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

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.

💡 Research Summary

This paper reports the first detection of the trans‑conformer of formic acid (t‑HCOOH) toward the hot molecular core G358.93‑0.03 MM1, a massive star‑forming region located at a distance of ~6.75 kpc. Using high‑resolution ALMA Band 7 observations (central frequency ≈ 291 GHz, angular resolution 0.41″ × 0.36″, spectral resolution 0.96 km s⁻¹), the authors identified six unblended rotational transitions of t‑HCOOH. LTE spectral fitting, performed with an MCMC approach, yields a column density of (8.13 ± 0.72) × 10¹⁵ cm⁻², an excitation temperature of 120 ± 15 K, and a line width of ~3.5 km s⁻¹. Assuming a source size of ~0.5″, the fractional abundance relative to H₂ is (2.62 ± 0.29) × 10⁻⁹. The derived abundance ratios t‑HCOOH/CH₃OH = 1.56 × 10⁻² and t‑HCOOH/H₂CO = 1.16 indicate that this core is unusually rich in formic acid compared with other well‑studied hot cores such as Sgr B2(N) and Orion KL.

The authors also present a comprehensive analysis of the dust continuum emission. Eight continuum cores are identified in the 291 GHz image; MM1 is the brightest, with a mass of ~26 M⊙, a luminosity of 7.8 × 10⁴ L⊙, a dust temperature of 175 K, and a volume density of ~10⁸ cm⁻³. Spectral energy distributions constructed from ALMA Bands 6 and 7 are fitted with radiative‑transfer models (Robitaille et al. 2007) to derive these physical parameters.

To interpret the observed formic‑acid abundance, the authors employed the three‑phase gas‑grain chemical code UCLCHEM, implementing a warm‑up scenario typical for high‑mass star formation: an initial cold phase (10 K, 10⁴ yr) followed by a gradual temperature rise to >100 K over 5 × 10⁴ yr at a density of 10⁷ cm⁻³. The model predicts that HCOOH forms efficiently on grain surfaces via the reaction HCO + OH → HCOOH. When the temperature exceeds the desorption threshold (~100 K), the ice‑mantle HCOOH is released into the gas phase, reproducing the observed abundance within a factor of 0.89. This close agreement supports grain‑surface chemistry as the dominant formation route in this environment.

The paper places these results in a broader astrochemical context. Formic acid is structurally similar to glycine, the simplest amino acid, and can act as a precursor in gas‑phase pathways such as HCOOH + CH₂NH → NH₂CH₂COOH (glycine). Although glycine itself was not detected in this source, upper limits are consistent with model predictions, suggesting that the observed HCOOH reservoir could feed subsequent prebiotic chemistry. The authors discuss the implications for the chemical evolution of hot cores, emphasizing that organic acids, despite their relatively low abundances, may play a pivotal role in the synthesis of more complex biomolecules.

Limitations are acknowledged: the LTE assumption may not capture non‑thermal excitation; a single temperature and source size simplify the likely complex structure; and the chemical network does not include high‑energy processes such as shocks or intense UV fields. Future work involving non‑LTE radiative transfer, higher‑resolution interferometry, and expanded reaction networks (including radiation chemistry) is recommended to refine abundance estimates and to trace the full pathway from simple acids to amino acids.

In summary, the study provides (i) the first robust detection of t‑HCOOH in G358.93‑0.03 MM1, (ii) quantitative LTE-derived column density and abundance, (iii) a successful three‑phase warm‑up chemical model that reproduces the observations, and (iv) a compelling case that grain‑surface formation of formic acid can seed prebiotic chemistry in massive star‑forming hot cores.