2022ApJ...925....3S

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 and 356 GHz. Through an analysis under the local thermodynamic equilibrium assumption, by means of rotational diagrams, we determined T _exc = 50 ± 1 K, N(CH₃CCH) = (7.5 ± 0.4) x 10¹⁵ cm², X[CH₃CCH/H₂] ≃ (0.8-2.8) x 10^–8, and X[CH₃CCH/CH₃OH] ≃ 0.42 ± 0.05 for an extended emitting region (∼10''). 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 nonnegligible 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.