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The Physics of Crystallization in a Dense Coulomb Plasma from Globular Cluster White Dwarf Stars

The Physics of Crystallization in a Dense Coulomb Plasma from Globular Cluster White Dwarf Stars. Don Winget Department of Astronomy and McDonald Observatory University of Texas and Department of Physcis UFRGS Brasil S.O. Kepler, Pierre Bergeron, Mike Montgomery,

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The Physics of Crystallization in a Dense Coulomb Plasma from Globular Cluster White Dwarf Stars

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  1. The Physics of Crystallization in a Dense Coulomb Plasma from Globular Cluster White Dwarf Stars Don Winget Department of Astronomy and McDonald Observatory University of Texas and Department of Physcis UFRGS Brasil S.O. Kepler, Pierre Bergeron, Mike Montgomery, Fabi Campos, Leo Girardi, Kurtis Williams

  2. OUTLINE • Historical & Astrophysical Context • Quantum mechanics, cosmochronology and the equation of state (EoS) of matter • II. What We Can Learn From the Disk • Obstacles remain, even after 20 years • White Dwarf Physics from Globular Clusters • Overcoming obstacles with globular clusters

  3. OUTLINE • Historical & Astrophysical Context • Quantum mechanics, cosmochronology and the equation of state (EoS) of matter • II. What We Can Learn From the Disk • Obstacles remain, even after 20 years • White Dwarf Physics from Globular Clusters • Overcoming obstacles with globular clusters

  4. Cat’s Eye: What Are White Dwarf Stars? White Dwarf Stars: Eddington’s “Impossible” Star • 1844: Bessel notices “wobble” in Sirius’ position • 1862: Alvan Clark directly observes a faint companion Sirius B

  5. Cat’s Eye: What Are White Dwarf Stars? White Dwarf Stars: Eddington’s “Impossible” Star • Dark companion is hot and compact, roughly the size of Earth and the mass of the Sun • Interior, even if made of the smallest atoms, must be ionized • “ … to cool the star must expand and do work against gravity …” Eddington. • Heisenberg uncertainty principle and Pauli exclustion principle to the rescue – Fowler 1926 • Chandrasekhar (1931) limit • Mestel (1952) Theory: develops understanding of decoupledmechanical and thermal properties: • ions  electrons

  6. White Dwarf Flavors

  7. Cat’s Eye: What Are White Dwarf Stars? White Dwarf Stars: • Endpoint of evolution for most stars • Homogeneous • Narrow mass distribution • Chemically pure layers • Uncomplicated • Structure • Composition • Evolution dominated bycooling: (oldest=coldest) They Shed Their Complexity!

  8. … and why are they interesting? • Representative (and personal) • 98% of all stars, including our sun, will become one • Archeological history of star formation in our galaxy => White Dwarf Cosmochronology • A way to find Solar Systems dynamically like ours • Exploration of Extreme physics • Matter at extreme densities and temperatures • 60% of the mass of the Sun compressed into star the size of the Earth • Chance to study important and exotic physical processes: plasmon neutrinos, search for dark matter in the form of axions , and study the physics of crystallization …

  9. White Dwarf Animation

  10. Observations of Coolest WDs White Dwarf Cosmoshronology • Observations: finding the coolest white dwarf stars in a population • Thin disk • Open clusters • Thick disk • Halo • Globular clusters

  11. Calculate the ages of the coolest white dwarf stars:White dwarf cosmochronology • Critical theoretical uncertainties for dating the coolest WDs • Outer layers • Convection, degeneracy, and radiative opacity control throttle • Interiors • Neutrino emission in the hot stars • Crystallization and phase separation in coolest • Compare with observed distribution, and repeat the cycle…

  12. (log P, log T) plane Hot pre-white dwarf model cool white dwarf model

  13. Physical Properties in White Dwarf Stars The DB “Gap” Various physical processes thought to occur in WDs as they cool

  14. OUTLINE • Historical & Astrophysical Context • Quantum mechanics, cosmochronology and the equation of state (EoS) of matter • II. What We Can Learn From the Disk • Obstacles remain, even after 20 years • White Dwarf Physics from Globular Clusters • Overcoming obstacles with globular clusters

  15. The Disk Luminosity Function Fontaine, Brassard, & Bergeron (2001)

  16. DeGennaro et al. (2008) Disk LF 3358 new SDSS WDs (with spectra)

  17. Going after the cool WDs: Mukremin Kilic …. shows the lower left portion of the reduced proper motion diagram from SDSS Data Release 2.

