G331.512-0.103: An Interstellar Laboratory for Molecular Synthesis. I. the Ortho-to-para Ratios for CH3OH and CH3CN

Abstract

Spectral line surveys reveal rich molecular reservoirs in G331.512-0.103, a compact radio source in the center of an energetic molecular outflow. In this first work, we analyse the physical conditions of the source by means of CH3OH and CH3CN. The observations were performed with the APEX telescope. Six different system configurations were defined to cover most of the band within (292–356) GHz; as a consequence we detected a forest of lines towards the central core. A total of 70 lines of A/E-CH3OH and A/E-CH3CN were analysed, including torsionally excited transitions of CH3OH (νt=1). In a search for all the isotopologues, we identified transitions of 13CH3OH. The physical conditions were derived considering collisional and radiative processes. We found common temperatures for each A and E symmetry of CH3OH and CH3CN; the derived column densities indicate an A/E equilibrated ratio for both tracers. The results reveal that CH3CN and CH3OH trace a hot and cold component with Tk ∼ 141 K and Tk ∼ 74 K, respectively. In agreement with previous ALMA observations, the models show that the emission region is compact (. 5.5 ′′) with gas density n(H2)=(0.7–1) × 107 cm−3 . The CH3OH/CH3CN abundance ratio and the evidences for pre-biotic and complex organic molecules suggest a rich and active chemistry towards G331.512-0.103.