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

TL;DR

This study uses Hubble photometry to identify and analyze about 300 pre-main sequence stars in NGC 602, revealing multiple star formation episodes over the past 60 million years and examining their accretion properties.

Contribution

It introduces a novel photometric method to determine physical parameters and accretion rates of PMS stars in NGC 602, highlighting multiple star formation episodes and their characteristics.

Findings

Star formation in NGC 602 has occurred over the last 60 Myr.

Two main star formation episodes identified: ~2 Myr ago and >30 Myr ago.

Mass accretion rate decreases with stellar age and mass.

Abstract

We have studied the young stellar populations in NGC 602, in the Small Magellanic Cloud, using a novel method that we have developed to combine Hubble Space Telescope photometry in the V, I, and Halpha bands. We have identified about 300 pre-main sequence (PMS) stars, all of which are still undergoing active mass accretion, and have determined their physical parameters (effective temperature, luminosity, age, mass and mass accretion rate). Our analysis shows that star formation has been present in this field over the last 60 Myr. In addition, we can recognise at least two clear, distinct, and prominent episodes in the recent past: one about 2 Myr ago, but still ongoing in regions of higher nebulosity, and one (or more) older than 30 Myr, encompassing both stars dispersed in the field and two smaller clusters located about 100 arcsec north of the centre of NGC 602. The relative locations of younger and older PMS stars do not imply a causal effect or triggering of one generation on the other. The strength of the two episodes appears to be comparable, but the episode occurring more than 30 Myr ago might have been even stronger than the current one. We have investigated the evolution of the mass accretion rate M_acc as a function 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 function of the metallicity.