Isotopomer-Specific Carbon Isotope Ratio of Complex Organic Molecules in Star-Forming Cores

Recent high-resolution and sensitivity ALMA observations have unveiled the carbon isotope ratios (¹²C/¹³C) of Complex Organic Molecules (COMs) in a low-mass protostellar source.

To understand the ¹²C/¹³C ratios of COMs, we investigated the carbon isotope fractionation of COMs from prestellar cores to protostellar cores with a gas-grain chemical network model.

We confirmed that the ¹²C/¹³C ratios of small molecules are bimodal in the prestellar phase: CO and species formed from CO (e.g., CH₃OH) are slightly enriched in ¹³C compared to the local ISM (by ∼ 10 %), while those from C and C⁺ are depleted in ¹³C owing to isotope exchange reactions. COMs are mainly formed on the grain surface and in the hot gas (> 100 K) in the protostellar phase. The ¹²C/¹³C ratios of COMs depend on which molecules the COMs are formed from.

In our base model, some COMs in the hot gas are depleted in ¹³C compared to the observations. Thus, We additionally incorporate reactions between gaseous atomic C and H₂O ice or CO ice on the grain surface to form H₂CO ice or C₂O ice, as suggested by recent laboratory studies.

The direct C-atom addition reactions open pathways to form ¹³C-enriched COMs from atomic C and CO ice. We find that these direct C-atom addition reactions mitigate ¹³C-depletion of COMs, and the model with the direct C-atom addition reactions better reproduces the observations than our base model. We also discuss the impact of the cosmic ray ionization rate on the ¹²C/¹³C ratio of COMs.

Ryota Ichimura, Hideko Nomura, Kenji Furuya, Tetsuya Hama, T. J. Millar

Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)
Report number: No.: sp-2025-00180e
Cite as: arXiv:2512.10516 [astro-ph.GA] (or arXiv:2512.10516v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2512.10516
Focus to learn more
Related DOI:
https://doi.org/10.1021/acsearthspacechem.5c00180
Focus to learn more
Submission history
From: Ryota Ichimura
[v1] Thu, 11 Dec 2025 10:38:09 UTC (1,310 KB)
https://arxiv.org/abs/2512.10516
Astrobiology, Astrochemistry,