  We have undertaken a systematic study of pre-main sequence (PMS) stars
spanning a wide range of masses (0.5 - 4 Msolar), metallicities (0.1 - 1
Zsolar) and ages (0.5 - 30 Myr). We have used the Hubble Space Telescope (HST)
to identify and characterise a large sample of PMS objects in several
star-forming regions in the Magellanic Clouds, namely 30 Dor and the SN 1987A
field in the LMC, and NGC 346 and NGC 602 in the SMC, and have compared them to
PMS stars in similar regions in the Milky Way, such as NGC 3603 and Trumpler
14, which we studied with the HST and Very Large Telescope (VLT). We have
developed a novel method that combines broad-band (V, I) photometry with
narrow-band Halpha imaging to determine the physical parameters (temperature,
luminosity, age, mass and mass accretion rate) of more than 3000 bona-fide PMS
stars still undergoing active mass accretion. This is presently the largest and
most homogeneous sample of PMS objects with known physical properties and
includes not only very young objects, but also PMS stars older than 10 - 20 Myr
that are approaching the main sequence (MS). We find that the mass accretion
rate scales roughly with the square root of the age, with the mass of the star
to the power of 1.5, and with the inverse of the cube root of the metallicity.
The mass accretion rates for stars of the same mass and age are thus
systematically higher in the Magellanic Clouds than in the Milky Way. These
results are bound to have important implications for, and constraints on our
understanding of the star formation process.
