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 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 (△V_1/2 ∼ 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 (△V_1/2 ∼ 10 1-2 km s^-1), and consists of two separate velocity components at V_LSR ∼ -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 ≲ 0″.5, 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 T_vib = 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.