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Astron. Astrophys. 339, 811-821 (1998)

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1. Introduction

S stars are a relatively rare class of red giants with optical spectra showing ZrO bands, a principal criterion defining the S spectral class. Analyses of their spectra reveal a photospheric C/O abundance ratio which is close to 1 (within 5%), and evidence of strong enrichment in s-process elements. In the past few years it has become clear that some S stars show technetium, while others do not. Jorrissen & Mayor (1992) and Van Eck et al. (1998) have shown that S stars without technetium are probably low-mass stars on the red giant or early asymptotic giant branch (AGB), whose surface abundances were altered by mass transfer from a binary companion. These stars are referred to as "extrinsic" S stars. In contrast, those S stars with technetium have luminosities placing them on the thermally pulsing (TP) AGB, with surface abundances altered by convective dredge-up (Van Eck et al. 1998). These objects are referred to as "intrinsic" S stars. The identification of technetium, which has no stable isotope, is a clear indicator of recent mixing of s-process elements to the surface. It has been suggested that S stars may be transition objects on the AGB, between the M-giants with C/O [FORMULA] 1, and the carbon stars (C/O [FORMULA] 1). Their relative rarity may imply a short timescale during which the photospheric C/O ratio passes from O-rich to C-rich, but this hypothesis is controversial (Zuckerman & Maddalena 1989; de Jong 1989; Chan & Kwok 1991).

A molecular line survey of 27 S stars by Bieging & Latter (1994, hereafter BL) detected CO J=1-0 or 2-1 emission from 13 stars, for which mass loss rates were derived. For the stars showing CO emission, BL also detected emission from HCN in 4 objects and from SiO in 8. The relatively high detection rates, and chemical model calculations for the photospheric abundances of HCN, SiO, and other chemical species, suggested that the chemical composition of the circumstellar envelope (CSE) is very sensitive to the C/O ratio at or near the stellar photosphere. In the simplest interpretation, the presence of SiO should be indicative of an O-rich composition, while HCN should point to a C-rich photosphere. Molecular line surveys of M- and C-stars by Bujarrabal et al. (1994), and Olofsson et al. (1993, 1998), as well as the S star study by BL, showed that both HCN and SiO are detectable in all categories of AGB stars. The observed ratio of emission line intensities for HCN (J=1-0) and SiO (J=2-1) are well-correlated with the C/O ratio of the stellar photosphere, as shown by Olofsson et al. (1998). In this respect, the HCN/SiO intensity ratios for S stars detected by BL are, in the mean, precisely between the mean ratios for M-type and C-type AGB stars, which is consistent with their values of C/O near unity, i.e., intermediate between M and C stars.

The molecular line survey of BL suggested that S stars have, on average, a dust/gas ratio in their CSEs which is a factor of 2 lower than for carbon stars. This result depends on the dust model used to derive dust masses from IR fluxes, though a separate study by Sahai & Liechti (1995) reached a similar conclusion, i.e., that the dust/gas ratio in S stars is lower than in other AGB stars. In this connection, it is significant that the S stars which have been observed with mid-IR interferometry show dust shells with inner radii which are significantly larger than is typical of M-type Miras or carbon stars (Danchi & Bester 1995). The S stars [FORMULA] Cyg and W Aql have strong emission from both SiO and HCN (see BL), which is inconsistent with standard chemical models for formation under equilibrium conditions. Sharp (1988-see also Sharp & Wasserburg 1995), in theoretical chemical models which include grain condensation, finds that HCN formation can be strongly enhanced in even a slightly O-rich stellar atmosphere, if the oxygen in the gas phase is sufficiently depleted by condensation of silicate grains. Alternatively, HCN may be a product of photochemical reactions in the outer envelope. In O-rich stars showing detectable HCN emission, previous studies have argued that HCN is produced by a circumstellar photochemistry (e.g., Nercessian et al. 1989, Willacy & Millar 1997), but this interpretation is not without difficulties (Olofsson et al. 1998).

Since S stars as a class seem to be characterized by a C/O ratio near unity, they are of particular interest in examining the interplay of elemental abundances, dust formation, and envelope chemical composition in mass-losing AGB stars. For that reason, we were motivated to extend the molecular line survey of BL to a sample of S stars in the southern hemisphere. In this paper, we report the results of observations made with the Swedish-ESO Submillimetre Telescope (SEST) 1 of a sample of southern S stars for emission from both HCN and SiO. From the observed line intensities, we attempt to constrain the molecular abundances using statistical equilibrium models for a reasonable combination of model parameters. Finally, we compare our results with those obtained by others for mass-losing M- and C-stars on the AGB.

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© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998