Discovery of Interstellar Heavy Water
We report the discovery of doubly deuterated water (D2O, heavy water) in the interstellar medium. Using the James Clerk Maxwell Telescope and the Caltech Submillimeter Observatory 10 m telescope, we detected the 1_10–1_01 transition of para-D2O at 316.7998 GHz in both absorption and emission toward the protostellar binary system IRAS 16293-2422. Assuming that the D2O exists primarily in the warm regions where water ices have been evaporated (i.e., in a "hot corino" environment), we determine a total column density of N(D2O) of 1.0x10^13 cm^-2 and a fractional abundance of D2O/H2 = 1.7x10^-10. The derived column density ratios for IRAS 16293-2422 are D2O/HDO = 1.7x10^-3 and D2O/H2O = 5x10^-5 for the hot corino gas. Steady state models of water ice formation, either in the gas phase or on grains, predict D2O/HDO ratios that are about 4 times larger than that derived from our observations. For water formation on grain surfaces to be a viable explanation, a larger H2O abundance than that measured in IRAS 16293-2422 is required. Alternatively, the observed D2O/HDO ratio could be indicative of gas-phase water chemistry prior to a chemical steady state being attained, such as would have occurred during the formation of this source. Future observations with the Herschel Space Observatory satellite will be important for settling this issue. ; The CSO is funded by the NSF through grant AST 22-09008. This work was supported by the NASA Goddard Center for Astrobiology and, through Cooperative Agreement NCC2-1412, by NASA's Long Term Space Astrophysics Program. J. R. Pardo and J. Cernicharo thank the Spanish MEC for funding support under grant AYA2003-02785 and the Madrid Community Government under grant S-0505 ESP-0237 (ASTROCAM). ; Peer reviewed