  A spectral survey of methyl acetylene (CH3CCH) was conducted toward the hot
molecular core/outflow G331.512-0.103. Our APEX observations allowed the
detection of 41 uncontaminated rotational lines of CH3CCH in the frequency
range between 172-356 GHz. Through an analysis under the local thermodynamic
equilibrium assumption, by means of rotational diagrams, we determined Texc =
50 \pm 1 K, N(CH3CCH) = (7.5 \pm 0.4) x 10^{15} cm^{-2}, X[CH3CCH/H2] ~
(0.8-2.8) x 10^{-8} and X[CH3CCH/CH3OH] ~ 0.42 \pm 0.05 for an extended
emitting region (~10 arcsec). The relative intensities of the K=2 and K=3 lines
within a given K-ladder are strongly negatively correlated to the transitions'
upper J quantum-number (r=-0.84). Pure rotational spectra of CH3CCH were
simulated at different temperatures, in order to interpret this observation.
The results indicate that the emission is characterized by a non-negligible
temperature gradient with upper and lower limits of ~45 and ~60 K,
respectively. Moreover, the line widths and peak velocities show an overall
strong correlation with their rest frequencies, suggesting that the warmer gas
is also associated with stronger turbulence effects. The K=0 transitions
present a slightly different kinematic signature than the remaining lines,
indicating that they might be tracing a different gas component. We speculate
that this component is characterized by lower temperatures, and therefore
larger sizes. Moreover, we predict and discuss the temporal evolution of the
CH3CCH abundance using a two-stage zero-dimensional model of the source
constructed with the three-phase Nautilus gas-grain code.
