Abstract
Using the Hubble Space Telescope (HST) we have characterised and compared the physical properties of a large sample of pre-main sequence (PMS) stars spanning a wide range of masses (0:5 - 4M<sub>ʘ</sub>), metallicities (0:1 - 1 Z<sub>ʘ</sub>) and ages (0:5 - 30 Myr). This is presently the largest and most homogeneous sample of PMS objects with known physical properties. The main results of this ongoing study are briefly summarised here.
Highlights
With a group of European colleagues, whose names are listed in the Acknowledgments, we have undertaken a systematic study of the star formation process in the Milky Way and Magellanic Clouds (MCs), with special emphasis on moderate mass stars, say 0.5 to 4 M
Our motivations are: i) Stars in this range account for most of the star formation in a galaxy, e.g. more than 65% for a Salpeter initial mass function (IMF); ii) Low mass stars can form in small parent clouds as well as in big ones; iii) As a consequence, low mass stars are forming near massive stars as well as in isolated groups
Using massive star diagnostics to measure the overall star formation rates is valid if and only if we are confident to be able to extrapolate correctly from what we see coming from a bunch of massive stars to the total amount of mass that is forming the bulk of stars, i.e. the low mass stars
Summary
With a group of European colleagues, whose names are listed in the Acknowledgments, we have undertaken a systematic study of the star formation process in the Milky Way and Magellanic Clouds (MCs), with special emphasis on moderate mass stars, say 0.5 to 4 M . Ground-based spectroscopic studies of nearby young star-forming regions (e.g. Taurus, Auriga, Ophiuchus) show that the mass accretion rate appears to decrease steadily with time, from about 10−8 M yr−1 at ages of ∼ 1 Myr to ∼ 10−9 M yr−1 at ∼ 10 Myr. At face value this is in line with the expected evolution of viscous discs, but the scatter of the data exceeds 2 dex at any given age (see Fig. 3).
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