The Initial Conditions of Clustered Star Formation. III. The Deuterium Fractionation of the Ophiuchus B2 Core

We present N₂D⁺ 3-2 (IRAM), and H₂D⁺ 1₁₁-1₁₀ and N₂H⁺ 4-3 (JCMT) maps of the small cluster-forming Ophiuchus B2 core in the nearby Ophiuchus molecular cloud. In conjunction with previously published N₂H⁺ 1-0 observations, the N₂D⁺ data reveal the deuterium fractionation in the high-density gas across Oph B2. The average deuterium fractionation R_D = N(N₂D⁺)/N(N₂H⁺) ~ 0.03 over Oph B2, with several small scale R_D peaks and a maximum R_D = 0.1. The mean R_D is consistent with previous results in isolated starless and protostellar cores. The column density distributions of both H₂D⁺ and N₂D⁺ show no correlation with total H₂ column density. We find, however, an anticorrelation in deuterium fractionation with proximity to the embedded protostars in Oph B2 to distances gsim0.04 pc. Destruction mechanisms for deuterated molecules require gas temperatures greater than those previously determined through NH₃ observations of Oph B2 to proceed. We present temperatures calculated for the dense core gas through the equating of non-thermal line widths for molecules (i.e., N₂D⁺ and H₂D⁺) expected to trace the same core regions, but the observed complex line structures in B2 preclude finding a reasonable result in many locations. This method may, however, work well in isolated cores with less complicated velocity structures. Finally, we use R_D and the H₂D⁺ column density across Oph B2 to set a lower limit on the ionization fraction across the core, finding a mean x _e,lim >~ few × 10^-8. Our results show that care must be taken when using deuterated species as a probe of the physical conditions of dense gas in star-forming regions.