The abundance of 28 Si 32 S, 29 Si 32 S, 28 Si 34 S, and 30 Si 32 S in the inner layers of the envelope of IRC+10216

• Published in: Monthly notices of the Royal Astronomical Society 2015-10, Vol.453 (1), p.439-449
• Main Authors: Fonfría, J P, Cernicharo, J, Richter, M J, Fernández-López, M, Prieto, L Velilla, Lacy, J H
• Format: Article
• Language: English
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stv1634

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 of Si S, 26 of Si S, 20 of Si S, and 15 of Si S in the frequency range 720 - 790 cm . These lines belong to bands = 1 - 0, 2 - 1, 3 - 2, 4-3, and 5-4, and involve rotational levels with ≲ 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 ≲ 35 (≃ 0″.7). The fits are compatible with an expansion velocity of 1+2.5( / -1) km s between 1 and 5 , 11 km s between 5 and 20 , and 14.5 km s outwards. The derived abundance profile of Si S with respect to H is 4.9 × 10 between the stellar photosphere and 5 , decreasing linearly down to 1.6 × 10 at 20 and to 1.3 × 10 at 50 . Si S 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 the Si S lines of band = 1 - 0 that cannot be found in the lines of bands = 2 - 1, 3 - 2, 4 - 3, and 5 - 4. This excess could be explained by an enhancement of the vibrational temperature around 20 behind the star. The derived isotopic ratios Si/ Si, and S/ S are 17 and 14, compatible with previous estimates.