Journal
ASTROPHYSICAL JOURNAL
Volume 929, Issue 1, Pages -Publisher
IOP Publishing Ltd
DOI: 10.3847/1538-4357/ac5893
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Funding
- National Science Foundation [AST-2009253]
- NASAs Planetary Science Division
- NASA Astrobiology Institute through the Goddard Center for Astrobiology
- Korea Astronomy and Space Science Institute
- Chinese Academy of Sciences [XDB09000000]
- Science and Technology Facilities Council of the United Kingdom - Smithsonian Institution
- Academia Sinica
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In this study, high-resolution CO observations and a new radiative transfer code were used to investigate the outgassing behavior of comet C/2016 R2. The total CO production rate was found to be in the range of (3.8 - 7.6) x 10(28) s(-1), with emission concentrated in a near-sunward jet and evidence of an extended source of CO emission.
The coma of comet C/2016 R2 (PanSTARRS) is one of the most chemically peculiar ever observed, in particular due to its extremely high CO/H2O and N-2(+)/H2O ratios, and unusual trace volatile abundances. However, the complex shape of its CO emission lines, as well as uncertainties in the coma structure and excitation, has lead to ambiguities in the total CO production rate. We performed high-resolution, spatially, spectrally, and temporally resolved CO observations using the James Clerk Maxwell Telescope and Submillimeter Array to elucidate the outgassing behavior of C/2016 R2. Results are analyzed using a new, time-dependent, three-dimensional radiative transfer code (SUBlimating gases in LIME; SUBLIME, based on the open-source version of the LIne Modeling Engine), incorporating for the first time, accurate state-to-state collisional rate coefficients for the CO-CO system. The total CO production rate was found to be in the range of (3.8 - 7.6) x 10(28 )s(-1 )between 2018 January 13 and February 1 (at r(H) = 2.8-2.9 au), with a mean value of (5.3 +/- 0.6) x 10(28) s(-1). The emission is concentrated in a near-sunward jet, with a half-opening angle of similar to 62 degrees and an outflow velocity of 0.51 +/- 0.01 km s(-1), compared to 0.25 +/- 0.01 km s(-1) in the ambient (and nightside) coma. Evidence was also found for an extended source of CO emission, possibly due to icy grain sublimation around 1.2 x 10(5) km from the nucleus. Based on the coma molecular abundances, we propose that the nucleus ices of C/2016 R2 can be divided into a rapidly sublimating apolar phase, rich in CO, CO2, N-2, and CH3OH, and a predominantly frozen (or less abundant), polar phase containing more H2O, CH4, H2CO, and HCN.
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