Observations of Binaries in Globular Clusters
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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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Observations of Binaries in Globular Clusters

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Observations of binaries in globular clusters

Observations of Binaries in Globular Clusters

Adrienne Cool San Francisco State University


Observations of binaries in globular clusters

Primordial

Observations of ^ Binaries in Globular Clusters

Adrienne Cool San Francisco State University


Observations of binaries in globular clusters

OUTLINE

  • Why bother?

  • What do we want to know?

  • How can we find out?

  • What’s new?

  • What’s next?


Observations of binaries in globular clusters

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


Observations of binaries in globular clusters

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


Observations of binaries in globular clusters

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


Observations of binaries in globular clusters

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


Observations of binaries in globular clusters

searches for radial velocity variables


Observations of binaries in globular clusters

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


Observations of binaries in globular clusters

searches for eclipsing binaries – selected results


Observations of binaries in globular clusters

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


Observations of binaries in globular clusters

searches in color-magnitude diagrams – selected early results


Observations of binaries in globular clusters

searches in color-magnitude diagrams -- more recent results


Observations of binaries in globular clusters

What fraction fb of globular cluster stars are binaries?

Does fb differ among clusters? Range?

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


Observations of binaries in globular clusters

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


Observations of binaries in globular clusters

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 %


Observations of binaries in globular clusters

photometric variables – global binary fractions

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

 consistent with vrad results


Observations of binaries in globular clusters

Albrow et al. 2001 – 47 Tuc


Observations of binaries in globular clusters

CMD outliers – binary fractions – “all q” subset


Observations of binaries in globular clusters

Zhao & Bailyn 2005 – M3


Observations of binaries in globular clusters

Zhao & Bailyn 2005 – M3


Observations of binaries in globular clusters

mass ratio

distribution

F(q)

core 1 – 2 rc

more low q

~ flat

more high q

q = 1

Zhao & Bailyn 2005 – M3


Observations of binaries in globular clusters

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??


Observations of binaries in globular clusters

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?


Observations of binaries in globular clusters

Pal 5 – compare 2 methods

Koch et al. 2004

Odenkirchen et al. 2002

CMDs: fb ~ 9  1%

vrad: fb ~ 40  20 %


Observations of binaries in globular clusters

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


Observations of binaries in globular clusters

effect of photometric errors on

perceived binary fraction in CMDs

fb = 10%

Hut et al. 1992


Observations of binaries in globular clusters

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


Observations of binaries in globular clusters

Clark et al. 2004 – Pal 13

Cool & Bolton 2002 – NGC 6397


Observations of binaries in globular clusters

effect of photometric errors on

perceived binary fraction in CMDs

fb = 100%

Hut et al. 1992


Observations of binaries in globular clusters

Are there signs of dynamical evolution of binaries?


Observations of binaries in globular clusters

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??


Observations of binaries in globular clusters

Bellazzini et al. 2002 – NGC 288


Observations of binaries in globular clusters

Bellazzini et al. 2002 – NGC 288


Observations of binaries in globular clusters

mass segregation in NGC 288

Bellazzini et al. 2002 – NGC 288


Observations of binaries in globular clusters

mass segregation of binaries in 47 Tuc

Albrow et al. 2001 – 47 Tuc

71 BY Dra stars in 47 Tuc!

an untapped resource


Observations of binaries in globular clusters

period segregation of eclipsing binaries

in 47 Tuc

contact all stars detached

Weldrake et al. 2004 – 47 Tuc


Observations of binaries in globular clusters

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?


Observations of binaries in globular clusters

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


Observations of binaries in globular clusters

Cool & Bolton 2002 – NGC 6397


Observations of binaries in globular clusters

Cool & Bolton 2002 – NGC 6397


Observations of binaries in globular clusters

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)


Observations of binaries in globular clusters

How many “primordial” binaries are really primordial??


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