Deuterated Molecules in Translucent and Dark Clouds

We present observations of 10 deuterated molecular species in the dark clouds TMC-1, L183, and the translucent object CB 17, as well as a subset of species in TMCNH₃ and other objects. With sensitive observations of the J = 1-0 and 2-1 transitions of N₂D⁺, DCN, and DNC, we have been able to derive molecular constants that include the complex nuclear quadrupole hyperfine splitting in these species, which is essential to determine accurate abundances. The spectroscopic results have required, in turn, new radiative transport techniques to handle the hyperfine effects. Our abundance determinations also utilize sensitive observations of secondary isotopomers involving ¹³C, ¹⁸O, and ¹⁵N. Compared with earlier molecular D/H ratios in the literature, these innovations have resulted in radically different values in some cases (N₂D⁺/N₂H⁺ in TMC-1 and TMCNH₃; NH₂D/NH₃ in TMC-1; DCN/HCN in CB 17), and important modifications in others (C₃HD/C₃H₂ in TMC-1). The new techniques usually produce deuteration ratios lower than those obtained earlier by simpler methods. Thus, in addition to the special cases noted above, our results are generally lower than previous ones by factors of typically 2. We also find that deuteration occurs only in regions of high density, while nondeuterated species generally reside at lower densities. A recently proposed model of translucent clouds as low-density objects containing embedded small, high-density fragments explains the observations.

     To study the chemistry of deuterated species, we have used the New Standard Model, modified to include all monodeuterated species, and now containing 9930 reactions and 610 species. Our models explore the dependence of the molecular D/H ratios upon temperature, density, ionization rate, extinction, epoch, and elemental abundances. Within the uncertainties, we find agreement between observed and modeled ratios for nearly all species in nearly all sources. Our results generally agree with those of Roberts & Millar in a recent, similar study. We find significantly higher ratios in L183 than in TMC, and intermediate values in CB 17. With our lower values in general, however, we believe that L183 is "normal" for a cold dark cloud, CB 17 is typical of a slightly warmer translucent object, and the TMC region is perhaps underdeuterated in general, certainly strongly so in the case of N₂H⁺ and NH₃. These "anomalous" cases have no plausible single explanation in terms of gas-phase chemistry at this time. Grain processes are implicated.