2023ApJ...953...77M

In the interstellar cold gas, the chemistry of formaldehyde (H₂CO) 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 H₂CO, the formyl cation (HCO⁺), and protonated formaldehyde (H₂COH⁺) 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 H₂CO, ${{\rm{H}}}_{2}^{13}$CO, HDCO, and H₂C¹⁸O. The formyl cation was detected via eight lines of HCO⁺, H¹³CO⁺, HC¹⁸O⁺, and HC¹⁷O⁺. Deuterium was clearly detected via HDCO, whereas DCO⁺ remained undetected. The H₂COH⁺ 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 H₂COH⁺ are primarily associated with colder gas (T ≲ 30 K). The reaction H₂CO+HCO⁺ → H₂COH⁺ + 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 H₂; their values at timescales of ∼10³ yr matched the observations in G331: [H₂CO] = (0.2-2) × 10^–8, [HCO⁺] = (0.5-4) × 10^–9, and [H₂COH⁺] = (0.2-2) × 10^–10. Based on the molecular evolution of H₂CO, HCO⁺, and H₂COH⁺, we hypothesized about the young lifetime of G331, which is consistent with the active gas-grain chemistry of massive protostellar objects.