Properties of the Bare Nucleus of Comet 96P/Machholz 1* * Based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia, Inovações e Comunicações do Brasil (MCTIC/LNA), the U.S. National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU). Also based on service observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group of Telescopes in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. Also based on observations collected at the European Southern Observatory under ESO program 0101.C-0709(A).
The Astronomical Journal American Astronomical Society 157:5 (2019) 186
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The Astronomical Journal American Astronomical Society 157:5 (2019) 170
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Properties of the Bare Nucleus of Comet 96P/Machholz 1
The Astronomical Journal, Volume 157, Number 5
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.