Photometric determination of the mass accretion rates of pre-main-sequence stars. VIII. Recent star formation in NGC 299

Abstract

We studied the properties of the young stellar populations in the NGC 299 cluster in the Small Magellanic Cloud using observations obtained with the Hubble Space Telescope in the $V,I$, and $H\alpha$ bands. We identified 250 stars with H$\alpha$ excess exceeding 5 $\sigma$ and an equivalent width of the H$\alpha$ emission line of at least 20 Å, indicating that these stars are still undergoing accretion and therefore represent bona fide pre-main-sequence (PMS) objects. For 240 of these stars, we derived the mass, age, and mass accretion rate by comparing the observed photometry with theoretical models. We find evidence for the existence of two populations of PMS stars, with median ages of 25 and 50 Myr respectively. The average mass accretion rate for these PMS stars is $\sim 5\times 10^{-9}$ M${}_{\odot }$ yr${}^{-1}$, which is comparable to the values found in other low-metallicity, low-density clusters in the Magellanic Clouds, but is about a factor of three lower than those measured for stars of similar mass and age in denser Magellanic Cloud stellar regions. Our findings support the hypothesis that both the metallicity and density of the forming environment can affect the mass accretion rate and thus the star formation process in a region. A study of the spatial distribution of both massive stars and (low-mass) PMS objects reveals that the former are clustered near the nominal centre of NGC 299, whereas the PMS stars are rather uniformly distributed over the field. To explore whether the stars formed in an initially more diffuse or compact structure, we studied the cluster's stellar density profile. We find a core radius $r_c\simeq 0.6$ pc and a tidal radius $r_t\simeq 5.5$ pc, with an implied concentration parameter $c\simeq 1$, suggesting that the cluster could be dispersing into the field.

14 pages, 11 figures, accepted for publication in A&A, final revisions included

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