INTERSTELLAR H3O+ AND ITS RELATION TO THE O2 AND H2O ABUNDANCES

We present an interstellar medium study of the three reasonably accessible low-lying submillimeter lines of the H3O+ molecular ion at 396, 364, and 307 GHz. Emission at the frequencies of the latter two lines has previously been reported, but only from the spectroscopically confused regions Orion/KL and SgrB2. This work reports the first study of the 396 GHz line and, possibly more significantly, the detection of H3O+ lines in less confused regions. In particular, there is a clear detection of H3O+ in the W3 IRS 5 cloud where, in the case of the 396 GHz line at least, no other spectral features are visible in the observing band. An analysis of the H3O+ line ratios shows that under high-density (n(H-2) almost-equal-to 10(6)-10(7) cm-3), high-temperature conditions (T(kin) greater than or similar to 50 K), the 396 GHz line is about a factor of 2 stronger than the 364 GHz line, with the 307 GHz line much weaker, in agreement with the observations on sources such as W3 IRS 5, Orion/KL, and G34.3 + 0.15. However, for lower densities, the excitation of the 364 GHz line can be very sensitive to dust radiation pumping and it is shown that this seems to be the case in Sgr B2, resulting in the 364 GHz line being a factor of 2-3 stronger than the 396 GHz line. Under almost all conditions the 307 GHz line will be weak, the exception being for densities n(H-2) greater than or similar to 10(7) CM-3. H3O+ is an important molecular ion, intimately involved in the chemistry of oxygen and water. The fractional interstellar abundance, x(H3O+), is shown to be typically in the range from 1 x 10(-10) to 3 x 10(-9), in agreement with, or slightly below the theoretical values of (1-4) x 10(-9). From a simple chemical model we show that if H3O+ is the main precursor Of 02 and H2O, x(H2O) almost-equal-to 10(3) x(H3O+) and X(O2) almost-equal-to 10(4) x (H3O+), SO providing order-of-magnitude predictions of the H2O and O2 abundances for the clouds observed in H3O+. The predicted X(O2) almost-equal-to 10(-6)-10(-5) are close to the measured upper limits for 02. The estimated water abundances, x(H2O) almost-equal-to 10(-7)-10(-6), are somewhat lower than those derived from other observations and suggest that additional processes may play a role in the water production. A possible mechanism is grain surface formation of H2O followed by release back into the gas phase when the grains are heated to sufficiently high temperatures by radiation from newly-formed stars.