Molecular line study of the S-type AGB star W Aquilae. ALMA observations of CS, SiS, SiO and HCN

Context. With the outstanding spatial resolution and sensitivity of the Atacama Large Millimeter/sub-millimeter Array (ALMA), molecular gas other than the abundant CO can be observed and resolved in circumstellar envelopes (CSEs) around evolved stars, such as the binary S-type asymptotic giant branch (AGB) star W Aquilae.
Aims: We aim to constrain the chemical composition of the CSE and determine the radial abundance distribution, the photospheric peak abundance, and isotopic ratios of a selection of chemically important molecular species in the innermost CSE of W Aql. The derived parameters are put into the context of the chemical evolution of AGB stars and are compared with theoretical models.
Methods: We employ one-dimensional radiative transfer modeling - with the accelerated lambda iteration (ALI) radiative transfer code-of the radial abundance distribution of a total of five molecular species (CS, SiS, ³⁰SiS, ²⁹SiO and H¹³CN) and determine the best fitting model parameters based on high-resolution ALMA observations as well as archival single-dish observations. The additional advantage of the spatially resolved ALMA observations is that we can directly constrain the radial profile of the observed line transitions from the observations.
Results: We derive abundances and e-folding radii for CS, SiS, ³⁰SiS, ²⁹SiO and H¹³CN and compare them to previous studies, which are based only on unresolved single-dish spectra. Our results are in line with previous results and are more accurate due to resolution of the emission regions.