Characterising the young Sco-Cen association

The young Sco-Cen association provides a unique astrophysical laboratory for the study of many different stellar properties. In this thesis we present the results of our characterisation of the young OB association Scorpius-Centaurus via four different avenues; the stellar membership of the association, the multiplicity of the high-mass stars, the prevalence of circumstellar disks among Sco-Cen members, and age-dating the association with close binary systems. These are presented in the form of four chapters, two of which are journal publications. In the first section we present an analysis of the WISE photometric data for 829 B, A and F-type stars in the Sco-Cen association, using the latest high-mass membership probabilities. We detect debris disks associated with 134 Sco-Cen stars, with a clear increase in IR excess fraction with membership probability. We determine that 41+/-5%of Sco-Cen BAF stars have IR excesses, compared to 1+/-4% of field stars, and do not see any change in excess fraction between the Sco-Cen subgroups. Within our sample, we have observed that B-type association members have a significantly smaller excess fraction than A and F-type association members. In the second section we present a search for new low-mass members of the Sco-Cen association, focussing on the Upper Scorpius subgroup. We developed a Bayesian kinematic selection method to prioritise candidate members, and spectroscopically confirmed 232 new Upper-Scorpius G to M-type members via their Li absorption with the WiFeS IFU. Among these new members we also identify eight companions in H-α emission using spectro-astrometric techniques, four of which are candidate wide gas-giant planets. Additionally, we observed the wide gas-giant planet host GSC-6214-0210 tohave a significantly reduced H-planets. Additionally, we observed the wide gas-giant planet host GSC-6214-0210 tohave a significantly reduced H-α equivalent width of --0:63Å, compared to the previous observation of --:51Å , suggesting that the rate of accretion onto the planetary companion has slowed or stopped. In the third section we present the first multiplicity-dedicated long baseline optical interferometric survey of Sco-Cen. We have surveyed 58 Sco-Cen B-type stars with the Sydney University Stellar Interferometer and detected 23 companions at separations ranging from 7-130 mas, 13 of which are new detections. We then apply a Bayesian analysis to all available information in the literature to determine the multiplicity distribution of the 58 stars in our sample, showing that the companion frequency is F = 1.35+0.27/-0.20 and the mass ratio distribution is best described by qγ with γ = --0:46,agreeing with previous Sco-Cen high-mass star work and differing significantly from lower-mass stars in Tau-Aur. Based on our analysis, we estimate that among young B-type stars in moving groups, up to 27% are apparently single. In the final section we present the results of a Keck NIRC2 aperture-masking program of 7 G to M-type members of the Upper Scorpius subgroup of the Sco-Cen OB association. We present orbital solutions for the binary systems we have monitored, and also determine the age, component masses, distance and reddening for each system using the orbital solutions and multi-band photometry using a Bayesian fitting procedure.We find that the age of the Upper Scorpius subgroup is 7+/-2 Myr, with some members as old as ~ 10 Myr. This is younger than the previous estimate of Pecaut et al. 2012, but supports the hypothesis that there is an age distribution among stars kinematically consistent with Upper-Scorpius membership stretching from < 5 Myr upto ~ 10 Myr. We propose that the current evidence for the age of Upper-Scorpius is consistent with the existence of two populations of stars in this part of Sco-Cen; one of ~15 Myr, which formed with the rest of greater Sco-Cen, and a younger population of age ~5 Myr, including the clearly young stars γ-Sco and ω-Sco, which formed through supernova triggered star formation from a separate molecular cloud.