Studies of the S-type AGB star W Aquilae : The circumstellar envelope and the fain companion

Licentiate thesis, 2013

Low- and intermediate-mass stars will all eventually enter the asymptotic giant branch (AGB) phase. AGB stars experience intense mass-loss, generating a significant fraction of the dust and molecular matter that enriches the ISM. AGB stars are also responsible for a the production of about half of all elements heavier than iron. AGB stars can be classified into two broad categories: oxygen-rich M stars and carbon-rich C stars. When a star leaves the main sequence and ascends the AGB, it will initially be oxygen-rich. Over time, for a particular subset of AGB stars, enough carbon will be dredged up from their interiors to enrich their atmospheres and, eventually, turn them into carbon-rich stars. S-type AGB stars are believed to be an intermediate evolutionary stage between M and C stars, with a C/O ratio close to 1. As transition objects they provide a unique window into the mass-loss mechanism(s) and chemistry of AGB stars. W Aql is an S-type AGB star with a binary companion. In this thesis we examine its mass-loss properties through a detailed analysis of the molecular emission in its circumstellar envelope (CSE). With new Herschel/HIFI observations which probe areas of the CSE closer to the star, we are able to better constrain mass-loss and CSE properties than previously possible. We detect molecular emission lines of CO, H2O, SiO, HCN and NH3, the latter for the first time in an S star. We find a mass-loss rate for W Aql of 3.5E-6 Msun/yr and present abundances of each molecular species. We also use optical observations of W Aql to determine the spectral type, and hence constrain the mass and temperature, of the fain companion to the AGB star. Our spectroscopic analysis puts the companion in the range F8V–G0V. Our photometric observations broadly agree with this result and indicate that the companion undergoes extensive extinction, most likely due to dust produced by the AGB star.