1999A&A...347..957J

We report IRAM 30-m observations of DCN and DCO⁺ towards the galactic center molecular clouds Sgr B2 and Sgr A. We compare our results with simultaneous measurements of H¹³CN and HC¹⁸O⁺ in order to derive estimates of the [DCN]/[HCN] and [DCO⁺]/[HCO⁺] abundance ratios. We mapped Sgr B2 in DCN(2-1) and DCN(3-2) and we compared HC¹⁵N with DCN for one position of our maps. The uncertainties in such measurements turn out to be large, due to optical depth effects, in the case of H¹³CN, and, in the case of Sgr B2, due to the strong foreground absorption in H¹³CN, HC¹⁵N and HC¹⁸O⁺. Taking these effects into account, we conclude that [DCN]/[HCN] towards the molecular ridge in Sgr B2 is 1.3x10^–4 (uncertain by a factor 2) and towards the Sgr A molecular cloud is <6x10^–4. Towards Sgr B2, we also estimate [DCO⁺]/[HCO⁺] to be 2x10^–4 again with a factor of 2 uncertainty. We have carried out model calculations to estimate the amount of deuterium enhancement in these species in these clouds. We find, for temperatures between 40 and 120K, that one expects D to be enhanced in DCN in the galactic center by a factor between 12 and 140 over the actual ratio of [D/H]. The corresponding enhancement for DCO⁺ would be between 23 and 460. Based on these results, we conclude that the ratio of [D/H] in the Sgr B2 cloud is 5x10^–6 with a factor of 10 uncertainty. This result, while it has large uncertainty, suggests that deuterium is roughly one order of magnitude less abundant in the galactic center than in the solar neighbourhood. We briefly discuss possible consequences of this for the stellar initial mass function in the galactic center.