H 3 O + Revisited

A line at 307.19241 GHz was tentatively identified with the (J,K)= (1,1)−→ (2, 1)+ line (hereafter called the P(2,1) line) of para-H3O+ (Wootten et al. 1986, Hollis et al. 1986). New observations of this line in OMC1 with the NRAO 12 m telescope show the emission source to be unresolved by the 30” beam. Upper limits to the strength of the (3,2)+→ (2,2)− transition of para-H3O+ at 364.797427 GHz have been obtained with the MWO 4.9m telescope. Study of the excitation of this symmetric top molecule indicates that the excitation temperature of the P(2,1) line normally lies well below the rotational temperature of the molecule, rendering the line abnormally weak. Previous abundance limits for H3O+, based upon LTE analyses, should probably be revised upward.