Sulphur-Bearing and Complex Organic Molecules in an Infrared Cold Core

TL;DR

This study analyzes ALMA observations of an infrared dark cloud, revealing complex organic molecules and sulfur-bearing species, indicating hot gas and potential protostar presence in the core region.

Contribution

It provides detailed molecular identification and temperature analysis of IRDC-C9 Main, highlighting the coexistence of hot and cold gas phases in a cold core.

Findings

Detection of complex organic molecules like CH3CHO and CH3CH2CN.

Identification of two temperature regions, 25 K and 450 K.

Evidence of a protostar in the core region.

Abstract

Since the start of ALMA observatory operation, new and important chemistry of infrared cold core was revealed. Molecular transitions at millimeter range are being used to identify and to characterize these sources. We have investigated the 231 GHz ALMA archive observations of the infrared dark cloud region C9, focusing on the brighter source that we called as IRDC-C9 Main. We report the existence of two sub-structures on the continuum map of this source: a compact bright spot with high chemistry diversity that we labelled as core, and a weaker and extended one, that we labelled as tail. In the core, we have identified lines of the molecules OCS(19-18), $^{13}$CS(5-4) and CH$_{3}$CH$_{2}$CN, several lines of CH$_{3}$CHO and the k-ladder emission of $^{13}$CH$_{3}$CN.We report two different temperature regions: while the rotation diagram of CH$_{3}$CHO indicates a temperature of 25 K, the rotation diagram of $^{13}$CH$_{3}$CN indicates a warmer phase at temperature of $\sim450$K. In the tail, only the OCS(19-18) and $^{13}$CS(5-4) lines were detected. We used the $Nautilus$ and the \textsc{Radex} codes to estimate the column densities and the abundances. The existence of hot gas in the core of IRDC-C9 Main suggests the presence of a protostar, which is not present in the tail.