Near-Infrared Imaging Survey of Bok Globules: Density Structure

On the basis of near-infrared imaging observations, we derived visualextinction (Av) distribution toward ten Bok globules through measurements ofboth the color excess (E_{H-K}) and the stellar density at J, H, and Ks (starcount). Radial column density profiles for each globule were analyzed with theBonnor-Ebert sphere model. Using the data of our ten globules and four globulesin the literature, we investigated the stability of globules on the basis ofxi_max, which characterizes the Bonnor-Ebert sphere as well as the stability ofthe equilibrium state against the gravitational collapse. We found that morethan half of starless globules are located near the critical state (xi_max =6.5 +/- 2). Thus, we suggest that a nearly critical Bonnor-Ebert spherecharacterizes the typical density structure of starless globules. Remainingstarless globules show clearly unstable states (xi_max > 10). Since unstableequilibrium states are not long maintained, we expect that these globules areon the way to gravitational collapse or that they are stabilized by non-thermalsupport. It was also found that all the star-forming globules show unstablesolutions of xi_max >10, which is consistent with the fact that they havestarted gravitational collapse. We investigated the evolution of a collapsinggas sphere whose initial condition is a nearly critical Bonnor-Ebert sphere. Wefound that the column density profiles of the collapsing sphere mimic those ofthe static Bonnor-Ebert spheres in unstable equilibrium. The collapsing gassphere resembles marginally unstable Bonnor-Ebert spheres for a long time. Wefound that the frequency distribution of xi_max for the observed starlessglobules is consistent with that from model calculations of the collapsingsphere.