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Observations of Binaries in Globular Clusters. Adrienne Cool San Francisco State University. Primordial. Observations of ^ Binaries in Globular Clusters. Adrienne Cool San Francisco State University. OUTLINE. Why bother? What do we want to know? How can we find out?

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slide1
Observations of Binaries in Globular Clusters

Adrienne Cool San Francisco State University

slide2
Primordial

Observations of ^ Binaries in Globular Clusters

Adrienne Cool San Francisco State University

slide3
OUTLINE
  • Why bother?
  • What do we want to know?
  • How can we find out?
  • What’s new?
  • What’s next?
slide4
Why bother?
  • Binary fraction is a fundamental parameter
  • Primordial binaries play a key role in cluster dynamics
  • Primordial binaries are implicated in the formation of
  • many more exotic populations
slide5
What do we want to know?
  • What fraction of globular cluster stars are binaries?
  • Do clusters have different binary fractions (fb)?
  • Any correlation with particular cluster parameters?
  • Are there signs of dynamical evolution of binaries?
  • How are the binaries distributed in…
      • period (Porb)
      • eccentricity (e)
      • mass ratio (q  m2/m1)
      • primary mass
slide6
How can we find out?

3 methods used so far…

outliers in

color-magnitude diagrams

photometric variables

radial velocity variables

Pryor / Hut et al. 1992 Kaluzny et al. 1999 Rubenstein & Bailyn 1997

slide7
Method 1: Radial velocity variables
  • ground-based spectroscopy
  • samples of ~30-300 giants
  • ~2-4 velocities per star
  • accuracies ~0.6–3 km/s
  • baselines ~ 1-20 years

Cote et al. 1996 – M22

sensitive to binaries with

Porb ~ days – years

fb estimates depend on

eccentricity distribution

slide9
Method 2: photometric variables
  • HST & ground-based imaging
  • ~ 2000 – 40,000 stars sampled
  • 250 – 1300 images
  • baselines ~ 1 week – 1 month

sensitive to binaries with

Porb ~ 0.1 day – few days

fb estimates depend on assumed

Porb, e, q distributions

Albrow et al. 2001 – 47 Tuc

slide11
Method 3: Outliers in color-magnitude diagrams
  • HST & gnd-based imaging
  • 100s – 1000s of stars
  • 2 filters is enough
  • no repeat measure required
  • high photometric accuracy

sensitive to binaries with…

any Porb, e, inclination!

fb estimates depend on

assumed q distribution, F(q)

Cool & Bolton 2002 – NGC 6397

slide14
What fraction fb of globular cluster stars are binaries?

Does fb differ among clusters? Range?

Do any clusters have fb = 0% ? 100% ??

slide15
radial velocity variables – global binary fractions
  • typical fb ~ 15% (~5% per decade)
  • trend toward lower fb for massive clusters (high , low Pcrit)
  •  disruption of soft binaries? Cote et al. 1996
slide16
Pal 5
  • MV = –5.2
  • c = 0.70
  • single epoch, VLT
  • 17 cluster stars
  • accuracy ~ 0.15 km/s

Odenkirchen et al. 2002 – Pal 5

broad pedestal under narrow peak = binaries?

 fb = 40  20 %

slide17
photometric variables – global binary fractions

47 Tuc: fb ~ 2 – 5 % per decade for hard binaries

 consistent with vrad results

slide22
mass ratio

distribution

F(q)

core 1 – 2 rc

more low q

~ flat

more high q

q = 1

Zhao & Bailyn 2005 – M3

slide23
CMD outliers – binary fractions – “all q” subset
  • all Porb, all e, any inclination… 4 - 5 decades in Porb
  •  why not higher fb ?
  • maybe M3 is okay, but NGC 288?
  • how come a post-collapse cluster has such a high fb??
slide24
CMD outliers – binary fractions – high q subset
  • 3 cases with fb ~ 25 – 30 % … extrapolate to all q (×3??)
  •  75 – 90 % ?!
  • NGC 2808 so high even far outside core ?
  • Pal 5 with tidal stripping… why not higher?
  • M4… why so low?
slide25
Pal 5 – compare 2 methods

Koch et al. 2004

Odenkirchen et al. 2002

CMDs: fb ~ 9  1%

vrad: fb ~ 40  20 %

slide26
M4 – compare 3 methods

CMDs: fb ~ 1–2 % (high q)

vrad: fb ~ 15  15 % Cote et al. 1996

variables: fb(observed) < 0.05%

(similar to 47 Tuc w/fb ~ 13%)

Richer et al. 2004

Ferdman et al. 2004

slide27
effect of photometric errors on

perceived binary fraction in CMDs

fb = 10%

Hut et al. 1992

slide28
Primordial binary fraction in globular clusters
  • all GCs studied have at least some binaries
  • not all GCs have same binary fraction… at present
  • low end: < 5 – 7 % ? (NGC 6397)
  • high end: ~ 30%  ~ 90% for all q ?? (E3, Pal 13)
  • fb = 100% is not ruled out for some poor clusters
  • fb = 0% is possible in outskirts of some clusters
  • trend toward higher fb for poorer clusters, with exceptions
slide29
Clark et al. 2004 – Pal 13

Cool & Bolton 2002 – NGC 6397

slide30
effect of photometric errors on

perceived binary fraction in CMDs

fb = 100%

Hut et al. 1992

slide32
Are there signs of dynamical evolution of binaries?
  • trends toward lower fb for higher mass
  • clusters consistent with destruction of
  • binaries beyond hard/soft boundary
  • … or are fb values in loose clusters just
  • enhanced by tidal stripping?
  • low fb in NGC 6397 and M30
  •  destruction in collapsed cores?
    • … but what about NGC 6752??
slide35
mass segregation in NGC 288

Bellazzini et al. 2002 – NGC 288

slide36
mass segregation of binaries in 47 Tuc

Albrow et al. 2001 – 47 Tuc

71 BY Dra stars in 47 Tuc!

an untapped resource

slide37
period segregation of eclipsing binaries

in 47 Tuc

contact all stars detached

Weldrake et al. 2004 – 47 Tuc

slide38
More signs of dynamical evolution…
  • segregation of binaries by mass is observed
  • 47 Tuc: shorter period binaries are more centrally
        • concentrated than long period binaries
        • mass effect?? binary hardening?
slide39
What about binary parameters?
  • Radial velocities can give Porb, e
  • and more… long-term tracking
  • of candidates required
  • Eclipsing binaries beginning to
  • give information on Porb
  • CMDs in principle can give
  • information on q = m1/m2
slide42
What’s next?
  • better constraints on binary fraction and distribution
  • * vrad – need larger samples! Fabry-Perot underway
  • * eclipsing – large samples are proven to work
  • * CMDs – exploit highest possible photometric accuracy
  • – look for MS-WD pairs too?
  • contraints on binary parameters? track candidates!
  • * vrad – already done for some
  • * CMDs – spectroscopy on MS-MS binaries?
  • – BY Dra stars: more complete sample??
  • HST very valuable, especially in crowded cluster cores
  • ground-based work equally powerful in sparse clusters
  • (e.g. Pal 13) or outskirts (e.g. 47 Tuc)
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