  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 broad-band V and I photometry with
narrow-band Halpha imaging to: (1) identify all stars with excess Halpha
emission; (2) derive their Halpha luminosity L(Halpha); (3) estimate the Halpha
emission equivalent width; (4) derive the accretion luminosity L_acc from
L(Halpha); and finally (5) obtain the mass accretion rate M_acc from L_acc and
the stellar parameters (mass and radius). By selecting stars with photometric
accuracy in Halpha better than 15%, the statistical uncertainty on the derived
M_acc is typically <17% and is dictated by the precision of the Halpha
photometry. Systematic uncertainties, of up to a factor of 3 on the value of
M_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 Halpha line analysis.
  As an application of our method, we study a field of 9.16 arcmin2 around
SN1987A, using existing HST photometry. We identify as bona-fide PMS stars a
total of 133 objects with a Halpha excess above the 4 sigma level and a median
age of 13.5 Myr. Their median mass accretion rate of 2.6x10-8 Msolar/yr is in
excellent agreement with previous determinations based on the U-band excess of
the stars in this field, and with the value measured for G-type PMS stars in
the Milky Way. Their L_acc shows a strong dependence on their distance from a
group of hot massive stars in the field and suggests that the UV radiation of
the latter is rapidly eroding the circumstellar discs around PMS stars.
