HCN/HNC ratio: A new chemical thermometer at 3 mm

Abstract

The gas kinetic temperature (TK) determines the physical and chemical evolution of the interstellar medium (ISM). This work explores the use of HCN/HNC (J=1–0) line ratio as new probe of the gas kinetic temperature in the molecular ISM. Based on a new set of IRAM-30m observations at 30 arcsec resolution towards Orion Nebula Cluster, we find a two-part linear correlation of the observed HCN/HNC line ratio and TK. This empirical calibration allows to obtain direct estimates of the TK values across a wide range of column densities (AV ≳ 5 mag) and up to scales of ~ 10 pc. Comparisons with additional studies highlight the potential use of the HCN/HNC line ratio as novel chemical thermometer at 3mm.