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.

arXiv comment12 pages, 8 figures (+26 pages appendix) Accepted for publication in Astronomy & Astrophysics

References in corpus (41)

• The chemical composition of the Sun
• Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles - II. CO line survey of evolved stars: derivation of mass-loss rate formulae
• Dust formation in the oxygen-rich AGB star IK Tau
• `Thermal' SiO radio line emission towards M-type AGB stars: a probe of circumstellar dust formation and dynamics
• Oxygen Chemistry in the Circumstellar Envelope of the Carbon-Rich Star IRC+10216
• Molecular abundances in the inner layers of IRC +10216
• A Detailed Analysis of the Dust Formation Zone of IRC+10216 Derived from Mid-IR Bands of C2H2 and HCN
• The first INTEGRAL-OMC catalogue of optically variable sources
• The inner wind of IRC+10216 revisited: New exotic chemistry and diagnostic for dust condensation in carbon stars
• 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 abundance of HCN in circumstellar envelopes of AGB stars of different chemical types
• Circumstellar molecular line emission from S-type AGB stars: Mass-loss rates and SiO abundances
• PO and PN in the wind of the oxygen-rich AGB star IK Tau
• 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
• Sulphur molecules in the circumstellar envelopes of M-type AGB stars
• A HIFI view on circumstellar H2O in M-type AGB stars: radiative transfer, velocity profiles, and H2O line cooling
• Carbon and oxygen isotopic ratios for nearby miras
• The Abundance of SiC2 in Carbon Star Envelopes: Evidence that SiC2 is a gas-phase precursor of SiC dust
• Detailed modelling of the circumstellar molecular line emission of the S-type AGB star W Aquilae
• Circumstellar shells and mass loss rates: Clues to the evolution of S stars
• Nucleosynthesis in AGB stars traced by oxygen isotopic ratios. I - Determining the stellar initial mass by means of the ${}^{17}$O/${}^{18}$O ratio
• Determining the effects of clumping and porosity on the chemistry in a non-uniform AGB outflow
• The Spitzer Spectroscopic Survey of S-type Stars
• Sulphur-bearing molecules in AGB stars I: The occurrence of hydrogen sulfide
• Study of CS, SiO, and SiS abundances in carbon star envelopes: Assessing their role as gas-phase precursors of dust
• When an old star smolders: On the detection of hydrocarbon emission from S-type AGB stars
• 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${}^{+}$
• Circumstellar environment of the M-type AGB star R Dor. APEX spectral scan at $159.0-368.5$ GHz
• Detection of `parent' molecules from the inner wind of AGB stars as tracers of non-equilibrium chemistry
• Sulphur-bearing molecules in AGB stars II: Abundances and distributions of CS and SiS
• Herschel/HIFI observation of highly excited rotational lines of HNC toward IRC +10 216
• Circumstellar ammonia in oxygen-rich evolved stars
• An ISO/SWS study of the dust composition around S stars
• Molecular line study of the S-type AGB star W Aquilae. ALMA observations of CS, SiS, SiO and HCN
• Herschel/HIFI observations of the circumstellar ammonia lines in IRC+10216
• Classifying the secondary component of the binary star W Aquilae
• Simultaneous 183 GHz H2O Maser and SiO Observations Towards Evolved Stars Using APEX SEPIA Band 5

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