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 and, hence, we carried out a comprehensive study of the isotopologues of H${}_2$CO and formyl cation (HCO${}^{+}$), and of protonated formaldehyde (H${}_2$COH${}^{+}$) through the APEX observations in the spectral window $\sim$159-356~GHz. We employed observational and theoretical methods to derive the physical properties of the molecular gas combining LTE and non-LTE analyses. Formaldehyde was characterized via 35 lines of H${}_2$CO, H${}_2^{13}$CO, HDCO and H${}_2$C${}^{18}$O. The formyl cation was detected via 8 lines of HCO${}^{+}$, H${}^{13}$CO${}^{+}$, HC${}^{18}$O${}^{+}$ and HC${}^{17}$O${}^{+}$. Deuterium was clearly detected via HDCO, whereas DCO${}^{+}$ remained undetected. H${}_2$COH${}^{+}$ 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 ($T\sim$20-90 K), while HCO${}^{+}$ and H${}_2$COH${}^{+}$ are primarily associated with a colder gas ($T\lesssim$ 30 K). The reaction H${}_2$CO+HCO${}^{+}\to$ H${}_2$COH${}^{+}$ + 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 H${}_2$ which values at time scales $\sim$10${}^3$ yr matched with the observations in G331: [H${}_2$CO] = (0.2-2) $\times$10${}^{-8}$, [HCO${}^{+}$] = (0.5-4) $\times$10${}^{-9}$ and [H${}_2$COH${}^{+}$] = (0.2-2) $\times$10${}^{-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.

arXiv comment24 pages, 8 figures, 8 tables. Accepted for publication in The Astrophysical Journal

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