Extended emission of D2H+ in a prestellar core

Context. In the past years, the H2D+ and D2H+ molecules have gained attention as probes of cold and depleted dense molecular cloud cores. These ions are 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 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 analyze 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 similar to 4800 AU. We derive column densities on the order of 10(12)-10(13) 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, contradicting 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 whether present chemical models investigating the basis of deuterium fractionation of molecules need to be revised.