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arXiv · 1903.10500

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' = 0.17 +/- 0.03, and i'-z' = 0.06 +/- 0.04. 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.

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Nora L. Eisner, Matthew M. Knight, Colin Snodgrass, Michael S. P. Kelley, Alan Fitzsimmons, Rosita Kokotanekova. 2019-04-10. Properties of the Bare Nucleus of Comet 96P/Machholz 1. https://doi.org/10.3847/1538-3881%2Fab0f42

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