The surprisingly carbon-rich environment of the S-type star W Aql

Abstract

W Aql is an asymptotic giant branch (AGB) star with an atmospheric elemental abundance ratio C/O$\approx$0.98 and reported circumstellar molecular abundances intermediate between those of M-type (C/O$<$1) and C-type (C/O$>$1) AGB stars. This intermediate status is considered typical for S-type stars, although our understanding of the chemical content of their circumstellar envelopes (CSEs) is currently rather limited. We performed observations in the frequency range 159-268 GHz with the APEX telescope and make abundance estimates through comparison to available spectra towards some well-studied AGB stars and based on rotational diagram analysis in the case of SiC2. We conclude that W Aql's CSE appears considerably closer to that of a C-type AGB star than to that of an M-type AGB star. In particular, we detect emission from C2H, SiC2, SiN, and HC3N, molecules previously only detected towards the CSEs of C-type stars. This conclusion, based on the chemistry of the gaseous component of the CSE, is further supported by reports in the literature on the presence of atmospheric molecular bands and spectral features of dust species typical for C-type AGB stars. Although our observations mainly trace species in the outer regions of the CSE, our conclusion matches closely that based on recent chemical equilibrium models for the inner wind of S-type stars: the atmospheric and circumstellar chemistry of S-type stars likely resembles that of C-type AGB stars much more closely than that of M-type AGB stars. Further observational investigation of the gaseous circumstellar chemistry of S-type stars is required to characterise its dependence on the atmospheric C/O. Non-equilibrium chemical models of the CSEs of AGB stars need to address the particular class of S-type stars and the chemical variety that is induced by the range in atmospheric C/O.

12 pages, 8 figures (+26 pages appendix) Accepted for publication in Astronomy & Astrophysics

References in corpus (14)

• Oxygen Chemistry in the Circumstellar Envelope of the Carbon-Rich Star IRC+10216
• A Detailed Analysis of the Dust Formation Zone of IRC+10216 Derived from Mid-IR Bands of C2H2 and HCN
• Density-enhanced gas and dust shells in a new chemical model for IRC+10216
• Chemical equilibrium in AGB atmospheres: Successes, failures, and prospects for small molecules, clusters, and condensates
• The millimeter IRAM-30m line survey toward IK Tau
• SEPIA - a new single pixel receiver at the APEX Telescope
• SiO in C-rich circumstellar envelopes of AGB stars: effects of non-LTE chemistry and grain adsorption
• Discovery of Time Variation of the Intensity of Molecular Lines in IRC+10216 in The Submillimeter and Far Infrared Domains
• Carbon and oxygen isotopic ratios for nearby miras
• Detailed modelling of the circumstellar molecular line emission of the S-type AGB star W Aquilae
• Sulphur-bearing molecules in AGB stars I: The occurrence of hydrogen sulfide
• Molecular ions in the O-rich evolved star OH231.8+4.2: HCO${}^{+}$,H${}^{13}$CO${}^{+}$ and first detection of SO${}^{+}$, N${}_2$H${}^{+}$, and H${}_3$O${}^{+}$
• Detection of `parent' molecules from the inner wind of AGB stars as tracers of non-equilibrium chemistry
• Classifying the secondary component of the binary star W Aquilae

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