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Observations of Binaries in Globular Clusters

Explore the significance of binary fractions in globular clusters, their impact on cluster dynamics, and the methods to study them through observations of period, eccentricity, mass ratio, and distribution. Learn about the latest findings and future directions in this field.

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Observations of Binaries in Globular Clusters

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  1. Observations of Binaries in Globular Clusters Adrienne Cool San Francisco State University

  2. Primordial Observations of ^ Binaries in Globular Clusters Adrienne Cool San Francisco State University

  3. OUTLINE • Why bother? • What do we want to know? • How can we find out? • What’s new? • What’s next?

  4. 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

  5. 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

  6. 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

  7. 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

  8. searches for radial velocity variables

  9. 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

  10. searches for eclipsing binaries – selected results

  11. 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

  12. searches in color-magnitude diagrams – selected early results

  13. searches in color-magnitude diagrams -- more recent results

  14. What fraction fb of globular cluster stars are binaries? Does fb differ among clusters? Range? Do any clusters have fb = 0% ? 100% ??

  15. 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

  16. 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 %

  17. photometric variables – global binary fractions 47 Tuc: fb ~ 2 – 5 % per decade for hard binaries  consistent with vrad results

  18. Albrow et al. 2001 – 47 Tuc

  19. CMD outliers – binary fractions – “all q” subset

  20. Zhao & Bailyn 2005 – M3

  21. Zhao & Bailyn 2005 – M3

  22. mass ratio distribution F(q) core 1 – 2 rc more low q ~ flat more high q q = 1 Zhao & Bailyn 2005 – M3

  23. 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??

  24. 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?

  25. Pal 5 – compare 2 methods Koch et al. 2004 Odenkirchen et al. 2002 CMDs: fb ~ 9  1% vrad: fb ~ 40  20 %

  26. 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

  27. effect of photometric errors on perceived binary fraction in CMDs fb = 10% Hut et al. 1992

  28. 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

  29. Clark et al. 2004 – Pal 13 Cool & Bolton 2002 – NGC 6397

  30. effect of photometric errors on perceived binary fraction in CMDs fb = 100% Hut et al. 1992

  31. Are there signs of dynamical evolution of binaries?

  32. 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??

  33. Bellazzini et al. 2002 – NGC 288

  34. Bellazzini et al. 2002 – NGC 288

  35. mass segregation in NGC 288 Bellazzini et al. 2002 – NGC 288

  36. mass segregation of binaries in 47 Tuc Albrow et al. 2001 – 47 Tuc 71 BY Dra stars in 47 Tuc! an untapped resource

  37. period segregation of eclipsing binaries in 47 Tuc contact all stars detached Weldrake et al. 2004 – 47 Tuc

  38. 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?

  39. 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

  40. Cool & Bolton 2002 – NGC 6397

  41. Cool & Bolton 2002 – NGC 6397

  42. 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)

  43. How many “primordial” binaries are really primordial??

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