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 B V I_c photometry of four BRC aggregates. The amounts of interstellar extinction and reddening for each star were estimated by using J H K_s photometry. We then constructed reddening-corrected V/V−I_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.