Cold dense clouds

Classification Cold dense cloud Outflowing C-rich circum-stellar envelope Warm molecular cloud/star-forming region Giant molecular cloud... 

In 1983 Pauls et al. detected emission from metastable transitions of NHs in the Orion-KL region, and concluded that it came from a hot, dense region (T jo = 150 K, n = 10 cm ). They calculated the fractional abundance of ammonia to be 10, two orders of magnitude larger than the typical value in cold, dense clouds, and suggested that the enhancement arose from the evaporation of molecular mantles on interstellar grains. 

The gas phase chemistry in the collapse phase will not be described here but is typical of cold, dense clouds (see Herbst, this volume and references therein). Accretion is allowed but, because of the uncertainty, no continually acting desorption mechanisms have been included. An atom, radical or molecule from the gas will collide with, and stick to, a dust grain at a rate given by ... 

This is similar to that found for cold, dense clouds (Brown and Charnley, 1988) and is in agreement with the fit of the 3 pm feature towards the BN object obtained by Knacke et a). (1982) using an ice mixture of water and ammonia in the ratio of 4 1. The CO is accreted from the gas phase solid CO has been widely observed in the Taurus complex (Whittet et al., 1985). 

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