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

¹ Institut UTINAM, UMR 6213 CNRS-Univ. Bourgogne Franche-Comté, OSU THETA, BP 1615 25010 Besançon Cedex, France
e-mail: philippe.rousselot@obs-besancon.fr
² GSMA, UMR CNRS 7331, UFR Sciences Exactes et Naturelles, Univ. of Reims Champagne-Ardenne, 51100 Reims, France
³ Institute for Astronomy, Univ. of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ UK
⁴ STAR Institute, Univ. of Liège, Allée du 6 Août 19c, 4000 Liège, Belgium

Received: 3 December 2021
Accepted: 14 February 2022

Abstract

Context. N₂ is rarely found in comets, or when it is present, it is found only in small quantities despite its abundance on the surfaces of different outer Solar System objects. A few comets presented N₂⁺ emission lines in their optical spectra. One of them, C/2016 R2, showed an unusually high abundance of this species in 2018, with dozens of bright N₂⁺ emission lines. A robust model of the N₂⁺ fluorescence in comets would permit us to perform a detailed quantitative analysis of this species and enlarge our comprehension of the surprisingly wide range of N₂ abundances in comets.

Aims. The goal of this work is to provide the necessary tools to interpret N₂⁺ spectra. Computing production rates for a cometary species necessitates a good knowledge of the number of molecules located along the line of sight of the spectrometer. This in turn requires a good modeling of the emission spectrum with detailed fluorescence efficiencies for the different bands.

Methods. We developed a model based on available laboratory data and new theoretical results relative to the N₂⁺ to compute the emission spectrum of this species observed in 8.2m Very Large Telescope high-resolution spectra of comet C/2016 R2. Because of some significant differences between spectra obtained on the nucleus and at a cometocentric distance of about 6000 km, it became apparent that a classic fluorescence equilibrium spectrum could not be used. A synthetic spectrum based on a Monte Carlo method and producing spectra at different times from an initial relative population was developed and compared to our observational data.

Results. Our modeling of the cometary N₂⁺ emission spectrum satisfactorily fits our observed spectra of comet C/2016 R2, leading to the first modeling at high resolution. Different fluorescence efficiency factors are computed.