Age Sequence in Small Clusters Associated with Bright-Rimmed Clouds

Abstract

Bright-rimmed clouds (BRCs) found in HII regions are probable sites of triggered star formation due to compression by ionization/shock fronts, and it is hypothesized that star formation proceeds from the exciting star(s) side outward of the HII region (``small-scale sequential star formation''). In order to quantitatively test this hypothesis we undertook $BVI_\mathrm{c}$ photometry of four BRC aggregates. The amounts of interstellar extinction and reddening for each star were estimated by using $JHK_\mathrm{s}$ photometry. We then constructed reddening-corrected $V/V-I_\mathrm{c}$ color-magnitude diagrams, where the age of each star was derived. All of the stars turned out to be a few tenths to a few Myr old. Although the scatters were large and the numbers of the sample stars are small, we found a clear trend that the stars inside or in the immediate vicinity of the bright rim are younger than those outside it in all four aggregates, confirming the hypothesis in question.