Gas phase Elemental abundances in Molecular cloudS (GEMS) XII. First look into the para-to-ortho ratio of c-C3D2 towards prestellar cores
Abstract
Symmetric molecules with identical nuclei form distinct nuclear spin isomers (ortho and para) that cannot interconvert via radiative or inelastic transitions, preserving clues about formation conditions. In the interstellar medium, cyclopropenylidene (c-C3H2) and its doubly-deuterated isotopologue (c-C3D2) provide a prime laboratory for studying these ratios. While measured for the main species, spin ratios for c-C3D2 remain unavailable. We present new detections of ortho and para transitions of c-C3D2 toward three Taurus molecular cloud sources from the GEMS Large program, and revisit four literature cores to investigate the para-to-ortho ratio of c-C3D2 for the first time. Column densities were computed using RADEX with dedicated collision rate coefficients, yielding total c-C3D2 column densities of (2.5-5.3) x 10e11 cm-2 for the new sources. The derived para-to-ortho ratios are 1.5(+0.7-0.6) (B213-C16-1), 1.3(+1.3-0.7) (TMC1-C), and 1.0(+1.4-0.7) (TMC1-NH3, statistical). For revisited sources L1544, TMC-1C, TMC-2, and L1517B, ratios are 0.4(+0.1-0.1), 0.5(+0.1-0.1), 0.6(+0.2-0.1), and 1.4(+0.7-0.5), respectively. Overall, four sources show statistical ratios and three show higher non-statistical values. This is the first observational determination of the para-to-ortho ratio in a doubly deuterated molecule using a detailed radiative transfer tool. The elevated ratios in some sources likely stem from p-D2H+ or D3+ driven reactions, and are independent of H2 and c-C3H2 ortho-to-para ratios. These findings offer an essential benchmark for chemical models investigating c-C3D2 formation pathways.
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BibTeX
@misc{asensio2026phase,
title = {{Gas phase Elemental abundances in Molecular cloudS (GEMS) XII. First look into the para-to-ortho ratio of c-C3D2 towards prestellar cores}},
author = {J. Ferrer Asensio and A. Fuente and E. Roueff and K. Furuya and B. Tercero and N. Sakai},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.24285},
eprint = {2609.24285},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.24285},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Gas phase Elemental abundances in Molecular cloudS (GEMS) XII. First look into the para-to-ortho ratio of c-C3D2 towards prestellar cores
AU - J. Ferrer Asensio
AU - A. Fuente
AU - E. Roueff
AU - K. Furuya
AU - B. Tercero
AU - N. Sakai
PY - 2026
DA - 2026/09/21
PB - arXiv
DO - 10.48550/arxiv.2609.24285
UR - https://arxiv.org/abs/2609.24285
AB - Symmetric molecules with identical nuclei form distinct nuclear spin isomers (ortho and para) that cannot interconvert via radiative or inelastic transitions, preserving clues about formation conditions. In the interstellar medium, cyclopropenylidene (c-C3H2) and its doubly-deuterated isotopologue (c-C3D2) provide a prime laboratory for studying these ratios. While measured for the main species, spin ratios for c-C3D2 remain unavailable. We present new detections of ortho and para transitions of c-C3D2 toward three Taurus molecular cloud sources from the GEMS Large program, and revisit four literature cores to investigate the para-to-ortho ratio of c-C3D2 for the first time. Column densities were computed using RADEX with dedicated collision rate coefficients, yielding total c-C3D2 column densities of (2.5-5.3) x 10e11 cm-2 for the new sources. The derived para-to-ortho ratios are 1.5(+0.7-0.6) (B213-C16-1), 1.3(+1.3-0.7) (TMC1-C), and 1.0(+1.4-0.7) (TMC1-NH3, statistical). For revisited sources L1544, TMC-1C, TMC-2, and L1517B, ratios are 0.4(+0.1-0.1), 0.5(+0.1-0.1), 0.6(+0.2-0.1), and 1.4(+0.7-0.5), respectively. Overall, four sources show statistical ratios and three show higher non-statistical values. This is the first observational determination of the para-to-ortho ratio in a doubly deuterated molecule using a detailed radiative transfer tool. The elevated ratios in some sources likely stem from p-D2H+ or D3+ driven reactions, and are independent of H2 and c-C3H2 ortho-to-para ratios. These findings offer an essential benchmark for chemical models investigating c-C3D2 formation pathways.
ER -