Upper limits on CN from exocomets transiting β Pictoris

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, BT7 1NN Belfast, UK
³ Institutionen för astronomi, Stockholms universitet, AlbaNova universitetscentrum, 106 91, Stockholm, Sweden
⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
⁵ LESIA, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen, 92190 Meudon, France

^★ Corresponding author: kenworthy@strw.leidenuniv.nl

Received: 7 February 2025
Accepted: 13 April 2025

Abstract

Context. The young (23 Myr) nearby (19.4 pc) star β Pictoris hosts an edge-on debris disk with two gas giant exoplanets in orbit around it. Many transient absorption features have been detected in the rotationally broadened stellar lines, which are thought to be the coma of infalling exocomets crossing the line of sight towards Earth.

Aims. In the Solar System, the molecule cynaogen (CN) and its associated ionic species are one of the most detectable molecules in the coma and tails of comets. We perform a search for cyanogen in the spectra of β Pic to detect or put an upper limit on this molecule’s presence in a young, highly active planetary system.

Methods. We divide twenty year’s worth of High Accuracy Radial Velocity Planet Searcher (HARPS) spectra into those with strong exocomet absorption features, and those with only stellar lines. The high signal-to-noise stellar spectrum normalises out the stellar lines in the exocomet spectra, which are then shifted and stacked on the deepest exocomet absorption features to produce a high signal-to-noise exocomet spectrum, and search for the CN band head using a model temperature dependent cross-correlation template.

Results. We do not detect CN in our data, and place a temperature and broadening dependent 5σ upper limit between 10¹² and 10¹³ cm⁻², to be compared to the typical 10⁹−10¹⁰ cm⁻² expected from scaling of the values in the Solar System comets.