Detection of Interstellar H[TINF]2[/TINF]D[TSUP]+[/TSUP] Emission

We report the detection of the 1 10 -1 11 ground-state transition of ortho-H 2 D + at 372.421 GHz in emission from the young stellar object NGC 1333 IRAS 4A. Detailed excitation models with a power-law temperature and density structure yield a beam-averaged H 2 D + abundance of 3 × 10 -12 with an uncertainty of a factor of 2. The line was not detected toward W33A, GL 2591, and NGC 2264 IRS (in the latter source at a level that is 3-8 times lower than previous observations). The H 2 D + data provide direct evidence in support of low-temperature chemical models in which H 2 D + is enhanced by the reaction of H and HD. The H 2 D + enhancement toward NGC 1333 IRAS 4A is also reflected in the high DCO + /HCO + abundance ratio. Simultaneous observations of the N 2 H + 4-3 line show that its abundance is about 50-100 times lower in NGC 1333 IRAS 4A than in the other sources, suggesting significant depletion of N 2 . The N 2 H + data provide independent lower limits on the H abundance that are consistent with the abundances derived from H 2 D + . The corresponding limits on the H column density agree with recent near-infrared absorption measurements of H toward W33A and GL 2591.