INTERSTELLAR DETECTION OF c-C3D2

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(0,3)-2(1,2), 3(1,3)-2(0,2), and 2(2,1)-1(1,0) 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)-2(1,2) and 3(1,3)-2(0,2) 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 C-13 off of the principal axis of the molecule, c-(HCC2H)-C-13. The lines of c-C3D2 have been observed with high signal-to-noise ratio, better than 7.5 sigma in TMC-1C and 9 sigma 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.