Abstract:
The aim of this paper is to quantitatively testify the ‘small-scale sequential star formation’
hypothesis in and around bright-rimmed clouds (BRCs). As a continuation of the recent attempt
by Ogura et al., we have carried out BV Ic photometry of four more BRC aggregates along with
deeper re-observations of two previously observed BRCs. Again, quantitative age gradients
are found in almost all the BRCs studied in the present work. Archival Spitzer/Infrared Array
Camera data also support this result. The global distribution of near-infrared excess stars in
each H II region studied here clearly shows evidence that a series of radiation-driven implosion
processes proceeded in the past from near the central O star(s) towards the peripheries of the
H II region. We found that in general weak-line T-Tauri stars (WTTSs) are somewhat older than
classical T-Tauri stars (CTTSs). Also the fraction of CTTSs among the T-Tauri stars (TTSs)
associated with the BRCs is found to decrease with age. These facts are in accordance with the
recent conclusion by Bertout, Siess & Cabrit that CTTSs evolve into WTTSs. It seems that in
general the equivalent width of Hα emission in TTSs associated with the BRCs decreases with
age. The mass function (MF) of the aggregates associated with the BRCs of the morphological
type ‘A’ seems to follow that found in young open clusters, whereas ‘B/C’-type BRCs show
significantly steeper MF.
