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Context. With the outstanding spatial resolution and sensitivity of theAtacama Large Millimeter/sub-millimeter Array (ALMA), molecular gas other thanthe 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 CSEand determine the radial abundance distribution, the photospheric peakabundance, and isotopic ratios of a selection of chemically important molecularspecies in the innermost CSE of W Aql. The derived parameters are put into thecontext of the chemical evolution of AGB stars and are compared withtheoretical models. Methods. We employ one-dimensional radiative transfermodeling - 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 modelparameters based on high-resolution ALMA observations as well as archivalsingle-dish observations. The additional advantage of the spatially resolvedALMA observations is that we can directly constrain the radial profile of theobserved line transitions from the observations. Results. We derive abundancesand e-folding radii for CS, SiS, 30SiS, 29SiO and H13CN and compare them toprevious studies, which are based only on unresolved single-dish spectra. Ourresults are in line with previous results and are more accurate due toresolution of the emission regions.

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