Coherent Dense Cores. I. NH3Observations

Abstract

In a study of the velocity dispersion of molecular gas in NH3 maps of four dense cores, we find that: (1) within the interiors of dense cores, the line widths are roughly constant at a value slightly but significantly 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 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 cores' environments, we find an apparent transition to this coherent regime from a more turbulent one, at about the size scale of a FWHM NH3 contour, or ~0.1 pc. 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, floating along in a turbulent flow. An appendix to this paper presents a new algorithm for predicting the intrinsic width of the 18 hyperfine lines in the NH3 inversion spectrum from a Gaussian fit to the main hyperfine blend and an estimate of the optical depth.