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 (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 and, hence, we carried out a comprehensive study of the isotopologues of H2CO and formyl cation (HCO+), and of protonated formaldehyde (H2COH+) through the APEX observations in the spectral window 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, H213CO, HDCO and H2C18O. The formyl cation was detected via 8 lines of HCO+, H13CO+, HC18O+ and HC17O+. Deuterium was clearly detected via HDCO, whereas DCO+ remained undetected. H2COH+ was detected through 3 clean lines. According to the radiative analysis, formaldehyde appears to be embedded in a bulk gas with a wide range of temperatures (T20-90 K), while HCO+ and H2COH+ are primarily associated with a colder gas (T 30 K). The reaction H2CO+HCO+ → H2COH+ + CO is crucial for the balance of the three species. We used Nautilus gas-grain code to predict the evolution of their molecular abundances relative to H2 which values at time scales 103 yr matched with 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.
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