Letter to the Editor

The similarity of the interstellar comet 2I/Borisov to Solar System comets from high-resolution optical spectroscopy

¹ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
e-mail: copi@roe.ac.uk
² Space sciences, Technologies & Astrophysics Research (STAR) Institute, University of Liège, Liège, Belgium
³ Koyama Astronomical Observatory, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto 603-8555, Japan
⁴ Institut UTINAM UMR 6213, CNRS, Univ. Bourgogne Franche-Comté, OSU THETA, BP 1615, 25010 Besançon Cedex, France
⁵ Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, CO, USA
⁶ Department of Astrophysics and Atmospheric Sciences, Faculty of Science, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto 603-8555, Japan
⁷ Astrophysics Research Centre, School of Mathematics and Physics, Queens University Belfast, Belfast BT7 1NN, UK
⁸ Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁹ ESA NEO Coordination Centre, Largo Galileo Galilei, 1, 00044 Frascati (RM), Italy
¹⁰ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
¹¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany

Received: 4 May 2021
Accepted: 6 June 2021

Abstract

Aims. 2I/Borisov (hereafter 2I) is the first visibly active interstellar comet observed in the Solar System, allowing us for the first time to sample the composition of a building block from another system. We report on the monitoring of 2I with the Ultraviolet-Visual Echelle Spectrograph, the high-resolution optical spectrograph of the ESO Very Large Telescope at Paranal, for four months from November 15, 2019 to March 16, 2020. Our goal is to characterise the activity and composition of 2I with respect to Solar System comets.

Methods. We collected high-resolution spectra at 12 different epochs from 2.1 au pre-perihelion to 2.6 au post-perihelion.

Results. On December 24 and 26, 2019, close to perihelion, we detected several OH lines of the 309 nm (0–0) band and derived a water production rate of 2.2 ± 0.2 × 10²⁶ molecules s⁻¹. The three [OI] forbidden oxygen lines were detected at different epochs and we derived a green-to-red doublet intensity ratio (G/R) of 0.31 ± 0.05 close to perihelion. The NH₂ ortho and para lines from various bands were measured and allowed us to derive an ortho-to-para abundance ratio (OPR) of 3.21 ± 0.15, corresponding to an OPR and spin temperature of ammonia of 1.11 ± 0.08 and 31₋₅⁺¹⁰ K, respectively. These values are consistent with the values usually measured for Solar System comets. Emission lines of the radicals NH (336 nm), CN (388 nm), CH (431 nm), and C₂ (517 nm) were also detected. Several FeI and NiI lines were identified and their intensities were measured to provide a ratio of log (NiI/FeI) = 0.21 ± 0.18, which is in agreement with the value recently found in Solar System comets.

Conclusions. Our high spectral resolution observations of 2I/Borisov and the associated measurements of the NH₂ OPR and the Ni/Fe abundance ratio are remarkably similar to Solar System comets. Only the G/R ratio is unusually high, but it is consistent with the high abundance ratio of CO/H₂O found by other investigators.