2000ApJ...544..881H

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 (ΔV_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 (ΔV_1/2∼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 ∼2-6x10¹⁶ cm^–2. The ground-state (v=0) column density is therefore on the order of 0.8-2x10¹⁸ cm^–2 for T_vib=500 K. The fractional abundance of CS relative to H₂ is consequently f∼3-7x10^–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.