A search for interstellar H3O+.

The P(2,1) line of H₃O⁺, a key species in ion-molecule chemistry, has been sought in the interstellar medium and in Halley's Comet. In OMC1 and Sgr B2, a line was detected which may possibly be attributed to H₃O⁺. Verification of this identification must be accomplished through observation of the P(3,2) line at 364 GHz, or detection of isotopic variants. If the detected line arises from H₃O⁺, it could be possible to deduce a fractional abundance X(H₃O⁺) in OMC1 and in Sgr B2 of ≡10^-9 and a production rate in Comet Halley of Q(H₃O⁺) < 10²⁸s^-1. These results would place H₃O⁺ among the more abundant molecular ions in the interstellar gas in agreement with theoretical predictions.