Abstract
Abstract 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) × 1015 cm2, X[CH3CCH/H2] ≈ (0.8–2.8) × 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.
Highlights
Star-forming regions play a key role in building the complex inventory of chemical species detected in astronomical environments, which in turn serve as powerful tools to study their surroundings
We have conducted a spectral survey of CH3CCH toward the Hot Molecular Core G331.512–0.103, resulting in the detection of 41 lines without contamination
The spectral analysis was performed through rotational diagrams, assuming Local Thermodynamic Equilibrium (LTE), from which we derived an averaged excitation temperature of ∼50 K for an extended emission
Summary
Star-forming regions play a key role in building the complex inventory of chemical species detected in astronomical environments, which in turn serve as powerful tools to study their surroundings. The formation process of those stars, is still less well-understood in comparison to the low-mass counterparts. Observational disadvantages, such as their complex cluster environments (n 100 pc−3) and large distances involved (d ≥ 1 kpc), together with a considerably shorter evolutionary timescale (tKH ≤ 104 yr for O-type stars), substantially impair the development of a solid massive starformation paradigm (Garay & Lizano 1999; Zinnecker & Yorke 2007; Tan et al 2014; Krumholz 2015; Silva et al 2017; Rosen et al 2020). One example are Hot Molecular Cores (HMCs), which are one of the first manifestations of massive star formation (Cesaroni 2005). HMCs are associated with a rich molecular emission spectrum, which carries information on their chemical and physical properties, as well as their morphology and evolutionary stage (e.g., Caselli et al 1993; Comito et al 2005; Herbst & van Dishoeck 2009; Allen et al 2018; Bonfand et al 2019; Jørgensen et al 2020; Gieser et al 2021)
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