Probable detection of H2D+ in the starless core Barnard 68

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 1(10)-1(11), the N2H+ J = 4-3 line, and the (HCO+)-C-13 4-3 line with the APEX telescope. Results. We report the probable detection of the o-H2D+ line at an intensity T-mb = 0.22 +/- 0.08 K and exclusively thermal line width, and find only upper limits to the N2H+ 4-3 and (HCO+)-C-13 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