G331.512-0.103: An Interstellar Laboratory for Molecular Synthesis I. The Ortho-to-para Ratios for CH${}_3$OH and CH${}_3$CN

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 CH${}_3$OH and CH${}_3$CN. 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 $A/E$-CH${}_3$OH and $A/E$-CH${}_3$CN were analysed, including torsionally excited transitions of CH${}_3$OH ($\hat{I}{½}_t$=1). In a search for all the isotopologues, we identified transitions of ${}^{13}$CH${}_3$OH. The physical conditions were derived considering collisional and radiative processes. We found common temperatures for each $A$ and $E$ symmetry of CH${}_3$OH and CH${}_3$CN; the derived column densities indicate an $A/E$ equilibrated ratio for both tracers. The results reveal that CH${}_3$CN and CH${}_3$OH trace a hot and cold component with $T_k\sim$ 141 K and $T_k\sim$ 74 K, respectively. In agreement with previous ALMA observations, the models show that the emission region is compact ($\lesssim$ 5.5 arcsec) with gas density $n$(H${}_2$)=(0.7-1) $\times$ 10${}^7$ cm${}^{-3}$. The CH${}_3$OH/CH${}_3$CN abundance ratio and the evidences for pre-biotic and complex organic molecules suggest a rich and active chemistry towards G331.512-0.103.

15 pages, 7 figures, 7 tables. Accepted for publication in The Astrophysical Journal