Abundance analysis of stars hosting gas-rich debris discs

¹ Tartu Observatory, University of Tartu, Observatooriumi 1, Tõravere 61602, Estonia
² Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
³ Institute for Theoretical and Computation Physics, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria
⁴ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁵ LIRA, Observatoire de Paris, Université PSL, CNRS, Université Paris Cité, Sorbonne Université, 5 place Jules Janssen, 92195 Meudon, France
⁶ Earth and Planets Laboratory, Carnegie Institution for Science, 5241 Broad Branch Road, NW, Washington, DC 20015, USA

^★ Corresponding author; sandipan.borthakur@ut.ee

Received: 31 October 2024
Accepted: 20 February 2025

Abstract

Context. Accretion from protoplanetary or debris discs can contaminate the stellar photosphere, especially in stars that have radiative envelopes. Due to the relatively slower photospheric mixing, these stars can exhibit clear contamination signatures. The contaminated photosphere reflects ongoing disc processes, which are detectable through stellar spectroscopy.

Aims. We investigate the composition of six gas-rich debris disc-hosting A-type stars to understand possible links with their debris disc or earlier accretion stages.

Methods. We used archival spectra and the ZEEMAN spectral synthesis code to estimate the stellar parameters and abundances of six debris disc-hosting A-type stars. We also estimated the stellar photospheric accretion contamination parameter, f_ph, using CAMSTARS, which indicates the fraction of accreting material on the stellar photosphere.

Results. The oxygen abundance in intermediate-mass stars decreases with age until the debris disc stage (<20 Myr), after which it could end up rising. The downward trend could result from H₂O ice accumulating in dust traps or the formation of hydrated asteroids in the protoplanetary disc, locking oxygen in solids and reducing its accretion onto the star. All the stars share similar volatile abundances (C, O), but HD 110058 and HD 32297 exhibit refractory depleted abundances. The near-zero f_ph values in the six stars suggest that any currently accreted gas would not overwhelm mixing in the photosphere and would not impact the observed composition. The refractory depleted abundances in HD 110058 and HD 32297 suggest residual or even chronic, accretion contamination from their earlier protoplanetary stages when the accretion rates were about five orders of magnitude higher. For HD 110058, with the highest refractory depletion, we estimated a lower limit on its earlier protoplanetary accretion rate of 9 × 10⁻⁸ M_⊙/yr, similar to other Herbig stars and equal to the Herbig star HD 100546. This supports our hypothesis that refractory depletion in HD 110058 originates from a prior phase of higher accretion of dust-poor material. We further develop this hypothesis by comparing HD 110058 with the HD 100546 protoplanetary disc system, which is of a similar age.