Sulphur-bearing molecules in AGB stars

I. The occurrence of hydrogen sulphide

¹ Department of Physics and AstronomyInstitute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
e-mail: taissa.danilovich@kuleuven.be
² Onsala Space Observatory, Department of Earth and Space Sciences, Chalmers University of Technology, 43992 Onsala, Sweden
³ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁴ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, University Road, Belfast BT7 1NN, UK

Received: 19 May 2017
Accepted: 14 July 2017

Abstract

Context. Sulphur is a relatively abundant element in the local Galaxy that is known to form a variety of molecules in the circumstellar envelopes of AGB stars. The abundances of these molecules vary based on the chemical types and mass-loss rates of AGB stars.

Aims. Through a survey of (sub-)millimetre emission lines of various sulphur-bearing molecules, we aim to determine which molecules are the primary carriers of sulphur in different types of AGB stars. In this paper, the first in a series, we investigate the occurrence of H₂S in AGB circumstellar envelopes and determine its abundance, where possible.

Methods. We surveyed 20 AGB stars with a range of mass-loss rates and different chemical types using the Atacama Pathfinder Experiment (APEX) telescope to search for rotational transition lines of five key sulphur-bearing molecules: CS, SiS, SO, SO₂, and H₂S. Here we present our results for H₂S, including detections, non-detections, and detailed radiative transfer modelling of the detected lines. We compared results based on various descriptions of the molecular excitation of H₂S and different abundance distributions, including Gaussian abundances, where possible, and two different abundance distributions derived from chemical modelling results.

Results. We detected H₂S towards five AGB stars, all of which have high mass-loss rates of Ṁ ≥ 5 × 10^-6 M_⊙ yr^-1 and are oxygen rich. H₂S was not detected towards the carbon or S-type stars that fall in a similar mass-loss range. For the stars in our sample with detections, we find peak o-H₂S abundances relative to H₂ between 4 × 10^-7 and 2.5 × 10^-5.

Conclusions. Overall, we conclude that H₂S can play a significant role in oxygen-rich AGB stars with higher mass-loss rates, but is unlikely to play a key role in stars of other chemical types or in lower mass-loss rate oxygen-rich stars. For two sources, V1300 Aql and GX Mon, H₂S is most likely the dominant sulphur-bearing molecule in the circumstellar envelope.