Analysis of the Herschel/HEXOS Spectral Survey Towards Orion South: A massive protostellar envelope with strong external irradiation

Abstract

We present results from a comprehensive submillimeter spectral survey toward the source Orion South, based on data obtained with the Heterodyne Instrument for the Far-Infrared instrument on board the Herschel Space Observatory, covering the frequency range of 480 to 1900 GHz. We detect 685 spectral lines with signal-to-noise ratios (S/Ns) > 3σ, originating from 52 different molecular and atomic species. We model each of the detected species assuming conditions of Local Thermodynamic Equilibrium. This analysis provides an estimate of the physical conditions of Orion South (column density, temperature, source size, and V_(LSR)). We find evidence for three different cloud components: a cool (T_(ex) ~ 20–40 K), spatially extended (>60"), and quiescent (ΔV_(FWHM) ~ 4 km s^(−1)) component; a warmer (T_(ex) ~ 80–100 K), less spatially extended (~30"), and dynamic (ΔV_(FWHM) ~ 8 km s^(−1)) component, which is likely affected by embedded outflows; and a kinematically distinct region (T_(ex) > 100 K; V_(LSR) ~ 8 km s^(−1)), dominated by emission from species that trace ultraviolet irradiation, likely at the surface of the cloud. We find little evidence for the existence of a chemically distinct "hot-core" component, likely due to the small filling factor of the hot core or hot cores within the Herschel beam. We find that the chemical composition of the gas in the cooler, quiescent component of Orion South more closely resembles that of the quiescent ridge in Orion-KL. The gas in the warmer, dynamic component, however, more closely resembles that of the Compact Ridge and Plateau regions of Orion-KL, suggesting that higher temperatures and shocks also have an influence on the overall chemistry of Orion South.

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