The star formation history and accretion-disc fraction among the K-type members of the Scorpius-Centaurus OB association

Abstract

We present results of a spectroscopic survey for new K- and M-type members of Scorpius- Centaurus (Sco-Cen), the nearest OB Association (~100-200 pc). Using an X-ray, proper motion and colour-magnitude selected sample, we obtained spectra for 361 stars, for which we report spectral classifications and Li and Hα equivalent widths. We identified 156 new members of Sco-Cen, and recovered 51 previously published members. We have combined these with previously known members to form a sample of 493 solar-mass (~0.7-1.3 M⊙) members of Sco-Cen. We investigated the star formation history of this sample, and reassessed the ages of the massive main-sequence turn-offand the G-type members in all three subgroups. We performed a census for circumstellar discs in our sample using WISE infrared data and find a protoplanetary disc fraction for K-type stars of 4.4-0.9+1.6 per cent for Upper Centaurus-Lupus and Lower Centaurus-Crux at~16 Myr and 9.0-2.2+4.0 per cent for Upper Scorpius at ~10 Myr. These data are consistent with a protoplanetary disc e-folding time-scale of ~4-5 Myr for ~1 M⊙ stars, twice that previously quoted, but consistent with the Bell et al. revised age scale of young clusters. Finally, we construct an age map of Scorpius-Centaurus which clearly reveals substructure consisting of concentrations of younger and older stars. We find evidence for strong age gradients within all three subgroups. None of the subgroups are consistent with being simple, coeval populations which formed in single bursts, but likely represents a multitude of smaller star formation episodes of hundreds to tens of stars each.