  Context: In the last years, the H2D+ and D2H+ molecules have gained great
attention as probes of cold and depleted dense molecular cloud cores. These
ions are at the basis of molecular deuterium fractionation, a common
characteristic observed in star forming regions. H2D+ is now routinely
observed, but the search for its isotopologue D2H+ is still difficult because
of the high frequency of its ground para transition (692 GHz). Aims: We have
observed molecular transitions of H2D+ and D2H+ in a cold prestellar core to
characterize the roots of deuterium chemistry. Methods: Thanks to the sensitive
multi-pixel CHAMP+ receiver on the APEX telescope where the required excellent
weather conditions are met, we not only successfully detect D2H+ in the H-MM1
prestellar core located in the L1688 cloud, but also obtain information on the
spatial extent of its emission. We also detect H2D+ at 372 GHz in the same
source. We analyse these detections using a non-LTE radiative transfer code and
a state-of-the-art spin-dependent chemical model. Results: This observation is
the first secure detection of D2H+ in space. The emission is moreover extended
over several pixels of the CHAMP+ array, i.e. on a scale of at least 40'',
corresponding to ~ 4800 AU. We derive column densities on the order of
1e12-1e13 cm-2 for both molecules in the LTE approximation depending on the
assumed temperature, and up to two orders of magnitude higher based on a
non-LTE analysis. Conclusions: Our modeling suggests that the level of CO
depletion must be extremely high (>10, and even >100 if the temperature of the
core is around 10 K) at the core center, in contradiction with CO depletion
levels directly measured in other cores. Observation of the H2D+ spatial
distribution and direct measurement of the CO depletion in H-MM1 will be
essential to confirm if present chemical models investigating the basis of
deuterium [...].
