Timescale of Mass Accretion in Pre-Main-Sequence Stars

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We present 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 ages between 1 - 50 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 10⁻¹¹ Msun yr-1. We compared the fraction of stars showing ongoing accretion (fₐcc) to the fraction of stars with near-to-mid infrared excess (fᵢRAC). In most cases we find fₐcc < fᵢRAC, 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 > 10⁻¹¹ Msun yr-1, while 20% of the stars show near-infrared excess emission. Assuming an exponential decay, we measure a mass accretion timescale (tₐcc) of 2.3 Myr, compared to a near-to-mid infrared excess timescale (tᵢRAC) of 2.9 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.