A line confusion limited millimeter survey of Orion KL I. Sulfur carbon chains

Abstract

We perform a sensitive (line confusion limited), single-side band spectral survey towards Orion KL with the IRAM 30 m telescope, covering the following frequency ranges: 80–115.5 GHz, 130–178 GHz, and 197–281 GHz. We detect more than 14 400 spectral features of which 10 040 have been identified up to date and attributed to 43 different molecules, including 148 isotopologues and lines from vibrationally excited states. In this paper, we focus on the study of OCS, HCS<sup>+<sup/>, H<sub>2<sub/>CS, CS, CCS, C<sub>3<sub/>S, and their isotopologues. In addition, we map the OCS <i>J<i/> = 18–17 line and complete complementary observations of several OCS lines at selected positions around Orion IRc2 (the position selected for the survey). We report the first detection of OCS <i>ν<i/><sub>2<sub/> = 1 and <i>ν<i/><sub>3<sub/> = 1 vibrationally excited states in space and the first detection of C<sub>3<sub/>S in warm clouds. Most of CCS, and almost all C<sub>3<sub/>S, line emission arises from the hot core indicating an enhancement of their abundances in warm and dense gas. Column densities and isotopic ratios have been calculated using a large velocity gradient (LVG) excitation and radiative transfer code (for the low density gas components) and a local thermal equilibrium (LTE) code (appropriate for the warm and dense hot core component), which takes into account the different cloud components known to exist towards Orion KL, the <i>extended ridge<i/>, <i>compact ridge<i/>, <i>plateau<i/>, and <i>hot core<i/>. The vibrational temperature derived from OCS <i>ν<i/><sub>2<sub/> = 1 and <i>ν<i/><sub>3<sub/> = 1 levels is 210 K, similar to the gas kinetic temperature in the hot core. These OCS high energy levels are probably pumped by absorption of IR dust photons. We derive an upper limit to the OC<sub>3<sub/>S, H<sub>2<sub/>CCS, HNCS, HOCS<sup>+<sup/>, and NCS column densities. Finally, we discuss the D/H abundance ratio and infer the following isotopic abundances: <sup>12<sup/>C/<sup>13<sup/>C = 45 <i>±<i/> 20, <sup>32<sup/>S/<sup>34<sup/>S = 20 <i>±<i/> 6, <sup>32<sup/>S/<sup>33<sup/>S = 75 <i>±<i/> 29, and <sup>16<sup/>O/<sup>18<sup/>O = 250 <i>±<i/> 135.