Molecular line study of the S-type AGB star W Aquilae

ALMA observations of CS, SiS, SiO and HCN

¹ Department for Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
e-mail: magdalena.brunner@univie.ac.at
² Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
³ Department of Physics and Astronomy, Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁴ Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden

Received: 29 January 2018
Accepted: 29 May 2018

Abstract

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.