A SEARCH FOR THE ROTATIONAL TRANSITIONS OF H2D+ AT 1370 GHZ AND H3O+ AT 985 GHZ

We have searched for the 1370 GHz lowest rotational transition of the molecular ion H2D+ in NGC 2264, W3, and the IRc2 region of M42. No emission lines were seen from any source, but an absorption feature was detected toward IRc2. If we identify the line as the 1(-1)-0(00) transition of para H-2D+, then the total column density is (6 +/- 3) x 10(13) cm-2 for T approximately 30 K, and the fractional abundance is 3 x 10(-11). The LSR velocity of 3.7 km s-1 and the measured line width of 13 km s-1 are consistent with the dynamical parameters of the hot core source. However, the physical parameters deduced from the data (T < 50 K, n(H2) < 10(6) cm-3, spatial extent >25'') differ from those derived from millimeter-wave observations of the hot core condensation. Our interpretation suggests that significant amounts of low-density gas are associated with this region and that the material is cold enough for enhanced deuterium fractionation to occur. A search was also made for the 985 GHz transition of ortho H3O+ in W3 and IRc2 with negative results.