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 ScorpiusCentaurus (Sco-Cen), the nearest OB Association (∼100-200 pc). Using an X-ray, proper motion and color-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 re-assessed the ages of the massive main-sequence turn-off and the G-type members in all three subgroups. We performed a census for circumstellar disks in our sample using WISE infrared data and find a protoplanetary disk fraction for K-type stars of 4.4 +1.6 −0.9 % for Upper Centaurus-Lupus and Lower Centaurus-Crux at ∼16 Myr and 9.0 +4.0 −2.2 % for Upper Scorpius at ∼10 Myr. These data are consistent with a protoplanetary disk e-folding timescale of ∼4-5 Myr for ∼1 M stars, twice that previously quoted (Mamajek 2009), 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.

The Greater Taurus–Auriga Ecosystem. I. There is a Distributed Older Population

“…However, both explanations still require the distributed population to be older than the concentrated disk-hosting population, indicating that Taurus is a long-lived structure that has produced multiple generations of comoving cospatial star formation, in analogy to the Sco-Cen OB association (Pecaut & Mamajek 2016). The measurement of stellar ages is fraught with uncertainty, due to a combination of measurement errors, unrecognized systematic uncertainties like multiplicity and excesses, and possibly even fundamental scatter in the properties of stars as a result of different assembly histories, rotation rates, or magnetic field strengths (e.g., Hartmann et al 1997;Baraffe & Chabrier 2010;Somers & Pinsonneault 2015a;Feiden 2016).…”

“…The mere existence of past generations of star formation would resolve the ongoing debate of molecular cloud lifetimes (e.g., Hartmann et al 2001;Krumholz & Tan 2007;Murray 2011;Federrath 2015), as kinematically related clouds must have existed since the first generation. Studies of nearby OB associations, most notably Scorpius-Centaurus, are revealing that age gradients and significant age spreads do indeed exist on scales of tens of parsecs (e.g., Pecaut & Mamajek 2016).…”

Origin of the wide-orbit circumbinary giant planet HD 106906

“…Our list of LCC members is based on previous surveys of this region (De Zeeuw et al 1999;Preibisch & Mamajek 2008;Song et al 2012;Pecaut & Mamajek 2016) and consists of 369 stars. In the following, we estimate the current density of stars around the planetary system and its evolution in time.…”

Orbiting a binary

“…Daemgen et al (2015) found that this star, together with several other objects known as bona-fide Taurus members, has similar space position and kinematics to those of the Taurus star forming region, but also shows evidence of a distinctly older age. They proposed an age of about 20 Myr for what is now called the Taurus-Ext Association (see Appendix A in Daemgen et al 2015, and references therein) from comparison of Lithium EW with that of the oldest Sco-Cen groups (Pecaut & Mamajek 2016). Such proposed age for the group is at the young edge of our age range.…”