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; 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, 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.