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Observations of Binaries in Globular Clusters

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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  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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