Abstract
We report the detection of the 110-111 ground-state transition of ortho-H2D+ 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 H2D+ 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 H2D+ data provide direct evidence in support of low-temperature chemical models in which H2D+ is enhanced by the reaction of H and HD. The H2D+ enhancement toward NGC 1333 IRAS 4A is also reflected 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 NGC 1333 IRAS 4A than in the other sources, suggesting significant depletion of N2. The N2H+ data provide independent lower limits on the H abundance that are consistent with the abundances derived from H2D+. The corresponding limits on the H column density agree with recent near-infrared absorption measurements of H toward W33A and GL 2591.
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