Organic Acid Chemistry in ISM: Detection of Formic Acid and its Prebiotic Chemistry in Hot Core G358.93$-$0.03 MM1

TL;DR

This study reports the first ALMA Band 7 detection of the $trans$ form of formic acid ($t$-HCOOH) toward the hot core G358.93--0.03 MM1, deriving $N_T=(8.10 ext{±}1.12) imes10^{15}$ cm$^{-2}$, $T_{rot} ext{≈}116$ K, and a fractional abundance $X(t ext{-HCOOH}) ext{≈}(2.62 ext{±}0.29) imes10^{-9}$; MM3 remains undetected for $t$-HCOOH. A three-phase warm-up chemical model using UCLCHEM shows that the observed abundances are reproduced within a factor of 0.89, supporting grain-surface formation of HCOOH via the HCO + OH reaction followed by desorption. The work also characterizes the dust continuum structure (eight cores, with MM1/MM3 as hot cores) and maps the spatial distribution of $t$-HCOOH, finding emission co-located with the dense inner region but unresolved at the current resolution. Correlation analyses reveal weak or non-significant links between HCOOH and proposed precursors CH$_3$OH and H$_2$CO, suggesting a more complex chemical network. Overall, the results underscore the role of $t$-HCOOH as a tracer of hot-core chemistry and its potential relevance to prebiotic molecule formation in high-mass star-forming regions.

Abstract

In the interstellar medium, formic acid (HCOOH) plays a significant role in the synthesis of the simplest amino acid, glycine (NH$_{2}$CH$_{2}$COOH). The presence of HCOOH suggests that oxygen-bearing molecules may be directly involved in the chemical and physical evolution of star formation regions, particularly in hot molecular cores. This paper presents the first detection of the rotational emission lines of the $trans$-conformer of HCOOH toward the hot molecular core G358.93$-$0.03 MM1, located in the massive star formation region G358.93$-$0.03. This study employed high-resolution observations from the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 7. The column density and excitation temperature of $t$-HCOOH are determined as $(8.13\pm0.72)\times10^{15}$ cm$^{-2}$ and $120\pm15$ K, respectively. The fractional abundance of $t$-HCOOH relative to H$_{2}$ is $(2.62\pm 0.29)\times 10^{-9}$. The column density ratios of $t$-HCOOH/CH$_{3}$OH and $t$-HCOOH/H$_{2}$CO are $(1.56 \pm 0.12)\times 10^{-2}$ and $(1.16 \pm 0.12)$, respectively. We computed a three-phase warm-up chemical model of HCOOH using the gas-grain chemical code UCLCHEM. We found that the observed and modelled abundances of HCOOH are almost identical, within a factor of 0.89. Based on chemical modelling, we showed that HCOOH may be formed through the reaction between HCO and OH on the grain surface, which is further released in the gas-phase.

Figures (13)

• Figure 1: Molecular structures of $cis$-HCOOH and $trans$-HCOOH. Carbon (C), hydrogen (H), and oxygen (O) atoms are represented by grey, white, and red spheres, respectively.
• Figure 2: The dust continuum emission image of the massive star formation region G358.93--0.03 at a frequency of 291.31 GHz. The synthesized beam size of the continuum image is 0.41$^{\prime\prime}$$\times$0.36$^{\prime\prime}$. The contour levels increase by a factor of $\surd$2 from the starting point of 2.5$\sigma$.
• Figure 3: SEDs of dust continuum cores from ALMA Band 6 and 7 observations. In the SED plots, orange and green data points represent the observed flux densities of the detected cores across different ALMA bands, with error bars indicating the measurement uncertainties. The black curve corresponds to the best-fit SED obtained using the radiative transfer model by rob07, while the blue-shaded region illustrates the 1$\sigma$ uncertainty range of the fit.
• Figure 4: Identified molecular emission lines $t$-HCOOH towards G358.93--0.03 MM1. The green lines are the observed spectra, and the blue lines are the LTE spectra of $t$-HCOOH. The black lines are the LTE spectra of the remaining molecules identified in the spectra of G358.93--0.03 MM1.
• Figure 5: Corner plots showing the covariances of the posterior probability distributions of the column density (log$_{10}$($N$)) in cm$^{-2}$, excitation temperature in K, and FWHM in km s$^{-1}$ of $t$-HCOOH.