2006A&A...454L..59H

The presence of H₂D⁺ in dense cloud cores underlies ion-molecule reactions that strongly enhance the deuterium fractionation of many molecular species. We determine the H₂D⁺ abundance in one starless core, Barnard 68, that has a particularly well established physical, chemical, and dynamical structure. 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. 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 H¹³CO⁺ 4-3 intensities. 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.