The Astrophysical Journal (Jan 2023)
Observations and Chemical Modeling of the Isotopologues of Formaldehyde and the Cations of Formyl and Protonated Formaldehyde in the Hot Molecular Core G331.512–0.103
Abstract
In the interstellar cold gas, the chemistry of formaldehyde (H _2 CO) can be essential to explain the formation of complex organic molecules. On this matter, the massive and energetic protostellar object G331 is still unexplored; hence, we carried out a comprehensive study of the isotopologues of H _2 CO, the formyl cation (HCO ^+ ), and protonated formaldehyde (H _2 COH ^+ ) through the APEX observations in a spectral window of ∼159–356 GHz. We employed observational and theoretical methods to derive the physical properties of the molecular gas combining LTE and non-LTE analyses. Formaldehyde was characterized via 35 lines of H _2 CO, ${{\rm{H}}}_{2}^{13}$ CO, HDCO, and H _2 C ^18 O. The formyl cation was detected via eight lines of HCO ^+ , H ^13 CO ^+ , HC ^18 O ^+ , and HC ^17 O ^+ . Deuterium was clearly detected via HDCO, whereas DCO ^+ remained undetected. The H _2 COH ^+ was detected through three clean lines. According to the radiative analysis, formaldehyde appears to be embedded in a bulk gas with a wide range of temperatures ( T ∼ 20–90 K), while HCO ^+ and H _2 COH ^+ are primarily associated with colder gas ( T ≲ 30 K). The reaction H _2 CO+HCO ^+ → H _2 COH ^+ + CO is crucial for the balance of the three species. We used the Nautilus gas–grain code to predict the evolution of their molecular abundances relative to H _2 ; their values at timescales of ∼10 ^3 yr matched the observations in G331: [H _2 CO] = (0.2–2) × 10 ^−8 , [HCO ^+ ] = (0.5–4) × 10 ^−9 , and [H _2 COH ^+ ] = (0.2–2) × 10 ^−10 . Based on the molecular evolution of H _2 CO, HCO ^+ , and H _2 COH ^+ , we hypothesized about the young lifetime of G331, which is consistent with the active gas–grain chemistry of massive protostellar objects.
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