Ground-based searches for interstellar H_(2)D^+

Abstract

We present ground-based searches for the 1_(10) - 1_(11) line of interstellar H_(2)D^(+) at 372 GHz which are more sensitive than those obtained from the Kuiper Airborne Observatory by factors of 3-4 for extended sources and by more than two orders of magnitude for compact sources. The line was not detected in a variety of interstellar clouds, including NGC 2264 toward which a possible detection had been suggested previously. The inferred H_(2)D^(+) abundance limits of 10^(-10) - 10^(-11) are still consistent with, but approach the abundances predicted by chemical models. Simultaneous observations of the DCO^(+) 3-2 and N_(2)H^(+) 4-3 lines have been used to place additional limits on the H_(3)^(+) abundance, and suggest 10^(-11) < x(H_(3)^(+))< 10^(-9). The N_(2)H^(+) data also indicate that for NGC 2264, but perhaps not for the other sources, gas-phase N_2 contains a substantial fraction of the available nitrogen in the cloud.