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
time
Primordial Binary Population
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 Msun pc–3
Nearby – 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
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

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

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

18. 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