ar X iv : a st ro - p h / 06 06 10 8 v 1 6 J un 2 00 6 Detection of H 2 D + in a massive prestellar core in Orion B ∗

Aims. The purpose of this study is to examine the prediction that the deuterated H + 3 ion, H 2 D + , can be found exclusively in the coldest regions of molecular cloud cores. This is also a feasibility study for the detection of the ground-state line of ortho-H 2 D + at 372 GHz with APEX. Methods. The (1 10 → 1 11) transition of H 2 D + at 372 GHz was searched towards selected positions in the massive star forming cloud OriB9, in the dark cloud L183, and in the low-to intermediate mass star-forming cloud R CrA. Results. The line was detected in cold, prestellar cores in the regions of OriB9 and L183, but only upper limits were obtained towards other locations which either have elevated temperatures or contain a newly born star. The H 2 D + detection towards OriB9 is the first one in a massive star-forming region. The fractional ortho-H 2 D + abundances (relative to H 2) are estimated to be ∼ 1 10 −10 in two cold cores in OriB9, and 3 10 −10 in the cold core of L183. Conclusions. The H 2 D + detection in OriB9 shows that also massive star forming regions contain very cold prestellar cores which probably have reached matured chemical composition characterized, e.g., by a high degree of deuterium fractionation. Besides as a tracer of the interior parts of prestellar cores, H 2 D + may therefore be used to put contraints on the timescales related to massive star formation.