Origin and Evolution of Short-period Comets

David Nesvorný1, David Vokrouhlický2, Luke Dones1, Harold F. Levison1, Nathan Kaib3, and Alessandro Morbidelli4
Astrophysical Journal 843, 120 Link to Article [https://doi.org/10.3847/1538-4357/aa7cf6]
1Department of Space Studies, Southwest Research Institute, 1050 Walnut St., Suite 300, Boulder, CO 80302, USA
2Institute of Astronomy, Charles University, V Holešovičkách 2, CZ-18000 Prague 8, Czech Republic
3H.L. Dodge Department of Physics and Astronomy, University of Oklahoma, Norman, OK 73019, USA
4Département Cassiopée, University of Nice, CNRS, Observatoire de la Côte d’Azur, Nice, F-06304, France

Comets are icy objects that orbitally evolve from the trans-Neptunian region into the inner solar system, where they are heated by solar radiation and become active due to the sublimation of water ice. Here we perform simulations in which cometary reservoirs are formed in the early solar system and evolved over 4.5 Gyr. The gravitational effects of Planet 9 (P9) are included in some simulations. Different models are considered for comets to be active, including a simple assumption that comets remain active for ${N}_{{\rm{p}}}(q)$ perihelion passages with perihelion distance $q\lt 2.5\,\mathrm{au}$. The orbital distribution and number of active comets produced in our model is compared to observations. The orbital distribution of ecliptic comets (ECs) is well reproduced in models with ${N}_{{\rm{p}}}(2.5)\simeq 500$ and without P9. With P9, the inclination distribution of model ECs is wider than the observed one. We find that the known Halley-type comets (HTCs) have a nearly isotropic inclination distribution. The HTCs appear to be an extension of the population of returning Oort-cloud comets (OCCs) to shorter orbital periods. The inclination distribution of model HTCs becomes broader with increasing ${N}_{{\rm{p}}}$, but the existing data are not good enough to constrain ${N}_{{\rm{p}}}$ from orbital fits. ${N}_{{\rm{p}}}(2.5)\gt 1000$ is required to obtain a steady-state population of large active HTCs that is consistent with observations. To fit the ratio of the returning-to-new OCCs, by contrast, our model implies that ${N}_{{\rm{p}}}(2.5)\lesssim 10$, possibly because the detected long-period comets are smaller and much easier to disrupt than observed HTCs.

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