Model of Taurus molecular cloud 1 based on HC3N observations

We have observed five rotational transitions of cyanoacetylene in the ark cloud TMC 1 and, using a Monte Carlo radiative transfer code, have produced a model of this source which is consistent with these observations. The interaction between radiative trapping and population equilibrium was solved for the 15 lowest rotational levels of HC3N, assuming a cylindrical geometry for the source. The model, which we envisage as representing a compact condensation embedded well within Heiles' Cloud 2, is characterized by a relatively high excitation core and a lower excitation halo. Although we cannot rule out the possibility that the higher excitation of the core can be attributed solely to enhanced density there, we were unable to fit the observations well without involving a temperature enhancement as well. Transitions higher in frequency than the J = 8→7 line originate predominantly in the core which is characterized by a kinetic temperature of 17 K and H2 density of 6 x 104 cm-3. Corresponding values for the halo are 7 K and 8 x 103. The column density of HC3N in TMC 1 is estimated to be 1.1 x 1014 cm-2