On the basis of near-infrared imaging observations , we derived visual extinction ( A _ { V } ) distribution toward ten Bok globules through measurements of both the color excess ( E _ { H - 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 ten 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 the gravitational collapse .
We found that more than half of starless globules are located near the critical state ( \xi { } _ { { max } } = 6.5 \pm 2 ) .
Thus , we suggest that a nearly critical Bonnor-Ebert sphere characterizes the typical density structure of starless globules .
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 non-thermal 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 ( C ^ { 18 } O and N _ { 2 } H ^ { + } ) toward the same ten 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 .