Analysis of ISO SWS01 spectra of S stars^*

¹ National Astronomical Observatories/Yunnan Observatory, Chinese Academy of Sciences, PO Box 110 Kunming, Yunnan Province 650011, PR China e-mail: [yangxh;chenps;jcwang;jinhuahe]@ynao.ac.cn
² Department of Physics, Chongqing University, Chongqing 400044, PR China
³ Graduate School of Chinese Academy of Sciences, Beijing 100049, PR China

Received: 20 February 2006
Accepted: 20 October 2006

Abstract

The Infrared Space Observatory (ISO) Short-Wavelength Spectrometer (SWS01) plays an important role in studying properties of S stars. We reduce and analyze the SWS01 spectra of 17 S stars, and identify the candidate carriers of molecular absorption features. The ISO Spectral Analysis Package (ISAP) developed by the LWS and SWS Instrument Teams and Data Centers is used to process and analyze the SWS01 spectra of 17 S stars. The ISO archives of 17 S stars are obtained from the ISO database. Of 17 S stars, two stars are extrinsic S stars, the others are intrinsic S stars. The 15 intrinsic S stars can be divided into three groups (6 stars for Group I, 7 stars for Group II, and 2 stars for Group III) according to their Infrared Astronomical Satellite (IRAS) low-resolution spectra (LRS) and dust mass-loss rate , where increases from Group I to II and III gradually. 17 S stars show the following properties: 1. two extrinsic S stars and 15 intrinsic S stars among different groups have different infrared properties; 2. two extrinsic S stars and 6 intrinsic S stars in Group I have similar ISO SWS01 spectra and their continua can be approximately described by a single blackbody representing the stellar photosphere, while some intrinsic S stars in Group I have 60 μm infrared excess; 3. for intrinsic S stars in Groups I, II, and III, their continua peak shifts toward longer wavelength from Groups I to II to III; 4. S stars in Groups II and III show obvious dust emission features in which the ~10 μm dust features seem to display two different shapes. Moreover, two S stars (IRAS 00192-2020 & IRAS 15492+4837) present the 13 μm feature; 5. molecules H₂O, CO, and CO₂ greatly affect the ISO SWS01 spectra of S stars. It is noted that the absorption features of molecules CS and HCN usually thought to exist only in C stars may appear in the S stars.