Photometric determination of the mass accretion rate of pre-main sequence stars. IV. Recent star formation in NGC 602

摘要

We have studied the young stellar populations in NGC 602, in the SmallMagellanic Cloud, using a novel method that we have developed to combine HubbleSpace Telescope photometry in the V, I, and Halpha bands. We have identifiedabout 300 pre-main sequence (PMS) stars, all of which are still undergoingactive mass accretion, and have determined their physical parameters (effectivetemperature, luminosity, age, mass and mass accretion rate). Our analysis showsthat star formation has been present in this field over the last 60 Myr. Inaddition, we can recognise at least two clear, distinct, and prominent episodesin the recent past: one about 2 Myr ago, but still ongoing in regions of highernebulosity, and one (or more) older than 30 Myr, encompassing both starsdispersed in the field and two smaller clusters located about 100 arcsec northof the centre of NGC 602. The relative locations of younger and older PMS starsdo not imply a causal effect or triggering of one generation on the other. Thestrength of the two episodes appears to be comparable, but the episodeoccurring more than 30 Myr ago might have been even stronger than the currentone. We have investigated the evolution of the mass accretion rate M_acc as afunction of the stellar parameters finding that log M_acc ~ -0.6 log t + log m+ c, where t is the age of the star, m its mass and c is a decreasing functionof the metallicity.