Timescale of mass accretion in pre-main-sequence stars

We present the initial result of a large spectroscopic survey aimed at measuring the timescale of mass accretion in young, pre-main-sequence stars in the spectral type range K0–M5. Using multi-object spectroscopy with VIMOS at the VLT we identified the fraction of accreting stars in a number of young stellar clusters and associations of the ages of between 1–30 Myr. The fraction of accreting stars decreases from ~60% at 1.5–2 Myr to ~2% at 10 Myr. No accreting stars are found after 10 Myr at a sensitivity limit of 10<sup>-11<sup/> . We compared the fraction of stars showing ongoing accretion () to the fraction of stars with near-to-mid infrared excess (). In most cases we find <i><<i/> , i.e. , mass accretion appears to cease (or drop below detectable level) earlier than the dust is dissipated in the inner disk. At 5 Myr, 95% of the stellar population has stopped accreting material at a rate of , while ~20% of the stars show near-infrared excess emission. Assuming an exponential decay, we measure a mass accretion timescale () of 2.3 Myr, compared to a near-to-mid infrared excess timescale () of 3 Myr. Planet formation and/or migration, in the inner disk might be a viable mechanism to halt further accretion onto the central star on such a short timescale.

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