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 ga s. High spectral resolution observations at 12 positions show the line to have a median intrinsic velocity dispersion of 0.088 km s-1 as com pared 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 l ine has an observed FWMH of only 0.14 km s-1, corresponding to an intr insic 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 coincidenc e, together with the similar velocities of the transitions, suggests t hat these two transitions are tracing the same material within these c ores. Adopting a two-component model for the velocity dispersion of th ese lines, we estimate the kinetic temperature and the nonthermal velo city dispersion in these cores. The mean kinetic temperature derived f rom the velocity dispersions in the cores is 9.2 +/- 0.2 K, in good ag reement with the value of 10.2 +/- 0.6 K determined from NH3 observati ons of the same objects. The kinetic temperature of 9.2 K is also simi lar to the mean excitation temperature of the HC3N line, 8.4 K. This s imilarity suggests that the transition is close to being thermalized a nd indicates that the number density in these cores is close to 3 x 10 (4) cm-3. These new temperature and nonthermal velocity dispersion det erminations indicate that the nonthermal velocity dispersion increases as the 0.6 +/- 0.1 power of the map size, while the total velocity di spersion increases as the 0.05 +/- 0.07 power of the map size. In an e quilibrium model, these relations correspond to a number density profi le n approximately r-1.9, which is very close to that expected for an isothermal sphere. The magnetic field strength whose energy density eq uals that of the nonthermal motions is typically 10 muG over the range of core sizes 0.02-0.11 pc.