Millimeter Observations of Vibrationally Excited CS toward IRC +10216: A New Circumstellar Maser?

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Abstract
New observations of vibrationally excited CS in its v = 1 state have been conducted toward the circumstellar 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 profiles that are narrower (ΔV1/2 ~ 10 km s-1) than those displayed by most molecules in IRC +10216, indicating an origin in the inner stellar envelope. The J = 3 → 2 line is particularly narrow (ΔV1/2 ~ 1-2 km s-1), and consists of two separate velocity components at VLSR ~ -26 and -22 km s-1. These characteristics are suggestive of weak maser action in this transition. The narrow line widths observed in the other transitions indicate a source size 05, which implies a CS column density in the v = 1 state of ~ 2-6 × 1016 cm-2. The ground-state (v = 0) column density is therefore on the order of 0.8-2 × 1018 cm-2 for Tvib = 500 K. The fractional abundance of CS relative to H2 is consequently f ~ 3-7 × 10-5 at ~ 14 R*. Such a high abundance close to the stellar photosphere is strong evidence that CS is a "parent" molecule in IRC +10216, as suggested by infrared studies of the v = 0 → 1 vibrational transition and interferometric observations.