c-C3H2 deuteration towards prestellar and starless cores in the Perseus Molecular Cloud

TL;DR

This work analyzes the deuteration of c-C$_{3}$H$_{2}$ and its isotopologues toward 16 starless and prestellar cores in the Perseus Molecular Cloud, using multi-telescope observations and non-LTE radiative transfer to derive column densities. It finds a median c-C$_{3}$H$_{2}$ otp of $3.5 \\pm 0.4$, with D/H and D$_{2}$/D statistically corrected medians of $1.5 \\pm 0.2 \\%$ and $25.9 \\pm 4.3 \\%$, respectively. A strong positive correlation between D/H and $n_{ m H_2}$ supports deuteration as an evolutionary tracer, while D$_{2}$/D correlates with $T_{ m kin}$, implying second deuteration is influenced by mildly endothermic gas-phase paths. Comparisons with Taurus and Chamaeleon suggest environment, rather than cloud identity, largely governs deuteration variability, and protostellar cores exhibit higher deuteration than starless cores, indicating partial inheritance from the prestellar phase.

Abstract

Deuterium fractionation becomes highly efficient in cold, dense cores where CO is frozen out. Cyclopropenylidene (c-C3H2), an early-formed carbon ring, and its deuterated isotopologues trace gas-phase deuteration in these environments. We present a statistical study of c-C3H2 deuteration in starless and prestellar cores of the Perseus Molecular Cloud using observations of c-C3H2, c-C3HD and c-C3D2 obtained with the Yebes 40 m, ARO 12 m and IRAM 30 m telescopes towards 16 cores. Gaussian fits and RADEX modeling yield column densities for the detected species. c-C3H2 is detected in 14/15 covered cores, c-C3HD in 15/16, and c-C3D2 in 9/16. Derived column densities range from 0.5-8.1 x 10^{13} cm^{-2} for c-C3H2, 0.2-2.1 x 10^{12} cm^{-2} for c-C3HD, and 0.6-1.6 x 10^{11} cm^{-2} for c-C3D2. The ortho-to-para ratio of c-C3H2 is obtained for all but one core, with a median value of 3.5\pm0.4. Statistically corrected D/H ratios span 0.5-9.2% (median 1.5\pm0.2%), and D2/D ratios 9-55% (median 25.9\pm4.3%). No trend is found between the c-C3H2 ortho-to-para ratio and core evolutionary stage traced by n(H2). The median D/H ratio in Perseus appears lower than values reported for Taurus and Chamaeleon, while the D2/D ratio agrees with Taurus within uncertainties. A positive correlation between D/H and n(H2) supports the use of D/H as an evolutionary tracer. D2/D does not correlate with n(H2), but shows a positive correlation with T_{kin}, suggesting that its formation is influenced by a mildly endothermic pathway.

Figures (16)

• Figure 1: Molecular hydrogen column density ($N_{\rm H_2}$) map of the Perseus Molecular Cloud in gray scale pezzuto:12sadavoy:12sadavoy:14. The starless and prestellar cores studied in this study are indicated by coloured circles corresponding to the regions within the molecular cloud where they are located. NGC1333 is plotted in teal, B1 in purple, IC348 in red and B5 in yellow.
• Figure 2: Observed spectra of the c-C$_{3}$H$_{2}$ transitions organised by core number. All of the transitions for the same core are plotted in the same subplot with vertical offsets for clarity. The Gaussian fit from pyspeckit are overplotted with a blue line. The spectra without a Gaussian fit indicate a non-detection. Core number labels with a colour background indicate c-C$_{3}$H$_{2}$ has been detected towards the source. Lastly, horizontal gray dashed lines indicates 3$\sigma$ levels.
• Figure 3: Same as Figure \ref{['occ3h2']} for c-C$_{3}$HD. Here the Gaussian fits and labels are plotted with the colour green.
• Figure 4: Same as Figure \ref{['occ3h2']} for c-C$_{3}$D$_{2}$. Here the Gaussian fits and labels are plotted with the colour orange.
• Figure 5: Observed spectra and the Gaussian fit from pyspeckit of c-C$_{3}$HD, in green, and c-C$_{3}$D$_{2}$, in orange. Horizontal gray dashed lines indicates 3$\sigma$ levels.