Photometric determination of the mass accretion rates of pre-main sequence stars. V. Recent star formation in the 30 Dor nebula

Abstract

We report on the properties of the low-mass stars that recently formed in the central ~ 2.7'x2.7' of 30 Dor including the R136 cluster. Using the photometric catalogue of De Marchi et al. (2011c), based on observations with the Hubble Space Telescope (HST), and the most recent extinction law for this field, we identify 1035 bona-fide pre-main sequence (PMS) stars showing Halpha excess emission at the 4 sigma level with Halpha equivalent width of 20 AA or more. We find a wide spread in age spanning the range ~ 0.1-50 Myr. We also find that the older PMS objects are placed in front of the R136 cluster and are separated from it by a conspicuous amount of absorbing material, indicating that star formation has proceeded from the periphery into the interior of the region. We derive physical parameters for all PMS stars, including masses m, ages t, and mass accretion rates M_acc. To identify reliable correlations between these parameters, which are intertwined, we use a multivariate linear regression fit of the type log M_acc = a log t + b log m + c. The values of a and b for 30 Dor are compatible with those found in NGC 346 and NGC 602. We extend the fit to a uniform sample of 1307 PMS stars with 0.5 < m/Msun < 1.5 and t < 16 Myr in six star forming regions in the Large and Small Magellanic Clouds and Milky Way with metallicities in the range 0.1-1.0 Zsun. We find a=-0.59+/-0.02 and b=0.78+/-0.08. The residuals are systematically different between the six regions and reveal a strong correlation with metallicity Z, of the type c = (-3.69+/-0.02) - (0.30+/-0.04) log Z/Zsun. A possible interpretation of this trend is that when the metallicity is higher so is the radiation pressure and this limits the accretion process, both in its rate and duration.