Astrochemistry

Piecing Together Formic Acid Isomerism In Dark Clouds. Detection Of Cis-formic Acid In TMC-1 And Astrochemical Modeling

By Keith Cowing
Status Report
astro-ph.GA
September 25, 2025
Filed under , , , , , , , ,
Piecing Together Formic Acid Isomerism In Dark Clouds. Detection Of Cis-formic Acid In TMC-1 And Astrochemical Modeling
Sensitivity analysis of our model to the oxygen atom diffusion energy for CO2 ice (dashed lines) and FA (suming c-t abundances). Values are represented with respect to H2O column density- The difussion Ediff energy is defined from the Ediff/Ebin parameter in our models. Ediff/Ebin is set to 0.25, 0.30 and 0.40 in those models. The pale blue zone represents the observationally constrained abundances of CO2 (Boogert et al. 2015; McClure et al. 2023) and the pale yellow zone represent the same for FA (Boogert et al. 2015; Rocha et al. 2024). We assume that FA and HCOO– abundances sum. — astro-ph.GA

The presence of molecular isomers in interstellar environments has become a topic of growing interest within the astrochemical community.

Contrary to predictions based on thermodynamic equilibrium, recent observations reveal a diverse array of high-energy isomers and conformers. One of the most iconic molecular isomers detected in space, formic acid (HCOOH, FA), has been the focus of extensive theoretical research aimed at understanding its speciation into cis and trans conformers in dark clouds and photodissociation regions.

In this work, we report the detection of c-FA, the higher-energy conformer, using ultrasensitive observations of TMC-1. This detection adds to previous findings in the Barnard-5 and L483 dark clouds.

The derived trans-to-cis isomer ratio in TMC-1, 17.5, closely matches those observed in other sources, suggesting that the same chemical processes are at play across these environments. To investigate this, we conducted detailed astrochemical gas-grain models tailored to formic acid isomerism to explain the observed ratios.

Our models successfully reproduce the observed trans/cis ratios and indicate that the presence of cis-formic acid can be attributed to the release of c-FA from grains, followed by isomerization driven by the excess energy released during the desorption process, a process that we name as isomerization upon desorption.

The models also show that the isomerization of t-FA to c-FA in the gas phase is negligible at 10 K, meaning the observed ratios are a direct consequence of the formation pathways of both isomers on the surface of dust grains. However, at higher temperatures, quantum tunneling mediated direct isomerization in the gas becomes significant, and the ratios converge toward the thermodynamic equilibrium value.

G. Molpeceres, M. Agúndez, M. Mallo, C. Cabezas, M. Sanz-Novo, V.M. Rivilla, J. García de la Concepción, I. Jiménez-Serra, J. Cernicharo

Comments: Accepted for publication in A&A
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2509.18315 [astro-ph.GA] (or arXiv:2509.18315v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2509.18315
Focus to learn more
Submission history
From: German Molpeceres
[v1] Mon, 22 Sep 2025 18:38:28 UTC (924 KB)
https://arxiv.org/abs/2509.18315
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

Follow on Twitter
MORE FROM Astrochemistry