Low-Mass Pre-Main Sequence Stars in the Large Magellanic Cloud - III: Accretion Rates from HST-WFPC2 Observations

Abstract

We have measured the present accretion rate of roughly 800 low-mass (~1-1.4 Mo) pre-Main Sequence stars in the field of Supernova 1987A in the Large Magellanic Cloud (LMC, Z~0.3 Zo). It is the first time that this fundamental parameter for star formation is determined for low-mass stars outside our Galaxy. The Balmer continuum emission used to derive the accretion rate positively correlates with the Halpha excess. Both these phenomena are believed to originate from accretion from a circumstellar disk so that their simultaneous detection provides an important confirmation of the pre-Main Sequence nature of the Halpha and UV excess objects, which are likely to be the LMC equivalent of Galactic Classical TTauri stars. The stars with statistically significant excesses are measured to have accretion rates larger than 1.5x10^{-8}Mo/yr at an age of 12-16 Myrs. For comparison, the time scale for disk dissipation observed in the Galaxy is of the order of 6 Myrs. Moreover, the oldest Classical TTauri star known in the Milky Way (TW Hydrae, with 10 Myrs of age) has a measured accretion rate of only 5x10^{-10} Mo/yr, ie 30 times less than what we measure for stars at a comparable age in the LMC. Our findings indicate that metallicity plays a major role in regulating the formation of low-mass stars.Comment: Accepted for publication in the Astrophysical Journal (10 June 2004), 28 pages, 9 figures. Typo corrected in the abstract on 21 February 200