Probable detection of H

Context. The presence of H2D+ in dense cloud cores underlies ion-molecule reactions that strongly enhance the deuterium fractionation of many molecular species. Aims.We determine the H2D+ abundance in one starless core, Barnard 68, that has a particularly well established physical, chemical, and dynamical structure. Methods.We observed the ortho-H2D+ ground-state line 110–111, the N2H+ $J=4$–3 line, and the H13CO+ 4–3 line with the APEX telescope. Results.We report the probable detection of the o-H2D+ line at an intensity $T_{\rm mb}=0.22\pm 0.08$ K and exclusively thermal line width, and find only upper limits to the N2H+ 4–3 and H13CO+ 4–3 intensities. Conclusions.Within the uncertainties in the chemical reaction rates and the collisional excitation rates, chemical model calculations and excitation simulations reproduce the observed intensities and that of o-H2D+ in particular