¹²CO⁺ and ¹³CO⁺ fluorescence models for measuring the ¹²C/¹³C isotopic ratio in comets

¹ Institut UTINAM – UMR 6213, CNRS / Univ. de Franche-Comté, OSU THETA, 41 bis Av. de l’Observatoire, BP 1615, 25010 Besançon Cedex, France
e-mail: philippe.rousselot@obs-besancon.fr
² STAR Institute, Univ. of Liège, Allée du 6 Août 19c, 4000 Liège, Belgium
³ Institute for Astronomy, Univ. of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
⁴ Laboratoire Interdisciplinaire Carnot de Bourgogne – UMR 6303, CNRS / Univ. de Bourgogne, 9 Av. A. Savary, BP 47870, 21078 Dijon Cedex, France

Received: 20 September 2023
Accepted: 1 December 2023

Abstract

Context. CO is an abundant species in comets, creating CO⁺ ion with emission lines that can be observed in the optical spectral range. A good modeling of its fluorescence spectrum is important for a better measurement of the CO⁺ abundance. Such a species, if abundant enough, can also be used to measure the ¹²C/¹³C isotopic ratio.

Aims. This study uses the opportunity of a high CO content observed in the comet C/2016 R2 (PanSTARRS), which created bright CO⁺ emission lines in the optical range, to build and test a new fluorescence model of this species and to measure the ¹²C/¹³C isotopic ratio in this chemical species for the first time with ground-based observations.

Methods. Thanks to laboratory data and theoretical works available in the scientific literature, we developed a new fluorescence model both for ¹²CO⁺ and ¹³CO⁺ ions. The ¹³CO⁺ model can be used for coadding faint emission lines and to obtain a sufficient signal-to-noise ratio to detect this isotopologue.

Results. Our fluorescence model provides a good modeling of the ¹²CO⁺ emission lines, allowing us to publish revised fluorescence efficiencies. Based on similar transition probabilities for ¹²CO⁺ and ¹³CO⁺, we derive a ¹²C/¹³C isotopic ratio of 73±20 for CO⁺ in comet C/2016 R2. This value is in agreement with the Solar System ratio of 89±2 within the error bars, but is also consistent with the ¹²C/¹³C ratio in local interstellar medium (68±15).