Sulphur-bearing molecules in AGB stars

Abstract

Context. Sulphur is a relatively abundant element in the local galaxy which 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 H 2 S in AGB circumstellar envelopes and determine its abundance, where possible. Methods. We have surveyed 20 AGB stars with a range of mass-loss rates and of different chemical types using the APEX telescope to search for rotational transition lines of five key sulphur-bearing molecules: CS, SiS, SO, SO 2 and H 2 S. Here we present our results for H 2 S, including detections, non-detections and detailed radiative transfer modelling of the detected lines. We compare results based on different descriptions of the molecular excitation of H 2 S and different abundance distributions, including Gaussian abundances, where possible, and two different abundance distributions derived from chemical modelling results. Results. We detected H 2 S towards five AGB stars, all of which have high mass-loss rates ofṀ ≥ 5 × 10 −6 M yr −1 and are oxygenrich. H 2 S 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-H 2 S abundances relative to H 2 between 4 × 10 −7 and 2.5 × 10 −5 . Conclusions. Overall, we conclude that H 2 S 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 the lower mass-loss rate oxygen-rich stars. For two sources, V1300 Aql and GX Mon, H 2 S is most likely the dominant sulphur-bearing molecule in the circumstellar envelope.

Efficient Production of S8 in Interstellar Ices: The Effects of Cosmic-Ray-driven Radiation Chemistry and Nondiffusive Bulk Reactions

“…Hydrogen sulfide, H 2 S, is a species often predicted to be a major reservoir of sulfur in dense regions (Esplugues et al 2014;Holdship et al 2016;Danilovich et al 2017;Vidal et al 2017), since it forms readily from sulfur atoms adsorbed onto grain surfaces via…”

An ALMA view of CS and SiS around oxygen-rich AGB stars

“…Sulphur is also the tenth most abundant element in the universe, meaning that when a significant portion is locked up in particular molecules, they are relatively easy to detect in observations. The most common sulphur-bearing molecules found in AGB CSEs are CS, SiS, SO, SO 2 and H 2 S. These have been most commonly studied using spatially unresolved observations of rotational transition lines, for example by Danilovich et al (2017b) for H 2 S.…”

Molecular Data Needs for Modelling AGB Stellar Winds and Other Molecular Environments

“…The matter is further complicated, as noted in [50], when these H 2 O collisional rates are used to approximate the collisional rates of other molecules (via Equation (6)) such as H 2 S. As well as some discrepancies in the energy ordering of the levels relative to the corresponding quantum numbering, there is the question of how accurate the scaled levels might be when the dipole moments of H 2 O and H 2 S differ by a factor of ∼2 with 1.8546 D for H 2 O [51] and 0.974 D for H 2 S [52]. The two different sets of collisional rates employed by [50] produced significantly different results for their H 2 S modelling, with very different resultant line intensities for the same circumstellar model depending on the choice of collisional rates.…”

“…The two different sets of collisional rates employed by [50] produced significantly different results for their H 2 S modelling, with very different resultant line intensities for the same circumstellar model depending on the choice of collisional rates. They found the difference in results between sets of collisional rates was more significant than the difference from the inclusion or not of vibrationally excited energy levels and radiative rates.…”