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R. Y. Shah (University of Virginia/NRAO), H. A. Wootten (NRAO)
We report on observations of the {\rm DCO+} and DCN J=5arrow 4, 4arrow 3, &\ 3arrow 2 transitions toward several well known Class 0/I objects in the Serpens, Ophiuchus, and Taurus molecular cloud/star--formation complexes using the Caltech Submillimeter Observatory and the NRAO 12 Meter Telescope. Our observations allow us to study both the electron abundance and deuterium fractionation in the densest material within protostellar cores.
Past studies of protostars have concentrated heavily on the low excitation energy transitions of {\rm DCO+}, {\rm HCO+}, and other ionic tracers of the electron fraction. However, such lines, most easily visible in the colder outer, quiescent envelopes of the ISM, clearly cannot sample the electron fraction in inner core regions of protostars. We employed high--excitation line observations of molecules to estimate the electron fraction in the central regions of protostars and discuss the relevance of these values to their evolution. At the high frequencies of these lines, the CSO beamwidth is only 20". Together with high excitation temperatures, this ensures that we observe only the densest regions within protostars.
We also compare the observations of {\rm DCO+} NH2D and DCN in several objects to ascertain how physical conditions in molecular cores and the onset of star--formation affect the chemistry of deuterated species. Observed deuterium fractionations of species such as {\rm NH3}, HDO, {\rm CH3OH}, {\rm HCO+}, and HCN in well--examined Galactic clouds range between 0.001 and 0.01. It has been shown that non--equilibrium processes, in addition to traditional ion--molecule chemistry, can contribute to the observed deuterium fractionation ratios. In cold clouds, the deuterated molecules should freeze onto grains at low temperatures. As stars form within the collapsing cloud, grains are warmed and the frozen mantles are released into the gas phase. Warm star-forming cores also display enhanced fractionation, as observed. {\rm DCO+} reflects the relative influence of ion--molecule chemistry; DCN is largely a grain product. A comparison between the two throughout regions of various temperatures and evolutionary stages provides important constraints on the overall chemical structure of the sources.
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