Photometric determination of the mass accretion rates of pre-main-sequence stars

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 Hα bands. We identified 250 stars with Hα excess exceeding 5σ and an equivalent width of the Hα emission line of at least 20 Å, which indicates that these stars are still undergoing active accretion and therefore represent bona fide pre-main-sequence (PMS) objects. For 240 of them, we derived physical stellar parameters such as the mass, age, and mass accretion rate by comparing the observed photometry with theoretical models. We find evidence that suggests the existence of two populations of PMS stars, one with a median age of around 25 Myr and the other about 50 Myr old. These ages are consistent with previously determined ages for the main population of the cluster. The average mass accretion rate for these PMS stars is ∼5 × 10−9 M⊙ yr−1, which is comparable to the values found with the same method 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. Although it is possible that the PMS stars formed in situ in a more diffuse manner than massive stars, it is also plausible that the PMS stars formed initially in a more compact structure together with the massive stars and were later dispersed due to two-body relaxation. To explore this possibility, we studied the cluster’s stellar density profile. We find a core radius rc ≃ 0.6 pc and a tidal radius rt ≃ 5.5 pc, with an implied concentration parameter c ≃ 1, suggesting that the cluster could be dispersing into the field.