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.

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 Jlow ≲ 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(-1) between 1 and 5R⋆, 11 km s(-1) between 5 and 20R⋆, and 14.5 km s(-1) outwards. The derived abundance profile of (28)Si(32)S with respect to H2 is 4.9 × 10(-6) between the stellar photosphere and 5R⋆, decreasing linearly down to 1.6 × 10(-6) at 20R⋆ and to 1.3 × 10(-6) at 50R⋆. (28)Si(32)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 (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 20R⋆ behind the star. The derived isotopic ratios (28)Si/(29)Si, and (32)S/(34)S are 17 and 14, compatible with previous estimates.