  18. HET Spectra of Cool White Dwarf Stars

  19. The Disk vs M4: Globular clusters are older than the disk …. Hansen & Liebert (2003)

  20. OUTLINE • Historical & Astrophysical Context • Quantum mechanics, cosmochronology and the equation of state (EoS) of matter • II. What We Can Learn From the Disk • Obstacles remain, even after 20 years • White Dwarf Physics from Globular Clusters • Overcoming obstacles with globular clusters

  21. White Dwarf Stars in Clusters • Explore white dwarf cooling ages as compared to main sequence isochrone ages • Open clusters help in establishing constraints on disk age • Older open clusters sample critical physics of white dwarf cooling • Minimize problems with birthrates • Globular Clusters: Finally, we can isolate masses and explore the physics!

  22. NGC 6397

  23. NGC 6397 with HST AC

  24. ComparingTheoreticalmodels:new(er) opacities, interior EOS and atmospheric boundary conditionsHansen & Liebert (2003)

  25. Fontaine 2001 models and Winget et al. 2008 models 0.5 Msun

  26. Conclusions from model comparisons • Mass – radius is consistent for all groups • EoS improvements ( Chabrier et al. 2000 over Lamb & Van Horn 1975 for interiors and Saumon Chabrier & Van Horn 1993 over Fontaine , Graboske & Van Horn 1977 for the envelope) do not produce (presently) observable differences in the models. • Improved atmospheric surface boundary condition is not as important as has been claimed in the literature … it produces no observable differences until bolometric luminosities below the largest magnitude globular cluster stars

  27. HST ObservationsHansen et al. 2007point sourcesonly

  28. Fixing the WD evolutionary tracks in the CMD by simultaneously fitting the main sequence and the WDsgives Z, (m-M) and E Data: proper motion screened sample from Richer et al. 2008, AJ, 135,2131

  29. What advantages do we have over the disk population? • The cooling sequences are “pinned” to the CMD by the main sequence and white dwarfs fitted together – sliding is not allowed. • If we ignore the observational errors, the CMD location of a star uniquely determines its mass and radius: setting the mechanical properties of the white dwarf determined independently of the thermal. • The mass range is verynarrow. • Ages provide some independent information … The terminus white dwarfs aren’t as old as you think!

  30. Oops … the CIA hook is in the wrong place!

  31. Luminosity Function for NGC 6397 proper motion screened WD sample

  32. Richer et al. 2008 (proper motion) Hansen et. Al. 2007

  33. Richer et al. 2008 completeness

  34. What physics might be relevant near the peak of theLuminosity Function (the “clump” in the CMD)? • Convective Coupling: The surface convection zone reaches the degeneracy boundary, reducing the insulation of the envelope • Crystallization: Ions crystallize with attendant latent heat and phase separation expected from theory

  35. Fontaine, Brassard & Bergeron (2001)

  36. Crystallization Visuallization a cartoon by M.H. Montgomery

  37. Ratio of Coulomb Energy to Ion Thermal Energy What is the expected value of Gamma at crystallization? (OCP) = 176 (Potekhin & Chabrier 2000, DeWitt et al. 2001, Horowitz, Berry & Brown 2007) • = 230 - 260(Horowitz, Berry & Brown 2007) • This is at the frontier of (brute force) molecular dynamics

  38. Ratio of Coulomb Energy to Ion Thermal Energy What is the value of Gamma at and near the “clump” in the observed CMD, or equivalently, the value of Gamma at and before (rise) the peak of the Luminosity Function? log rho = 6.32 log T = 6.40 … nearly independent of composition! (peak) = 194 (carbon) = 313 (oxygen) (rise) = 182 (carbon) = 291 (oxygen)

  39. Richer et al. 2008 completeness

  40. Conclusions from NGC 6397 • Confirm that crystallization occurs • Confirm that Debye cooling occurs • We can measuretheGammaof crystallization • Low metallicity clusters may not produce significant O in cores of some of the 0.5Msun stars … or Brown and collaborators are right and Gamma = 230 - 260 • We find the first empirical evidence that Van Horn’s 1968 prediction is correct: Crystallization is a first order phase transition

  41. The End Thank you

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