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The White Dwarf Age of NGC 2477

A Bayesian Approach to Measuring Cluster Ages. The White Dwarf Age of NGC 2477. Elizabeth Jeffery Space Telescope Science Institute Collaborators: Ted von Hippel, Steven DeGennaro, David van Dyk, Nathan Steinn, W.H. Jefferys, D.E. Winget, Kurtis Williams White Dwarf European Workshop

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The White Dwarf Age of NGC 2477

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  1. A Bayesian Approach to Measuring Cluster Ages The White Dwarf Age of NGC 2477 Elizabeth Jeffery Space Telescope Science Institute Collaborators: Ted von Hippel, Steven DeGennaro, David van Dyk, Nathan Steinn, W.H. Jefferys, D.E. Winget, Kurtis Williams White Dwarf European Workshop Tübingen, Germany August 17, 2010

  2. Talk Outline Stellar Ages: Main Sequence and White Dwarf Ages  Calibration using open clusters Clusters for study Hubble Space Telescope data  WD ages New Bayesian technique analysis of clusters

  3. vs. Alcaino et al. 1998 Winget, et al. 1987 Ages of Stellar PopulationsThe Question of When Fundamental property in astronomy Two main ways: Main Sequence Isochronesvs. the White Dwarf Luminosity Function

  4. Comparing Ages in Open Star ClustersWhy do both? Calibrate MSTO and WD ages Why not use white dwarfs in halo globular clusters? More difficult to observe (3 to date) Gradually increase calibration; thoroughly understand physics WDs provide independent check of MS models, and vice versa increase understanding of both MS models have uncertainties up to 20% between models. WD models have uncertainties up to 5% – 20%

  5. Observing Cluster White Dwarfs • Cluster white dwarfs are FAINT, requiring large or space-based telescopes (Hubble) • This is expensive • This is time consuming • … but it’s possible! …

  6. Open Clusters AgesWhite Dwarf Techniques 0.5 Mo 0.8 Mo Simulated Cluster, 3 Gyr Mv Relationship between a WD’s luminosity and cooling time (i.e., age) Location of the terminus of the cluster WD sequence is determined by the age

  7. NGC 3960 NGC 2660 NGC 2360 NGC 188* Current Agreement * see Poster 72 by Williams, Jeffery, & For von Hippel 2005

  8. Talk Outline Stellar Ages : Main Sequence and White Dwarf Ages  Calibration using open clusters Clusters for study Hubble Space Telescope data  WD ages New Bayesian technique analysis of clusters

  9. HST, ACS and WFPC2: Deep observations of the white dwarfs Data – The Observations CTIO 1m/Y4kCam CCD : Observations of cluster turn off, giants, and upper main sequence

  10. Data – The ObservationsField of View Comparison – NGC 2477 (WFPC2)

  11. Data – Deep Color-Magnitude Diagrams NGC 2360 NGC 2477 NGC 2660 • Things to Note: • Data go deep • Cluster Main Sequence • Many field stars / background galaxies NGC 3960 NGC 188

  12. CMD Features: • Cluster Main Sequence • CMD Features: • Cluster Main Sequence • Background population • Background galaxies • White Dwarfs • CMD Features: • Cluster Main Sequence • Background population • Background galaxies • White Dwarfs • CMD Features: • Cluster Main Sequence • Background population • Background galaxies • White Dwarfs 1 Data – Deep Color-Magnitude Diagrams 2 4 3 NGC 2477

  13. Data – Deep Color-Magnitude Diagrams NGC 2477

  14. 0.5 Gyr 1.0 Gyr 1.5 Gyr Fitting White Dwarf IsochronesNGC 2477

  15. Talk Outline Stellar Ages : Main Sequence and White Dwarf Ages  Calibration using open clusters Clusters for study Hubble Space Telescope data  WD ages New Bayesian technique analysis of clusters

  16. New MachineryUsing Bayesian Statistics Bayesian Statistics – Bayes Theorem: Posterior α Likelihood * Prior Use Markov Chain Monte Carlo (MCMC) technique to numerically sample posterior probability distributions Simultaneously fit age, distance, reddening, and metallicity by modeling photometry. Run with different models: DSED, Y2, Girardi • von Hippel, T., et al. 2006, ApJ, 645, 1436 • Jeffery, E. et al. 2007, ApJ, 658, 391 • DeGennaro S. et al. 2009, ApJ, 696, 12

  17. Data – Deep Color-Magnitude Diagrams NGC 2477

  18. DSED (Dotter et al. 2008) Y2 (Yi et al. 2001) Girardi et al. (2000) Applying MCMCNGC 2477 Posterior Distributions

  19. WD Age (Gyr) : 1.035 ± 0.054 ± 0.087 DSED (Dotter et al. 2008) Y2 (Yi et al. 2001) Girardi et al. (2000) Applying MCMCNGC 2477 Weighted Average (“Answer”) Error among models (“external”) Error within models (“internal”) MSTO Age = 1.0 Gyr (Kassis et al. 1997)

  20. DSED (Dotter et al. 2008) Y2 (Yi et al. 2001) Girardi et al. (2000) Applying MCMCNGC 2477 WD Age (Gyr) : 1.035 ± 0.054 ± 0.087

  21. MS Fit WD Fit Further Power of the TechniqueComparing complete distributions NGC 2360

  22. A Calibration of the AgesWhere we are now

  23. Open clusters provide ideal environment for calibration of MS ages and WD ages, testing theory against theory • Analysis of six clusters • New Bayesian Algorithm • High precision fits allows us to pin down problems in the models • Compare distributions – not just single numbers • Incorporate different combinations of model ingredients, pushing the models to their limits • Good agreement found for clusters included here Concluding Remarks

  24. Improving Error BarsNGC 2420 2 +/- 0.2 Gyr von Hippel & Gilmore 2000

  25. Applying MCMCNGC 2420

  26. Applying MCMCThe Example of NGC 2420

  27. Applying MCMCThe Example of NGC 2420 1.83 +/- 0.12 Gyr

  28. Applying MCMCThe Example of NGC 2420 – MSTO

  29. DSED vs. Y2 isochrones What does the best fit demonstrate? Which is right?  Compare age with the White Dwarfs White dwarf age is 1.83 +/- 0.12 Gyr. Discrepancy in age is likely caused by inadequate incorporation of metallicity in the models.

  30. Ages from the Bright Cluster WDs Rationale of the Idea

  31. A Test Case for the Bright White Dwarf IdeaThe Hyades White Dwarfs DeGennaro S. von Hippel, T., Jefferys, W.H., Stein, N., van Dyk, D., & Jeffery, E., 2008, in prep

  32. Data – Color-Magnitude Diagrams NGC 2360 NGC 2477 NGC 2660 NGC 188 NGC 3960 NGC 2420

  33. Things to Note: • Data go deep • Cluster Main Sequence • Many field stars / background galaxies Data – Deep Color-Magnitude Diagrams NGC 2477

  34. Things to Note: • Data go deep • Cluster Main Sequence • Many field stars / background galaxies Data – Deep Color-Magnitude Diagrams NGC 2660

  35. Things to Note: • Data go deep • Cluster Main Sequence • Many field stars / background galaxies Data – Deep Color-Magnitude Diagrams NGC 188

  36. Fitting White Dwarf Isochrones NGC 2360 NGC 2477 NGC 2660 NGC 3960 NGC 188

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