Age sequence in small clusters associated with bright-rimmed clouds

Abstract

Bright-rimmed clouds (BRCs) found in H II 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 H II region (“small-scale sequential star formation”). In order to quantitatively test this hypothesis we undertook ВVIc photometry of four BRC aggregates. The amounts of interstellar extinction and reddening for each star were estimated by using JHKs photometry. We then constructed reddening-corrected V ⁄ V –Ic 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.