Publication Date:
2015-08-15
Description:
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 28 Si 32 S, 26 of 29 Si 32 S, 20 of 28 Si 34 S, and 15 of 30 Si 32 S in the frequency range 720–790 cm –1 . These lines belong to bands v = 1–0, 2–1, 3–2, 4–3, and 5–4, and involve rotational levels with J low 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 R * (~=0.7 arcsec). The fits are compatible with an expansion velocity of 1 + 2.5( r / R * – 1) km s –1 between 1 and 5 R * , 11 km s –1 between 5 and 20 R * , and 14.5 km s –1 outwards. The derived abundance profile of 28 Si 32 S with respect to H 2 is 4.9 x 10 –6 between the stellar photosphere and 5 R * , decreasing linearly down to 1.6 x 10 –6 at 20 R * and to 1.3 x 10 –6 at 50 R * . 28 Si 32 S seems to be rotationally under local thermodynamic equilibrium (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 28 Si 32 S lines of band v = 1–0 that cannot be found in the lines of bands v = 2–1, 3–2, 4–3, and 5–4. This excess could be explained by an enhancement of the vibrational temperature around 20 R * behind the star. The derived isotopic ratios 28 Si/ 29 Si, and 32 S/ 34 S are 17 and 14, compatible with previous estimates.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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