Gas phase Elemental abundances in Molecular cloudS (GEMS)

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Abstract
Context. Symmetric molecules that contain several identical nuclei give rise to distinct isomers that are characterized by the total nuclear spin and produce separate ortho and para species that cannot be exchanged through radiative or inelastic transitions and whose relative populations can preserve information about formation conditions. In the interstellar medium, cyclopropenylidene (c-C 3 H 2 ) and its doubly deuterated isotopologue provide a prime laboratory for studying spin isomer ratios, which have been measured for the main species but are yet unavailable for c-C 3 D 2 . Aims. We present new detections of both ortho and para transitions of c-C 3 D 2 towards three sources in the Taurus molecular cloud from the GEMS Large program, and revisit previous detections towards four other cores from the literature in order to investigate the para-to-ortho ratio for doubly deuterated c-C 3 D 2 for the first time. Methods. We computed the ortho and para c-C 3 D 2 column densities towards all the sources using the non-local thermodynamic equilibrium (non-LTE) software RADEX thanks to the recent availability of dedicated collision rate coefficients. Results. The total derived column densities of c-C 3 D 2 for the new sources present a range of (2.5–5.3) × 10 11 cm −2 . The para-to-ortho ratio derived for B213-C16-1 and TMC1-C, 1.5 −0.6 +0.7 and 1.3 −0.7 +1.3 , respectively, are higher than the statistical ratio of 0.5, while for TMC1-NH3 the ratio is statistical within errors, 1.0 −0.7 +1.4 . The para-to-ortho ratio for the revisited sources from the literature, L1544, TMC-1C, TMC-2, and L1517B are 0.4 −0.1 +0.1 , 0.5 −0.1 +0.1 , 0.6 −0.1 +0.2 , and 1.4 −0.5 +0.7 , respectively. In this case, the first three cores show para-to-ortho statistical values, and the latter a non-statistical one. Thus, from all the sources studied, four have statistical c-C 3 D 2 para-to-ortho ratios and three sources show higher non-statistical ratios. Conclusions. This is the first observational determination of the para-to-ortho ratio in a doubly deuterated molecule derived through a detailed radiative transfer tool. The observed larger-than-statistical c-C 3 D 2 para-to-ortho ratio values for some of the sources may result from p-D 2 H + or D 3 + driven reactions. The actual ratio is not directly related to the ortho-to-para ratios of H 2 and c-C 3 H 2 . These findings provide an important reference point for future chemical models to investigate the formation pathways of c-C 3 D 2 .

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