Photometric determination of the mass accretion rates of pre-main sequence stars. IX. Recent star formation in the periphery of NGC 346

Original Abstract

We studied the properties of star formation and the characteristics of young stars in a quiet region located beyond the outskirts of the prominent star-forming cluster NGC 346 in the Small Magellanic Cloud (SMC). Utilising observations from the Hubble Space Telescope across the broad V and I bands, as well as the narrow Halpha band, we identified populations with ages of roughly 10, 60, 400 Myr and of 5 Gyr through isochrone comparison. We successfully identified 137 bona fide pre-main sequence (PMS) candidates exhibiting Halpha excess with a significance level of 5 sigma, accompanied by an Halpha line emission equivalent width exceeding 20 Å. Physical parameters for these PMS stars were determined, including mass, age, accretion luminosity, and mass accretion rate. Most PMS stars have an age around 16 Myr and an average mass of 0.80 \pm 0.16 M_sun. The median mass accretion rate for all 137 PMS stars is estimated to be about 8.0 x 10^(-9) M_sun/yr. While this rate is lower than that observed in the NGC 346 cluster itself, it is comparable with those measured for PMS stars in low-density star-forming regions in the SMC, despite the absence of apparent clustering and nebulosity. Furthermore, our analysis reveals that the ratios of accreting and non-accreting PMS stars to non-PMS stars and their mass accretion rate correlate with their distance from a group of hot massive stars in the vicinity. This suggests that the ultraviolet radiation emitted by these massive stars might erode the circumstellar discs of nearby PMS stars. Lastly, the overlap between our studied region and observations from the James Webb Space Telescope reveals that some of the identified PMS stars display near-infrared excess.