Abundances of refractory ions in Beta Pictoris exocomets

¹ Institut d’Astrophysique de Paris, CNRS, UMR 7095, Sorbonne Université, 98 bis boulevard Arago, 75014 Paris, France
² Department of Physics, University of Warwick, Coventry CV4 7AL, UK
³ LESIA, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen, 92190 Meudon, France

^★ Corresponding author; vrignaud@iap.fr

Received: 20 December 2024
Accepted: 18 March 2025

Abstract

β Pictoris is a young A5V star known for harbouring a large number of cometary-like objects (or exocomets) that frequently transit the star and create variable absorption signatures in its spectrum. The physical and chemical properties of these exocomets can be probed by the recently introduced curve of growth approach, which enables column densities measurements in cometary tails using absorption measurements in numerous spectral lines. Through this approach, we present a new study of archival spectra of β Pic obtained with the Hubble Space Telescope, the HARPS spectrograph, and at the Mont John University Observatory aimed at constraining the abundance of refractory ions in β Pic exocomets. We studied 29 individual objects, all of which were observed in Fe II lines (used as a reference ion) and at least one other species (e.g. Ni II, Ca II, Cr II). We find that the refractory composition of β Pic exocomets is stable overall, especially for singly ionised species, and consistent with solar abundances. This outcome validates the use of the curve of growth approach to study exocometary composition. We also show that some ions, such as Ca II, are significantly depleted compared to solar abundances, which allowed us to constrain the typical ionisation state in β Pic exocomets. We find that most refractory elements (e.g. Mg, Ni, Fe) are split into similar fractions between their first and second ionisation states, with the exception of Ca, which is mostly ionised twice. A strong correlation between the Al III/Fe II ratio and radial velocity is also found, showing that the most redshifted exocomets tend to be more ionised. These results open the way for further modelling of exocomets in order to unveil their composition and the physical processes that affect their tails.