THE LINE WIDTH-SIZE RELATION IN MASSIVE CLOUD CORES

We report (CO)-C-13 and (CO)-O-18 line observations and maps in Orion A (L1641) and B (L1630). Together with already published observations, these data are used to study the line width-map size relation in massive star-forming regions. The nonthermal component of the line width (Delta upsilon(NT)) in Orion cores follows the trend Delta upsilon(NT)similar to R(q) with q = 0.21 +/- 0.03, significantly different from q = 0.53 +/- 0.07 found in low-mass cores. These relations are analyzed in the context of an equilibrium model of a spherically symmetric dense core which incorporates both thermal and nonthermal (''TNT'') motions. The internal consistency of the TNT model and Delta upsilon(NT)-R data is shown. We present general formulae for the TNT model and apply them to the observational data. Differences in the slope and in the intercept of the log Delta upsilon(NT)-log R relation between massive and low-mass cores imply significant differences in density structure, pressure profile, mass infall rate, and probably in the masses of stars which form. In particular, massive cores are denser and have steeper density profiles than low-mass cores. Visual extinction values predicted by the TNT model for low mass and massive cores (3.3 and 16 mag, respectively) are in good agreement with available observational estimates for similar objects. The higher density and pressure in massive cores lead to values for the infall time for 1 M. of similar to 7 x 10(4) yr, similar to 6 times shorter than in low-mass cores. Massive dense cores associated with embedded young stellar objects have physical properties almost identical to neighboring massive starless cores. Thus, the formation of a star or a small group of stars does not significantly affect the initial physical conditions of the associated molecular cloud core. On the other hand, line widths of ammonia cores become narrower as the distance from embedded young stellar clusters increases. In particular, the massive core farthest away from embedded clusters is mostly thermal and its kinetic temperature is similar to 10 K, similar to 2 times lower than the typical kinetic temperature of massive cores.