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

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?

  • What’s new?

  • What’s next?


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


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


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


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


searches for radial velocity variables


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


searches for eclipsing binaries – selected results


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


searches in color-magnitude diagrams – selected early results


searches in color-magnitude diagrams -- more recent results


What fraction fb of globular cluster stars are binaries?

Does fb differ among clusters? Range?

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


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


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 %


photometric variables – global binary fractions

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

 consistent with vrad results


Albrow et al. 2001 – 47 Tuc


CMD outliers – binary fractions – “all q” subset


Zhao & Bailyn 2005 – M3


Zhao & Bailyn 2005 – M3


mass ratio

distribution

F(q)

core 1 – 2 rc

more low q

~ flat

more high q

q = 1

Zhao & Bailyn 2005 – M3


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??


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?


Pal 5 – compare 2 methods

Koch et al. 2004

Odenkirchen et al. 2002

CMDs: fb ~ 9  1%

vrad: fb ~ 40  20 %


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


effect of photometric errors on

perceived binary fraction in CMDs

fb = 10%

Hut et al. 1992


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


Clark et al. 2004 – Pal 13

Cool & Bolton 2002 – NGC 6397


effect of photometric errors on

perceived binary fraction in CMDs

fb = 100%

Hut et al. 1992


Are there signs of dynamical evolution of binaries?


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??


Bellazzini et al. 2002 – NGC 288


Bellazzini et al. 2002 – NGC 288


mass segregation in NGC 288

Bellazzini et al. 2002 – NGC 288


mass segregation of binaries in 47 Tuc

Albrow et al. 2001 – 47 Tuc

71 BY Dra stars in 47 Tuc!

an untapped resource


period segregation of eclipsing binaries

in 47 Tuc

contact all stars detached

Weldrake et al. 2004 – 47 Tuc


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?


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


Cool & Bolton 2002 – NGC 6397


Cool & Bolton 2002 – NGC 6397


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)


How many “primordial” binaries are really primordial??


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