Low NH${}_3$/H${}_2$O ratio in comet C/2020 F3 (NEOWISE) at 0.7 au from the Sun

Abstract

A lower-than-solar elemental nitrogen content has been demonstrated for several comets, including 1P/Halley and 67P/C-G with independent in situ measurements of volatile and refractory budgets. The recently discovered semi-refractory ammonium salts in 67P/C-G are thought to be the missing nitrogen reservoir in comets. The thermal desorption of ammonium salts from cometary dust particles leads to their decomposition into ammonia and a corresponding acid. The NH${}_3$/H${}_2$O ratio is expected to increase with decreasing heliocentric distance with evidence for this in near-infrared observations. NH${}_3$ has been claimed to be more extended than expected for a nuclear source. Here, the aim is to constrain the NH${}_3$/H${}_2$O ratio in comet C/2020 F3 (NEOWISE) during its July 2020 passage. OH emission from comet C/2020 F3 (NEOWISE) was monitored for 2 months with NRT and observed from GBT on 24 July and 11 August 2020. Contemporaneously with the 24 July 2020 OH observations, the NH${}_3$ hyperfine lines were targeted with GBT. The concurrent GBT and NRT observations allowed the OH quenching radius to be determined at $\left(5.96\pm 0.10\right)\times 10^4$ km on 24 July 2020, which is important for accurately deriving $Q(\text{OH})$. C/2020 F3 (NEOWISE) was a highly active comet with $Q({\text{H}}_2\text{O})\approx 2\times 10^{30}$ molec s${}^{-1}$ one day before perihelion. The $3\sigma$ upper limit for $Q_{{\text{NH}}_3}/Q_{{\text{H}}_2\text{O}}$ is $<0.29\%$ at $0.7$ au from the Sun. The obtained NH${}_3$/H${}_2$O ratio is a factor of a few lower than measurements for other comets at such heliocentric distances. The abundance of NH${}_3$ may vary strongly with time depending on the amount of water-poor dust in the coma. Lifted dust can be heated, fragmented, and super-heated; whereby, ammonium salts, if present, can rapidly thermally disintegrate and modify the NH${}_3$/H${}_2$O ratio.

Accepted for publication in A&A; 18 pages, 8 figures, 6 tables

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