Photometric determination of the mass accretion rates of pre-main sequence stars. VII. The low-density cluster NGC 376 in the Small Magellanic Cloud

Abstract

Aims. We study the properties of low-mass stars recently formed in the field of the NGC 376 cluster in the Small Magellanic Cloud (SMC). Methods. Using photometric observations acquired with the Hubble Space Telescope (HST) in the V, I and Halpha bands, we identify 244 candidate pre-main sequence (PMS) stars showing Halpha excess emission at the 5 sigma level and with Halpha equivalent width of 20 Å or more. We derive physical parameters for all PMS stars, including masses, ages, and mass accretion rates. We compare the effective mass accretion rate of stars in NGC 376 to that of objects in the NGC 346 cluster, which features similar metallicity but higher total mass and gas density. Results. We find a median age of 28 Myr for this population (with 25 and 75 percentiles at about 20 and 40 Myr, respectively), in excellent agreement with previous studies of massive stars in the same field. The PMS stars are rather uniformly distributed across the field, whereas massive stars are more clustered. The spatial distribution of PMS objects is compatible with them having formed in the centre of the cluster and then migrated outwards. We find that in NGC 376 the mass accretion rate is systematically lower than in NGC 346 for stars of the same mass and age. This indicates that, besides metallicity, there are other environmental factors affecting the rate of mass accretion onto PMS stars. Our observations suggest that the gas density in the star-forming region might play a role.

arXiv comment10 pages, 6 figures, text updated as published

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