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

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

TL;DR

The study introduces a position-specific isotopologue astrochemical model that explicitly tracks the location of $^{13}$C within COMs across gas, surface, and mantle phases during the evolution of star-forming cores. By expanding the reaction network to preserve carbon-atom positions, the authors reveal isotopomer-specific $^{12}$C/$^{13}$C ratios that depend on formation pathways and environmental conditions, with differences up to around 40% between isotopomers. Results show cold-phase mantle radiolysis and warm-phase diffusion both imprint distinct isotopomer signatures, while diffusion barriers strongly modulate these patterns; some COMs retain early-epoch isotopic memory in the desorbed gas. The work provides a framework for using isotopomer-resolved observations to constrain formation pathways (surface versus mantle versus gas-phase) and emphasizes the need for precise measurements and improved microphysical ice parameters to interpret isotopic patterns in star-forming regions.

Abstract

The recent observation of complex organic molecules (COMs) in interstellar ices by the James Webb Space Telescope (JWST), along with previous gas-phase detections, underscores the importance of grain surface and ice mantle chemistry in the synthesis of COMs. In this study, we investigate the formation and carbon isotope fractionation of COMs by constructing a new astrochemical reaction network that distinguishes the position of $^{13}$C within species (e.g., H$^{13}$COOCH$_3$ and HCOO$^{13}$CH$_3$ are distinguished). We take into account the position of $^{13}$C in each species in gas and solid phase chemistry. This new model allows us to resolve isotopomer-specific $^{12}$C/$^{13}$C ratios of COMs formed in the star-forming cores. We consider thermal diffusion-driven radical-radical reactions on the ice surface and non-thermal radiolysis chemistry in the bulk (surface + mantle) ice. We find that carbon isotope fractionation of the functional groups in COMs appears through both non-thermal radiolysis in cold environments and thermal diffusion in warm environments, depending on the COMs. In particular, COMs containing methyl groups show isotopomer differences in $^{12}$C/$^{13}$C ratios that reflect their formation pathways and environments. These isotopomer-resolved fractionation patterns provide a diagnostic tool to probe the origins of COMs in star-forming cores. Our results suggest that future comparisons between high-sensitivity isotopic observations and isotopomer-specific models will be helpful for constraining the relative contributions of thermal and non-thermal formation processes of COMs.

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

TL;DR

The study introduces a position-specific isotopologue astrochemical model that explicitly tracks the location of C within COMs across gas, surface, and mantle phases during the evolution of star-forming cores. By expanding the reaction network to preserve carbon-atom positions, the authors reveal isotopomer-specific C/C ratios that depend on formation pathways and environmental conditions, with differences up to around 40% between isotopomers. Results show cold-phase mantle radiolysis and warm-phase diffusion both imprint distinct isotopomer signatures, while diffusion barriers strongly modulate these patterns; some COMs retain early-epoch isotopic memory in the desorbed gas. The work provides a framework for using isotopomer-resolved observations to constrain formation pathways (surface versus mantle versus gas-phase) and emphasizes the need for precise measurements and improved microphysical ice parameters to interpret isotopic patterns in star-forming regions.

Abstract

The recent observation of complex organic molecules (COMs) in interstellar ices by the James Webb Space Telescope (JWST), along with previous gas-phase detections, underscores the importance of grain surface and ice mantle chemistry in the synthesis of COMs. In this study, we investigate the formation and carbon isotope fractionation of COMs by constructing a new astrochemical reaction network that distinguishes the position of C within species (e.g., HCOOCH and HCOOCH are distinguished). We take into account the position of C in each species in gas and solid phase chemistry. This new model allows us to resolve isotopomer-specific C/C ratios of COMs formed in the star-forming cores. We consider thermal diffusion-driven radical-radical reactions on the ice surface and non-thermal radiolysis chemistry in the bulk (surface + mantle) ice. We find that carbon isotope fractionation of the functional groups in COMs appears through both non-thermal radiolysis in cold environments and thermal diffusion in warm environments, depending on the COMs. In particular, COMs containing methyl groups show isotopomer differences in C/C ratios that reflect their formation pathways and environments. These isotopomer-resolved fractionation patterns provide a diagnostic tool to probe the origins of COMs in star-forming cores. Our results suggest that future comparisons between high-sensitivity isotopic observations and isotopomer-specific models will be helpful for constraining the relative contributions of thermal and non-thermal formation processes of COMs.

Paper Structure

This paper contains 17 sections, 6 equations, 10 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Model Description
  3. Overview
  4. Carbon Isotope Chemistry
  5. Cosmic-ray-induced Non-thermal Reaction
  6. Other Modifications and Parameters
  7. Results
  8. Static Phase
  9. Collpase Phase
  10. Discussion
  11. Dependence on the Initial Condition of Carbon
  12. Comparison to the Full-Scrambling Model
  13. Isotopomer Differences in COMs Governed by the $^{12}$C/$^{13}$C Ratios of Precursor Molecules
  14. Effects of Thermal Radical Diffusion
  15. Comparisons with Observations
  16. ...and 2 more sections

Figures (10)

  • Figure 1: Conversion of chemical formula to SMILES format representation (upper). Comparison between the full scrambling (lower left) and position-conserved (lower right) models taking the reaction for the formation of ethanol (C$_2$H$_5$OH), CH$_3$ + CH$_2$OH $\rightarrow$ C$_2$H$_5$OH as an example.
  • Figure 2: Temporal variation of the number density of hydrogen nuclei (solid line) and gas temperature (dashed line) of a fluid parcel along a streamline in a gravitationally collapsing core.
  • Figure 3: Temporal variation of the molecular abundances and $^{12}$C/$^{13}$C ratios for gaseous species (solid lines), the bulk (surface + mantle) ice (dash-dot lines) during the static phase. The horizontal black dashed line represents the elemental $^{12}$C/$^{13}$C ratio.
  • Figure 4: Temporal variation of the abundances (left panels) and $^{12}$C/$^{13}$C ratios (middle and right panels) of selected COMs in the gas phase (solid lines) and in the bulk (surface + mantle) ice (dash-dotted lines) during the static phase (upper panels) and the collapse phase (lower panels). The middle panels show $^{12}$C/$^{13}$C ratios for H$^{13}$COOCH$_{3}$, HOCH$_{2}$$^{13}$CHO, $^{13}$CH$_{3}$CHO, and $^{13}$CH$_{3}$CH$_{2}$OH, while the right panels show those for HCOO$^{13}$CH$_{3}$, HO$^{13}$CH$_{2}$CHO, CH$_{3}$$^{13}$CHO, and CH$_{3}$$^{13}$CH$_{2}$OH. The vertical axis is shared between the middle and right panels, and the horizontal black dashed line represents the elemental $^{12}$C/$^{13}$C ratio.
  • Figure 5: Temporal variation of the abundance ratio of isotopomers in the gas phase (solid lines) and in the bulk (surface + mantle) ice (dash-dot lines) during the static phase (left panel) and the collapse phase (right panel). The isotopomer ratios shown are H$^{13}$COOCH$_{3}$/HCOO$^{13}$CH$_{3}$, HOCH$_{2}$$^{13}$CHO/HO$^{13}$CH$_{2}$CHO, $^{13}$CH$_{3}$CHO/CH$_{3}$$^{13}$CHO, and $^{13}$CH$_{3}$CH$_{2}$OH/CH$_{3}$$^{13}$CH$_{2}$OH.
  • ...and 5 more figures