Deuterated Molecules in Translucent and Dark Clouds

Abstract

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.