Timescale of Mass Accretion in Pre-Main-Sequence Stars

TL;DR

This study measures how quickly young stars stop accreting material, finding that accretion typically ceases around 2.3 million years, which is earlier than dust dissipation in the disk, impacting planet formation theories.

Contribution

It provides the first large-scale measurement of mass accretion timescales in pre-main-sequence stars using spectroscopic data across multiple clusters.

Findings

Accretion fraction decreases from 60% at 1.5-2 Myr to 2% at 10 Myr.

Mass accretion typically stops around 2.3 Myr.

Accretion ceases earlier than dust dissipation in the disk.

Abstract

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 (f_acc) to the fraction of stars with near-to-mid infrared excess (f_IRAC). In most cases we find f_acc < f_IRAC, 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 (t_acc) of 2.3 Myr, compared to a near-to-mid infrared excess timescale (t_IRAC) 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.