Detection of interstellar H2D+ emission

We report the detection of the 1(10)-1(11) ground-state transition of ortho -H2D+ at 372.421 GHz in emission from the young stellar object NGC 1333 IRA S 4A. Detailed excitation models with a power-law temperature and density s tructure yield a beam-averaged H2D+ abundance of 3 x 10(-12) with an uncert ainty 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 H2D+ data provide direct evidence in support o f low-temperature chemical models in which H2D+ is enhanced by the reaction of H-3(+) and HD. The H2D+ enhancement toward NGC 1333 IRAS 4A is also ref lected in the high DCO+/HCO+ abundance ratio. Simultaneous observations of the N2H+ 4-3 line show that its abundance is about 50-100 times lower in NG C 1333 IRAS 4A than in the other sources, suggesting significant depletion of N-2. The N2H+ data provide independent lower limits on the H-3(+) abunda nce that are consistent with the abundances derived from H2D+. The correspo nding limits on the H-3(+) column density agree with recent near-infrared a bsorption measurements of H-3(+) toward W33A and GL 2591.