THERMAL MATERIAL IN DENSE CORES - A NEW NARROW-LINE PROBE AND TECHNIQUE OF TEMPERATURE DETERMINATION

A survey of dense cores in nearby dark clouds indicates that the J = 4 --> 3 transition of HC3N is a good tracer of very quiescent, dense gas. High spectral resolution observations at 12 positions show the line to have a median intrinsic velocity dispersion of 0.088 km s-1 as compared to 0.100 km s-1 for new high spectral resolution measurements of the NH3 (1, 1) transition at the same positions. The narrowest HC3N line has an observed FWMH of only 0.14 km s-1, corresponding to an intrinsic velocity dispersion of only 0.058 +/- 0.002 km s-1. Maps of two regions show the HC3N emission to be spatially coincident with the NH3 emission and to be comparable, or smaller, in extent. This coincidence, together with the similar velocities of the transitions, suggests that these two transitions are tracing the same material within these cores. Adopting a two-component model for the velocity dispersion of these lines, we estimate the kinetic temperature and the nonthermal velocity dispersion in these cores. The mean kinetic temperature derived from the velocity dispersions in the cores is 9.2 +/- 0.2 K, in good agreement with the value of 10.2 +/- 0.6 K determined from NH3 observations of the same objects. The kinetic temperature of 9.2 K is also similar to the mean excitation temperature of the HC3N line, 8.4 K. This similarity suggests that the transition is close to being thermalized and indicates that the number density in these cores is close to 3 x 10(4) cm-3. These new temperature and nonthermal velocity dispersion determinations indicate that the nonthermal velocity dispersion increases as the 0.6 +/- 0.1 power of the map size, while the total velocity dispersion increases as the 0.05 +/- 0.07 power of the map size. In an equilibrium model, these relations correspond to a number density profile n approximately r-1.9, which is very close to that expected for an isothermal sphere. The magnetic field strength whose energy density equals that of the nonthermal motions is typically 10 muG over the range of core sizes 0.02-0.11 pc.