1999ApJ...521L..67S

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 3x10^–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⁺₃ 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⁺₃ abundance that are consistent with the abundances derived from H₂D⁺. The corresponding limits on the H⁺₃ column density agree with recent near-infrared absorption measurements of H⁺₃ toward W33A and GL 2591.