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Session 20 - Chemistry & Physical Process in the ISM.
Display session, Wednesday, January 07
We present a molecular cloud survey of 86 GHz and 110 GHz emission lines of NH_2D observed with the NRAO 12--meter telescope. We compare the observations of NH_2D and NH_3 in several objects to ascertain how physical conditions in molecular cores and the onset of star--formation affect the chemistry of deutero-ammonia. Observed abundances relative to NH_3 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, contribute to the observed NH_2D/NH_3 ratios. In cold clouds, the abundant deutero-ammonia we observe should freeze onto grains at low temperatures. As stars form within the collapsing cloud, grains are warmed and the frozen deutero-ammonia is relased into the gas phase. Warm star-forming cores also display enhanced fractionation, as observed.
Cold cores such as those in TMC1 provide ideal laboratories to explore the issues of cloud chemistry because of their relative structural simplicity and lack of an embedded source. Furthermore, one core shows enhanced ammonia emission while an adjacent core harbors high abundances of cyanopolyynes. Study of the relative deuterium fractionation of the cores can help to understand these differences. Preliminary results for the TMC1 cores indicate that the [NH_2D]/[NH_3] is less than or of order 0.001, lower than in other dark cloud cores or OMC1. The extent of deuterium fractionation offers a useful aid in diagnosing different core chemistries.
Program listing for Wednesday