Direct Simulation Monte Carlo Modeling of Ammonia in Comet C/2014 Q2 (Lovejoy)

• Published in: The Astronomical journal 2023-11, Vol.166 (5), p.207
• Main Authors: Kawakita, Hideyo, Dello Russo, Neil, Vervack, Ronald J., DiSanti, Michael A., Bonev, Boncho P., Kobayashi, Hitomi, Boice, Daniel C., Shinnaka, Yoshiharu
• Format: Article
• Language: English
• Subjects: Ammonia; Ammonium; Ammonium chloride; Ammonium salts; Comae; Comet heads; Comet nuclei; Comet volatiles; Cometary coma; Comets; Direct simulation Monte Carlo method; Hydrogen cyanide; Ionization; Photodissociation; Solar ultraviolet radiation; Sublimation; Ultraviolet radiation
• ISSN: 0004-6256; 1538-3881
• DOI: 10.3847/1538-3881/acfee7

Ammonia (NH 3 ), likely the most abundant nitrogen-bearing molecule in cometary ices followed by hydrogen cyanide, is believed to be stored in the nucleus predominantly as a parent ice. However, spatial profiles of NH 3 observed in comet C/2014 Q2 (Lovejoy) in the near-infrared region are consistent with a distributed source contribution (Dello Russo et al. 2022). We developed the direct simulation Monte Carlo model of ammonia in cometary coma and applied it to comet C/2014 Q2 (Lovejoy). Results suggest that NH 3 molecules in the coma of C/2014 Q2 (Lovejoy) can plausibly originate from a combination of parent molecules of NH 3 in the coma and a NH 3 nucleus source. We demonstrate that the parents of NH 3 having photodissociation lifetimes of several hundreds of seconds or longer (at 1 au from the Sun) can explain the observed spatial profile of NH 3 in comet C/2014 Q2 (Lovejoy). Even though ammonia salts are possible candidates for parents of NH 3 , some simple ammonium salts such as NH 4 CN or NH 4 Cl may dissociate thermally within very short lifetimes after sublimation from the nucleus, so the contribution from those ammonium salts may be indistinguishable from the nucleus source. The lack of experimental data on photoprocesses for potential NH 3 parent molecules prevent us from identifying the origin of NH 3 in comets. Experimental and theoretical studies of photodissociation/ionization reactions of potential NH 3 parent molecules by the solar UV radiation field are encouraged for the future identification of NH 3 parents in comets.