MILLIMETER OBSERVATIONS OF VIBRATIONALLY EXCITED CS TOWARD IRC]10216: A NEW CIRCUMSTELLAR MASER?

New observations of vibrationally excited CS in its v \ 1 state have been conducted toward the cir-cumstellar shell of IRC]10216 using the NRAO 12 m telescope and the CSO. The J \ 3] 2, 6] 5, and 7] 6 transitions in the v \ 1 level at 146, 292, and 340 GHz have been detected toward this object, and the J \ 2] 1 and 5] 4 lines were also reobserved. After accounting for contamination from other species, these CS transitions all exhibit line proÐles that are narrower km s~1) than those(*V1@2D 10displayed by most molecules in IRC]10216, indicating an origin in the inner stellar envelope. The J \ 3] 2 line is particularly narrow km s~1), and consists of two separate velocity com-(*V1@2 D 1È2ponents at and [22 km s~1. These characteristics are suggestive of weak maser action inVLSR D [26this transition. The narrow line widths observed in the other transitions indicate a source size [ 0A.5, which implies a CS column density in the v \ 1 state of D 2È6] 1016 cm~2. The ground-state (v \ 0) column density is therefore on the order of 0.8È2] 1018 cm~2 for K. The fractional abun-Tvib \ 500dance of CS relative to is consequently f D 3È7] 10~5 at D 14 Such a high abundance close toH2 R*.the stellar photosphere is strong evidence that CS is a parent ÏÏ molecule in IRC]10216, as suggested by infrared studies of the v \ 0] 1 vibrational transition and interferometric observations