2013ApJ...769L..19S

We report the first interstellar detection of c-C₃D₂. Doubly deuterated cyclopropenylidene, a carbene, has been detected toward the starless cores TMC-1C and L1544 using the IRAM 30 m telescope. The J_Ka,Kc = 3_1, 3_–20, 2_, and 2_2, 1_–11, 0_ transitions of this species have been observed at 3 mm in both sources. The expected 1:2 intensity ratio has been found in the 3_0, 3_–21, 2_ and 3_1, 3_–20, 2_lines, belonging to the para and ortho species, respectively. We also observed lines of the main species, c-C₃H₂, singly deuterated c-C₃HD, and the species with one ¹³C off of the principal axis of the molecule, c-H¹³CC₂H. The lines of c-C₃D₂ have been observed with high signal-to-noise ratio, better than 7.5σ in TMC-1C and 9σ in L1544. The abundance of doubly deuterated cyclopropenylidene with respect to the normal species is found to be 0.4%-0.8% in TMC-1C and 1.2%-2.1% in L1544. The deuteration of this small hydrocarbon ring is analyzed with a comprehensive gas-grain model, the first including doubly deuterated species. The observed abundances of c-C₃D₂ can be explained solely by gas-phase processes, supporting the idea that c-C₃H₂ is a good indicator of gas-phase deuteration.