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