title: The abundance of28Si32S,29Si32S,28Si34S, and30Si32S in the inner layers of the envelope of IRC+10216.
abstract: We present high spectral resolution mid-IR observations of SiS towards the C-rich AGB star IRC+10216 carried out with the Texas Echelon-cross-Echelle Spectrograph mounted on the NASA Infrared Telescope Facility. We have identified 204 ro-vibrational lines of28Si32S, 26 of29Si32S, 20 of28Si34S, and 15 of30Si32S in the frequency range 720 - 790 cm-1. These lines belong to bandsv= 1 - 0, 2 - 1, 3 - 2, 4-3, and 5-4, and involve rotational levels withJlow≲ 90. About 30 per cent of these lines are unblended or weakly blended and can be partially or entirely fitted with a code developed to model the mid-IR emission of a spherically symmetric circumstellar envelope composed of expanding gas and dust. The observed lines trace the envelope at distances to the star ≲ 35R⋆(≃ 0″.7). The fits are compatible with an expansion velocity of 1+2.5(r/R⋆-1) km s-1between 1 and 5R⋆, 11 km s-1between 5 and 20R⋆, and 14.5 km s-1outwards. The derived abundance profile of28Si32S with respect to H2is 4.9 × 10-6between the stellar photosphere and 5R⋆, decreasing linearly down to 1.6 × 10-6at 20R⋆and to 1.3 × 10-6at 50R⋆.28Si32S seems to be rotationally under LTE in the region of the envelope probed with our observations and vibrationally out of LTE in most of it. There is a red-shifted emission excess in the28Si32S lines of bandv= 1 - 0 that cannot be found in the lines of bandsv= 2 - 1, 3 - 2, 4 - 3, and 5 - 4. This excess could be explained by an enhancement of the vibrational temperature around 20R⋆behind the star. The derived isotopic ratios28Si/29Si, and32S/34S are 17 and 14, compatible with previous estimates.
authors.affiliation: Departamento de Estrellas y Medio Interestelar, Instituto de Astronomía, UNAM, Ciudad Universitaria, 04510, Mexico City (Mexico).
authors.forename: J P
authors.initials: JP
authors.lastname: Fonfría
authors.affiliation: Grupo de Astrofísica Molecular, Instituto de Ciencia de Materiales de Madrid, CSIC, C/ Sor Juana Inés de la Cruz 3, 28049, Cantoblanco, Madrid (Spain).
authors.forename: J
authors.initials: J
authors.lastname: Cernicharo
authors.affiliation: Physics Dept. - UC Davis, One Shields Ave., Davis, CA 95616 (USA).
authors.forename: M J
authors.initials: MJ
authors.lastname: Richter
authors.affiliation: Instituto Argentino de Radioastronomía, CCT-La Plata (CONICET), C.C.5, 1894, Villa Elisa (Argentina).
authors.forename: M
authors.initials: M
authors.lastname: Fernández-López
authors.affiliation: Grupo de Astrofísica Molecular, Instituto de Ciencia de Materiales de Madrid, CSIC, C/ Sor Juana Inés de la Cruz 3, 28049, Cantoblanco, Madrid (Spain).
authors.forename: L Velilla
authors.initials: LV
authors.lastname: Prieto
authors.affiliation: Astronomy Dept., University of Texas, Austin, TX 78712 (USA).
authors.forename: J H
authors.initials: JH
authors.lastname: Lacy
keywords: circumstellar matter; infrared: stars; line: identification; line: profiles; post-AGB; stars: AGB; stars: individual: IRC+10216