Near-infrared imaging survey of Bok globules: Density structure

On the basis of near-infrared imaging observations, we derived the visual extinction ( AV) distribution toward 10 Bok globules through measurements of both the color excess (EH-K) and the stellar density at J, H, and K-s ( star count). Radial column density profiles for each globule were analyzed with the Bonnor-Ebert sphere model. Using the data of our 10 globules and four globules in the literature, we investigated the stability of globules on the basis of xi(max), which characterizes the Bonnor-Ebert sphere, as well as the stability of the equilibrium state against gravitational collapse. We found that more than half the starless globules are located near the critical state (xi(max) = 6.5 +/- 2). Thus, we suggest that a nearly critical Bonnor-Ebert sphere characterizes the typical density structure of starless globules. The remaining starless globules show clearly unstable states (xi(max) > 10). Since unstable equilibrium states are not long maintained, we expect that these globules are on the way to gravitational collapse or that they are stabilized by nonthermal support. It was also found that all the star-forming globules show unstable solutions of xi(max) > 10, which is consistent with the fact that they have started gravitational collapse. We investigated the evolution of a collapsing gas sphere whose initial condition is a nearly critical Bonnor-Ebert sphere. We found that the column density profiles of the collapsing sphere mimic those of the static Bonnor-Ebert spheres in unstable equilibrium. The collapsing gas sphere resembles marginally unstable Bonnor-Ebert spheres for a long time. We found that the frequency distribution of xi(max) for the observed starless globules is consistent with that from model calculations of the collapsing sphere. In addition to the near-infrared observations, we carried out radio molecular line observations ( (CO)-O-18 and N2H+) toward the same 10 globules. We confirmed that most of the globules are dominated by thermal support. The line width of each globule was used to estimate the cloud temperature including the contribution from turbulence, with which we estimated the distance to the globules from the Bonnor-Ebert model fitting.