Coherent Dense Cores

In a study of the velocity dispersion of molecular gas in maps of four dense cores, we Ðnd that : 3 (1) within the interiors of dense cores, the line widths are roughly constant at a value slightly but signiÐcantly higher than a purely thermal line width ; and (2) at the edges of the maps of the dense cores, it appears that the line width starts to increase. We suggest that these dense cores are ““ coherent ÏÏ in that the nonthermal, turbulent, contributions to the line width are so small that observed velocity dispersion is nearly independent of scale within the cores. In the second paper of this series (Goodman et al.), by analyzing maps of the coresÏ environments, we Ðnd an apparent transition to this coherent regime from a more turbulent one, at about the size scale of a FWHM contour, or D0.1 pc. 3 Analysis of velocity gradients in dense cores and their environs indicates that the cores appear to spin independently of their surroundings, along an axis not obviously related to their shape. Comparison of gradients implied by the relative velocities of high-density cores in complexes and gradients in the extended low-density gas in these complexes suggests a picture in which the coherent cores behave as (independently spinning) test particles, Ñoating along in a turbulent Ñow. An appendix to this paper presents a new algorithm for predicting the intrinsic width of the 18 hyperÐne lines in the inversion spectrum from a Gaussian Ðt to the main hyperÐne blend and an esti3 mate of the optical depth. Subject headings : ISM: clouds È ISM: kinematics and dynamics È ISM: molecules È ISM: structure È turbulence