The abundance of S in IRC+10216 and its production in the Galaxy^*

¹ Instituto de Radioastronomía Milimétrica, Avda. Divina Pastora 7, Local 20, 18012 Granada, Spain e-mail: mauers@iram.es
² Max-Planck-Institut für Chemie, Becherweg 27, 55128 Mainz, Germany
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ Instituto de la Estructura de la Materia, Dept. de Astronomía Molecular e Infrarroja, Serrano 113, 28006 Madrid, Spain
⁵ Dipartimento di Fisica Generale dell'Università di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
⁶ Centre for Stellar and Planetary Sciences, Monash University, Melbourne 3800, Australia

Received: 16 March 2004
Accepted: 21 June 2004

Abstract

The  and  rotational lines of the rare isotopomer C³⁶S and the  and  transitions of Si³⁶S were detected in the carbon star IRC+10216 (CW Leo). These are the first detections of ³⁶S-bearing molecules in a star and the first spectroscopic detection of Si³⁶S. From a comparison of  ³⁴S- and ³⁶S-bearing isotopomers, the  ³⁴S/³⁶S isotopic ratio is 107(). This value is comparable to values in the interstellar medium of the inner Galactic disk (115) but is smaller than the solar value of 288 (Ding et al. [CITE]). The increase of the  ³⁶S abundance relative to ³⁴S only qualitatively follows model predictions of a low mass AGB star. Quantitative agreement of the observed ³⁴S/³⁶S ratio with the stellar models can be reached if the age of IRC+10216 and Galactic chemical evolution are taken into account. Other less likely possibilities are the presence of considerable inhomogeneities in the interstellar medium and either IRC+10216 or the Sun started with a peculiar ³⁶S abundance. Other production mechanisms potentially capable of enhancing the Galactic interstellar medium are discussed. From the observed line density toward IRC+10216 and toward Galactic star forming regions, we estimate the confusion limit toward those sources.