The primordial binary population in Sco OB2 Thijs Kouwenhoven Anthony Brown (Leiden) Lex Kaper (Amsterdam) Simon Portegies Zwart (Amsterdam) Hans Zinnecker (Potsdam) Multiplicity in star formation, University of Toronto, 16 May 2007

The primordial binary population Hydrodynamical simulations time Primordial Binary Population N-body simulations

The primordial binary population Star formation binaries/multiples dominant mode of star formation? IMF, f(q), f(P), f(e), binary fraction, triples, .... Evolution and fate of stellar groupings OB associations, star clusters, YMCs Origin of field stars, OB runaway stars Exotic objects GRB, XRB, blue stragglers, ... End point for hydrodynamical simulations Initial conditions for N-body simulations

Finding the primordial binary population Observed Binary Population stellar and dynamical evolution True Binary Population selection effects Primordial Binary Population

Scorpius-Centaurus (Sco OB2) Young 5 – 20 Myr Low density 0.01 Msun pc–3 Nearby 120 – 150 pc Binary population close to primordial Membership and stellar content known (de Zeeuw et al. 1999) B stars to brown dwarfs Cleared of gas Well-studied association

The binary fraction in Sco OB2 All Sco OB2 binaries in literature (visual, spectroscopic, astrometric, eclipsing, …) Does the binary fraction vary with spectral type? Brown (2001), Kouwenhoven et al. (2005)

ADONIS/NACO observations Adaptive optics + Infrared ADONIS (ESO 3.6m) survey All 200 B4V-A9V stars, in KS 0.2”–13.5” (40–2000 AU) 6.4 < KS < 15.5 mag NACO (VLT) survey 22 A/B stars, JHKS bands 0.1”–11” (13–1430 AU) 5.4 < KS < 17 mag HIP53701, HIP58416, HIP76001, HIP80799; KS -band, 13”x13” (Kouwenhoven et al. 2005, 2007a)

ADONIS/NACO survey results ADONIS (All 200 B4V-A9V association members) 74 companion stars (41 new) 77 background stars NACO (22 B4V-A9V association members) 29 companion stars: 18 confirmed (3 new) and 11 candidates 33 background stars Companion/background criterion correct Observed binary fraction among A/B stars increased significantly “Brown dwarf desert” (130–520 AU) for A/B stars

ADONIS/NACO survey results 44 new companions (Kouwenhoven et al. 2005, 2007a) Binary fraction of A/B-stars significantly increased Selection effects partially removed

ADONIS/NACO survey results 44 new companions (Kouwenhoven et al. 2005, 2007a) Binary fraction of A/B-stars significantly increased Selection effects partially removed

Finding the true binary population Projection Sample bias Instrument bias Observed binary population comparison Simulated binary population Simulated observations Simulated projection Sample bias model Instrument bias model

Binarity among A/B stars in Sco OB2 Combination with all available literature data (visual, spectroscopic, astrometric, eclipsing binaries) Removal of selection effects (sample selection, instrumental biases, contamination) Binary fraction ~100% Mass ratio f(q) = q–0.4±0.1 (random pairing excluded) Period f(log P) ≈ flat Observed “brown dwarf desert” follows from f(q) (Kouwenhoven et al. 2007c, submitted to A&A)

The primordial binary population in Sco OB2 Evolution of the binary population since birth dynamical evolution: only widest binaries affected stellar evolution: only most massive binaries affected practically no binary formation by capture Sco OB2 expanding  evolution of the binary population halted shortly after formation? Sco OB2 likely contains a fossil record of the star forming process

Summary Near-infrared AO surveys with ADONIS/NACO All 200 A/B members of Sco OB2 surveyed 77 companions confirmed (44 new) The current binary population in Sco OB2 Binary fraction 100% f(q) = q–0.4±0.1 ; random pairing excluded “Brown dwarf desert” (130–520 AU) for A/B stars; consistent with f(q) for stellar companions  brown dwarfs form like stars The primordial binary population of Sco OB2 Binary population mildly affected by stellar/dynamical evolution  fossil record of the star forming process

The color-magnitude diagram □ companion star  background star Kouwenhoven et al. (2007a)

Close brown dwarf companions Stars Brown dwarfs + known companion ● new companion  background star 1” ≈ 130 AU

Finding the primordial binary population t = now Primordial binary population Current binary population Dynamical evolution Stellar evolution Binary evolution comparison Initial simulated cluster Evolved simulated cluster N-body simulations Including stellar and binary evolution