Properties of the Bare Nucleus of Comet 96P/Machholz 1

Nora L. Eisner, Matthew M. Knight, Colin Snodgrass, Michael S. P. Kelley, Alan Fitzsimmons, Rosita Kokotanekova

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Abstract

We observed comet 96P/Machholz 1 on a total of 9 nights before and after perihelion during its 2017/2018 apparition. Both its unusually small perihelion distance and the observed fragmentation during multiple apparitions make 96P an object of great interest. Our observations show no evidence of a detectable dust coma, implying that we are observing a bare nucleus at distances ranging from 2.3 AU to 3.8 AU. Based on this assumption we calculated its color, and found average values of g'-r' = 0.50 +/- 0.04, r'-i' = 0.037 +/- 0.34, and i'-z' = 0.10 +/- 0.08. These are notably more blue than those of the nuclei of other Jupiter family and long period comets. Furthermore, assuming a bare nucleus, we found an equivalent nuclear radius of 3.4 +/- 0.2 km with an axial ratio of at least 1.6 +/- 0.1. The lightcurve clearly displays one large peak, one broad flat peak, and two distinct troughs, with a clear asymmetry that suggests that the shape of the nucleus deviates from that of a simple triaxial ellipsoid. This asymmetry in the lightcurve allowed us to constrain the nuclear rotation period to 4.10 +/- 0.03 hours and 4.096 +/- 0.002 hours before and after perihelion, respectively. Within the uncertainties, 96P's rotation period does not appear to have changed throughout the apparition, and we conclude a maximum possible change in rotation period of 130 seconds. The observed properties were compared to those of comet 322P and interstellar object 1I/'Oumuamua in an attempt to study the effects of close perihelion passages on cometary surfaces and their internal structure, and the potential interstellar origin of 96P.
Original languageEnglish
Article number186
Number of pages14
JournalAstronomical Journal
Volume157
Issue number5
DOIs
Publication statusPublished - 22 Apr 2019

Bibliographical note

13 pages text, 3 tables, 8 figures

Keywords

  • astro-ph.EP

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    Eisner, N. L., Knight, M. M., Snodgrass, C., Kelley, M. S. P., Fitzsimmons, A., & Kokotanekova, R. (2019). Properties of the Bare Nucleus of Comet 96P/Machholz 1. Astronomical Journal, 157(5), [186]. https://doi.org/10.3847/1538-3881/ab0f42