We report the first interstellar detection of c-C _ { 3 } D _ { 2 } .
The doubly deuterated cyclopropenylidene , a carbene , has been detected toward the starless cores TMC-1C and L1544 using the IRAM 30m telescope .
The J _ { K _ { a } ,K _ { c } } = 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-C _ { 3 } H _ { 2 } , the singly deuterated c-C _ { 3 } HD , and the species with one ^ { 13 } C off of the principal axis of the molecule , c-H ^ { 13 } CC _ { 2 } H. The lines of c-C _ { 3 } D _ { 2 } 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 analysed with a comprehensive gas-grain model , the first including doubly deuterated species .
The observed abundances of c-C _ { 3 } D _ { 2 } can be explained solely by gas-phase processes , supporting the idea that c-C _ { 3 } H _ { 2 } is a good indicator of gas-phase deuteration .