Comet C/2017 K2 (PANSTARRS): dynamically old or new?
0000-0002-5319-5716]Raúl de la Fuente Marcos
\move@AU\move@AF\@affiliationAEGORA Research Group
Facultad de Ciencias Matemáticas
Universidad Complutense de Madrid
Ciudad Universitaria, E-28040 Madrid, Spain
0000-0003-3894-8609]Carlos de la Fuente Marcos
\move@AU\move@AF\@affiliationUniversidad Complutense de Madrid
Ciudad Universitaria, E-28040 Madrid, Spain
At discovery time, C/2017 K2 (PANSTARRS) was the second most distant (16 au) inbound active comet ever observed (Meech et al., 2017). The fact that the comet was active beyond the crystallization zone was interpreted as evidence in favor of having an origin in the Oort Cloud and of being in the process of crossing the inner Solar System for the first time (Jewitt et al., 2017). This conjecture was further explored by Hui et al. (2018), arriving to the conclusion that the comet was most probably from the Oort cloud (with a probability of having an interstellar provenance 1.7%), but leaving open the answer to the question of the comet being dynamically new or old. Meech et al. (2017) assumed that C/2017 K2 is a dynamically new Oort cloud comet, but Królikowska & Dybczyński (2018) have carried out extensive calculations to conclude that it is dynamically old.
Here, we present numerical evidence suggesting that C/2017 K2 is dynamically old. Our results are based on the latest orbital solution avaliable from JPL’s Small-Body Database, which is different from the ones used by Hui et al. (2018) and Królikowska & Dybczyński (2018). Our full -body calculations have been carried out as described by de la Fuente Marcos & de la Fuente Marcos (2012) —with input state vectors computed as discussed by de la Fuente Marcos & de la Fuente Marcos (2015)— this approach and tools are also different from the ones used by Hui et al. (2018) and Królikowska & Dybczyński (2018). The current orbit determination of C/2017 K2 (epoch JDTDB 2457961.5, 27-July-2017) computed by D. Farnocchia (4-April-2018) is based on 491 observations for a data-arc span of 1770 d and has perihelion distance, =1.810660.00009 au, eccentricity, =1.000330.00003, inclination, =8755332000008, longitude of the ascending node, =88185500007, and argument of perihelion, =2360220003; with an absolute magnitude of 6.30.8, C/2017 K2 may be under 18 km wide (Jewitt et al., 2017).
The orbit determination of C/2017 K2 is hyperbolic at the 11 level, but it is somewhat similar in terms of and to those of the long-period comets C/1969 O1-A (Kohoutek), =1.72 au, =0.9991, =863, C/1998 M5 (LINEAR), =1.74 au, =0.9960, =822, and C/2003 T2 (LINEAR), =1.79 au, =0.9997, =875. Although the present-day orbital solution of C/2017 K2 is slightly hyperbolic, it may have followed an elliptical path in the past, and -body simulations can confirm or reject this hypothesis. Figure 1, top panel, shows the evolution of the barycentric distance of the nominal orbit of the four comets; the output cadence is 100 yr. Consistent with the interpretation made by Królikowska & Dybczyński (2018), C/2017 K2 is not a dynamically new Oort cloud comet and its future evolution is rather chaotic; its past evolution matches that of C/2003 T2. The analysis of 200 control orbits of C/2017 K2 compatible with the observations and integrated backwards in time for 3 Myr shows (see Figure 1, bottom panel) that most of them, 67%, are consistent with a bound and dynamically old comet, but about 29% of the studied orbits are compatible with an interstellar origin.
Our independent analysis confirms the results in Królikowska & Dybczyński (2018), but the probability of this object having an interstellar origin seems to be significantly higher than that obtained by Hui et al. (2018), although their integrations only reach 1 Myr into the past. Comet C/2017 K2 joins the growing sample of dynamically old Oort cloud comets that, in the case of currently hyperbolic ones, corresponds to the subsample of bound objects discussed by de la Fuente Marcos et al. (2018).
We thank S. J. Aarseth for providing the code used in this research and A. I. Gómez de Castro for providing access to computing facilities. This work was partially supported by the Spanish MINECO under grant ESP2015-68908-R. In preparation of this Note, we made use of the NASA Astrophysics Data System and the MPC data server.
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