.95 − 4.17 × 1028 1.09-1.03

IAU GA 2015 - Fern´andez& Sosa 11 Computed fraction f : Faint comets

Most of these comets do not have estimated QH2O values, so we have to rely on indirect methods to estimate their fractions f, either dust production rates Qd or absolute total magnitudes. We consider a set of faint comets on ”asteroidal” (stable) orbits.

Faint JFCs on ”asteroidal” orbits

Comet R (km) method source 28 2P/Encke 3.0 QH2O = 0.26 − 5.5 × 10 mol/s several at r = 1.2 − 0.34 au 162P/Siding Spring 3.6 total magnitude (1) < −4 169P/NEAT 2.3 Qd → f ∼ 10 Kasuga et al. (2010) 189P/NEAT 0.5 total magnitude (1) 25 209P/LINEAR 0.87 QH2O = 2.5 × 10 mol/s at r = 0.99 au Schleichter (2014) 300P/Catalina 0.7 Qd Harmon et al. (2006) 317P/WISE 0.5 total magnitude (1)

(1) Estimate of the ”active fraction” f from the total magnitude

We assume that the coma light comes from scattering of sunlight by dust particles of typical size a¯. The total mass of dust, Md in the coma at a certain time is (e.g. Jewitt 2012)

IAU GA 2015 - Fern´andez& Sosa 12 4ρ a¯   M = d S 10∆m/2.5 − 1 d 3 where S = πR2 is the geometric cross-section of the , ∆m the difference between the nuclear and total magnitudes, and ρd is the mass density of the dust particles.

The dust production rate is given by

Mdvej M˙ d = RC where vej is the ejection velocity of the dust particles, and RC the coma radius. By assuming a certain dust to gas ratio D/G we can estimate the gas production rate. We adopt conservatively:

D = 5 G (Jewitt et al. 2014 and other references therein).

IAU GA 2015 - Fern´andez& Sosa 13 The results: fraction f versus heliocentric distance r

100 103P 46P 1P 45P 21P 73P 10−1 67P 6P 26P 7P

189P 10−2 2P 162P fraction

−3 317P 10 209P 300P

169P 10−4 0.0 0.5 1.0 1.5 heliocentric distance (au)

IAU GA 2015 - Fern´andez& Sosa 14 Computed fraction f as a function of the comet radius

100 46P 103P 1P 45P 21P 73P 10−1 67P 6P 7P 26P 189P 10−2 2P 162P fraction

317P −3 10 209P 300P

169P 10−4 0 1 2 3 4 5 6 radius (km)

IAU GA 2015 - Fern´andez& Sosa 15 Computed fraction f versus the capture time

100 46P 103P

45P 21P 73P 10−1 67P 6P 26P 7P

162P 10−2 189P fraction

−3 317P 10 209P 300P

169P 10−4 100 101 102 103 104 105

tcap (yr)

IAU GA 2015 - Fern´andez& Sosa 16 Open questions

> 4 Why JFCs in asteroidal orbits can last for so long ( ∼ 5 × 10 yr) in the near-Earth region? 1) Are they the (quasi)-devolatized fragments of larger parent comets sharing the same origin as the rest of the active JFCs (trans-neptunian belt)?

This could be the case of the small members (sub-km radius). Example : 289P/Blanpain (radius ∼ 0.15 km)

Or,

2) Do they come from the belt? This could explain their rather stable dynamics and their low activity as due to their mostly rocky composition with only a minor fraction of volatile components.

Arguments in favor of the latter interpretation: Bodies of different sizes (from ∼ 0.5 km to ∼ 4 km), so all of them are unlikely to be the fragments and debris of larger parent bodies. On the other hand, several of them are small enough to make difficult the buildup of insulating dust mantles so close to the Sun.

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