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The primordial binary population in Sco OB2

This study investigates the primordial binary population in the Sco OB2 association through hydrodynamical simulations and N-body simulations. It examines the formation, evolution, and fate of binary systems, as well as their impacts on star formation and the overall population of the association.

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The primordial binary population in Sco OB2

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  1. 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

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

  3. 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

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

  5. 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

  6. 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)

  7. 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)

  8. 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

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

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

  11. Finding the true binary population 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

  12. 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)

  13. 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

  14. 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

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

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

  17. Finding the primordial binary population t = 0 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

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