Emission from HCN and CH₃OH in comets

Abstract

Aims. The aim of this work is to characterise HCN and CH3OH emission from recent comets. Methods. We used the Onsala 20-m telescope to search for millimetre transitions of HCN towards a sample of 11 recent and mostly bright comets in the period from December 2016 to November 2019. Also, CH3OH was searched for in two comets. The HCN sample includes the interstellar comet 2I/Borisov. For the short-period comet 46P/Wirtanen, we were able to monitor the variation of HCN emission over a time-span of about one month. We performed radiative transfer modelling for the observed molecular emission by also including time-dependent effects due to the outgassing of molecules. Results. HCN was detected in six comets. Two of these are short-period comets and four are long-period. Six methanol transitions were detected in 46P/Wirtanen, enabling us to determine the gas kinetic temperature. From the observations, we determined the molecular production rates using time-dependent radiative transfer modelling. For five comets, we were able to determine that the HCN mixing ratios lie near 0.1% using contemporary water production rates, ${Q_{{{\rm{H}}_2}{\rm{O}}}}$, taken from other studies. This HCN mixing ratio was also found to be typical in our monitoring observations of 46P/Wirtanen but here we notice deviations of up to 0.2% on a daily timescale which could indicate short-time changes in outgassing activity. From our radiative transfer modelling of cometary comae, we find that time-dependent effects on the HCN level populations are of the order of 5–15% when ${Q_{{{\rm{H}}_2}{\rm{O}}}}$ is around 2 × 1028 mol s−1. The effects may be stronger for comets with lower ${Q_{{{\rm{H}}_2}{\rm{O}}}}$. The exact details of the time-dependent effects depend on the amount of neutral and electron collisions, radiative pumping, and molecular parameters such as the spontaneous rate coefficient.

Activity and composition of the hyperactive comet 46P/Wirtanen during its close approach in 2018

“…Our CN/OH ratios are in agreement with the HCN/H 2 O ratios derived directly from IR observations around the same dates, namely, Dec. 16-23, 2018 (see Table 2). In addition, the HCN production rates (1.06±0.03)×10 25 molec/s obtained from millimeter (Biver et al 2021) and (1.60±0.10)×10 25 molec/s from sub-millimeter observations (Bergman et al 2022) show an agreement with our Q(CN) values obtained during the same nights of Dec. 15 and 20. This indicates that the observed CN abundances are consistent with HCN being the dominant parent.…”

“…Moulane-corr (McKay et al 2021;Khan et al 2021;Roth et al 2021a), search for icy grains in the coma (Protopapa et al 2021;Bonev et al 2021), and coma features from optical imaging (Knight et al 2021). Millimeter and submillimeter observations of 46P have reported on the detection of several complex organic molecules and showed that the comet is relatively rich in methanol, but relatively depleted in CO, CS, HNC, HNCO, and HCOOH (Biver et al 2021;Roth et al 2021b;Coulson et al 2020;Bergman et al 2022). Using both TRAPPIST-North (TN) and TRAPPIST-South (TS), we collected photometric observation over almost a year.…”

Comprehensive Study of the Chemical Composition and Spatial Outgassing Behavior of Hyperactive Comet 46P/Wirtanen Using Near-IR Spectroscopy during its Historic 2018 Apparition

“…On 2018 December 21, we obtained a mixing ratio of ∼0.2% using comeasured Q(H 2 O). The HCN production rate of (1.57 ± 0.12) × 10 25 mol s −1 obtained by Bergman et al (2022) on 2018 December 20 was within the range of values that we obtained for 2018 December 21. Biver et al (2021) obtained an average HCN mixing ratio of (0.11 ± 0.01)% from millimeterwave observations of 46P spanning 2018 December ∼12-25.…”

“…Hydrogen cyanide (HCN) is a precursor in the synthesis of amino acids, and thus an important organic for understanding the emergence of life. Bergman et al (2022) used millimeterwave observations and radiative transfer modeling to obtain HCN mixing ratios of (0.10 ± 0.01)% and (0.19 ± 0.02)% in 46P on 2018 December 21, using H 2 O production rates from Combi et al (2020) and Bonev et al (2021), respectively. On 2018 December 21, we obtained a mixing ratio of ∼0.2% using comeasured Q(H 2 O).…”

The effect of water and electron collisions in the rotational excitation of HF in comets

“…It should be noted that similar results were recently observed by Bergman et al (2022) in the case of HCN. By running time-dependent radiative transfer calculations, these authors were able to quantify these effects and they have shown that the photodissociation and expansion processes do actually modify the HCN level populations by less than 15%.…”