A physico-chemical model to study the ion density distribution in the inner coma of comet C/2016 R2 (Pan-STARRS)

Abstract

The recent observations show that comet C/2016 R2 (Pan-Starrs) has a unique and peculiar composition when compared with several other comets observed at 2.8 au heliocentric distance. Assuming solar resonance fluorescence is the only excitation source, the observed ionic emission intensity ratios are used to constrain the corresponding neutral abundances in this comet. We developed a physico-chemical model to study the ion density distribution in the inner coma of this comet by accounting for photon and electron impact ionization of neutrals, charge exchange and proton transfer reactions between ions and neutrals, and electron–ion thermal recombination reactions. Our calculations show that CO2+ and CO+ are the major ions in the inner coma, and close to the surface of nucleus CH3OH+, CH3OH2+, and O2+ are also important ions. By considering various excitation sources, we also studied the emission mechanisms of different excited states of CO+, CO2+, N2+, and H2O+. We found that the photon and electron impact ionization and excitation of corresponding neutrals significantly contribute to the observed ionic emissions for radial distances smaller than 300 km and at larger distances, solar resonance fluorescence is the major excitation source. Our modelled ion emission intensity ratios are consistent with the ground-based observations. Based on the modelled emission processes, we suggest that the observed ion emission intensity ratios can be used to derive the neutral composition in the cometary coma only when the ion densities are significantly controlled by photon and photoelectron impact ionization of neutrals rather than by the ion–neutral chemistry.

N2+fluorescence spectrum of comet C/2016 R2 (PanSTARRS)

“…The revised values for the fluorescence efficiencies computed in this work imply some revisions for the relative abundances of N + 2 compared to CO + , for which they have been published and based on the calculations of Lutz et al (1993). 2019) reported a N + 2 /CO + ratio of 0.06 ± 0.01, which agrees well with Cochran & McKay (2018b,a) and Raghuram et al (2021). Because this ratio is proportional to the ratio of the CO + fluorescence efficiency to the efficiency of N + 2 , the final N + 2 /CO + ratio is expected to change from 6% to 8.8% if g N + 2 changes from 7 × 10 −2 to 4.77 × 10 −2 photon s −1 ion −1 (i.e., the value computed for the time of observation scaled to 1 au).…”

“…The N + 2 ions located along the line of sight correspond to different elapsed times since their creation, mainly by photoionization (see Raghuram et al 2021 for more details about an estimate of the different processes leading to the formation of N + 2 ions in the inner coma of comet C/2016 R2). Modeling the resulting spectrum that is created by ions with different emission spectra is therefore difficult.…”

Iron and nickel atoms in cometary atmospheres even far from the Sun

“…For the CO2 + A2Π −X2Π (0,0) band near 3500 Å we assumed a simple Haser profile with one component, CO2 being the parent. The time scale and fluorescence efficiency are taken from 38,39 . The profile distribution was integrated over the area of the slit of the spectrograph and compared to the observed flux of the bands to yield the production rate.…”

Cometary science with CUBES

“…Whether the ions can be used to derive the abundance of neutral species also depends on if the ions are mainly produced through photoionization of the corresponding neutrals or if they are produced through other processes like ion-neutral chemistry. For example, for comet C/2016 R2 (PANSTARRS), it has be shown that the CO + 2 was also produced through charge exchange between CO + and CO 2 , indicating that for comets with very high CO abundances, CO + 2 might not be a reliable indicator of the CO 2 abundance [37]. Nevertheless, in spite of these caveats, ions emission bands observed at optical wavelengths have proven very useful in the past to estimate the relative abundance of difficult to observe neutrals in the coma of comets, and in particular the N 2 /CO ratio [31,32,33].…”