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

TL;DR

This study uses Herschel/HEXOS spectral data to analyze the physical and chemical properties of Orion South, revealing multiple cloud components with distinct temperatures, dynamics, and irradiation effects, and comparing its chemistry to other Orion regions.

Contribution

First comprehensive spectral survey of Orion South with detailed modeling of multiple cloud components and their physical and chemical characteristics.

Findings

Identification of three distinct cloud components with different temperatures and dynamics.

Evidence of UV irradiation effects on the surface region of Orion South.

Chemical composition similarities with Orion-KL's quiescent ridge and shock-affected regions.

Abstract

We present results from a comprehensive submillimeter spectral survey toward the source Orion South, based on data obtained with the HIFI instrument aboard the \textit{Herschel Space Observatory}, covering the frequency range 480 to 1900 GHz. We detect 685 spectral lines with S/N $>$ 3$\sigma$, 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, \& V$_{LSR}$). We find evidence for three different cloud components: a cool (T$_{ex} \sim 20-40$ K), spatially extended ($> 60"$), and quiescent ($\Delta V_{FWHM} \sim 4$ km s $^{-1}$) component; a warmer (T$_{ex} \sim 80-100$ K), less spatially extended ($\sim 30"$), and dynamic ($\Delta V_{FWHM} \sim 8$ km s $^{-1}$) component, which is likely affected by embedded outflows; and a kinematically distinct region (T$_{ex}$ $>$ 100 K; V$_{LSR}$ $\sim$ 8 km s $^{-1}$), dominated by emission from species which 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 \textit{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.