The distribution of silicon sulfide in the envelope of IRC + 10216

The J = 5-4 and J = 6-5 transitions of SiS toward the carbon star IRC + 10216 have been mapped using the Berkely-Illinois-Maryland Array. The distribution of the emission is strongly peaked at the position of the star and shows no sign of shell structure. The radial SiS abundance distribution which best fits both the aperture synthesis maps and existing single-dish data has been derived for several rotational lines, and it is concluded that inside a radius of 3 x 10 to the 15th cm, the SiS abundance is 7.5 x 10 to the -6th with respect to H2, but falls to 6.5 x 10 to the -7th outside that radius. Beyond a radius of 4 x 10 to the 16th cm, the SiS molecule is photodissociated by ambient interstellar UV, with an outer cutoff of about 8 x 10 to the 16th cm. More than 98 percent of the SiS may be removed from the gas phase within about 60 yr from the time the gas leaves the star. The depletion time scale appears to be consistent with recent model calculations which include the effects of molecular adsorption onto dust grains.