AAS Meeting #193 - Austin, Texas, January 1999
Session 71. Molecular Clouds
Display, Friday, January 8, 1999, 9:20am-6:30pm, Exhibit Hall 1

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[71.08] Submillimeter Observations of Protostars: High Excitation Transition studies of DCO+ and DCN

R.Y. Shah (University of Virginia \& NRAO), H.A. Wootten (NRAO)

We present new high frequency data towards low--mass star forming regions. Our experiment uses high--frequency line observations of ionized molecular radicals to estimate the electron fraction in the central regions of protostars. Such values bear direct relevance to ambipolar diffusion models for protostellar collapse in the ISM.

Current 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. Lacking high spatial resolution, one must use submillimeter transitions of ionized radicals such as {\rm DCO+} and {\rm H13CO+} to properly estimate the ionization fraction in molecular cores.

We have observed the {\rm DCO+} and DCN J=5arrow 4, 4arrow 3, &\ 3arrow 2 transitions toward several well known low--mass Class 0/I objects in the Serpens, Ophiuchus, and Taurus molecular cloud/star--formation complexes using the Caltech Submillimeter Observatory. Combined with existing {\rm H13CO+} measurements (from previous unpublished work as well as from the literature) we will present estimates of electron fractions in the regions of protostars where infall is likely to occur according to standard inside-out collapse models. Furthermore, we will ascertain the relative influence of grain versus ion--molecule chemistry in these low--mass protostars. This provides important contraints on chemical evolution models.

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