PHOTOMETRIC DETERMINATION OF THE MASS ACCRETION RATES OF PRE-MAIN-SEQUENCE STARS. I. METHOD AND APPLICATION TO THE SN 1987A FIELD

We have developed and successfully tested a new self-consistent method to reliably identify pre-main-sequence (PMS) objects actively undergoing mass accretion in a resolved stellar population, regardless of their age. The method does not require spectroscopy and combines broadband V and I photometry with narrowband H alpha imaging to (1) identify all stars with excess H alpha emission, (2) convert the excess H alpha magnitude into H alpha luminosity L(H alpha), (3) estimate the H alpha emission equivalent width, (4) derive the accretion luminosity L-acc from L(H alpha), and finally (5) obtain the mass accretion rate. (M) over dot(acc) from L-acc and the stellar parameters (mass and radius). By selecting stars with an accuracy of 15% or better in the H alpha photometry, the statistical uncertainty on the derived (M) over dot(acc) is typically less than or similar to 17% and is dictated by the precision of the H alpha photometry. Systematic uncertainties, of up to a factor of 3 on the value of (M) over dot(acc), are caused by our incomplete understanding of the physics of the accretion process and affect all determinations of the mass accretion rate, including those based on a spectroscopic H alpha line analysis. As an application of our method, we study the accretion process in a field of 9.16 arcmin(2) around SN 1987A, using existing Hubble Space Telescope photometry. We identify as bona fide PMS stars a total of 133 objects with a H alpha excess above the 4 sigma level and a median age of 13.5 Myr. Their median mass accretion rate of 2.6 x 10(-8) M-circle dot yr(-1) is in excellent agreement with previous determinations based on the U-band excess of the stars in the same field, as well as with the value measured for G-type PMS stars in the Milky Way. The accretion luminosity of these PMS objects shows a strong dependence on their distance from a group of hot massive stars in the field and suggests that the ultraviolet radiation of the latter is rapidly eroding the circumstellar disks around PMS stars.