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

OUTLINE I.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 III.White Dwarf Physics from Globular Clusters Overcoming obstacles with globular clusters

OUTLINE I.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 III.White Dwarf Physics from Globular Clusters Overcoming obstacles with globular clusters

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

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 decoupled mechanical and thermal properties: ions  electrons

White Dwarf Flavors

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

… 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 …

White Dwarf Animation

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

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…

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

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

OUTLINE I.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 III.White Dwarf Physics from Globular Clusters Overcoming obstacles with globular clusters

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

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

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

HET Spectra of Cool White Dwarf Stars

MS Age vs WD Age

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

OUTLINE I.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 III.White Dwarf Physics from Globular Clusters Overcoming obstacles with globular clusters

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!

NGC 6397

NGC 6397 with HST AC

Comparing Theoretical models: new(er) opacities, interior EOS and atmospheric boundary conditions Hansen & Liebert (2003)

Fontaine 2001 models and Winget et al models 0.5 Msun

Conclusions from model comparisons Mass – radius is consistent for all groups – EoS improvements ( Chabrier et al 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

HST Observations Hansen et al point sources only

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

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 very narrow. Ages provide some independent information … The terminus white dwarfs aren’t as old as you think!

Oops … the CIA hook is in the wrong place!

Luminosity Function for NGC 6397 proper motion screened WD sample

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

Richer et al completeness

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

Fontaine, Brassard & Bergeron (2001)

Crystallization Visuallization a cartoon by M.H. Montgomery

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) (MIX)= (Horowitz, Berry & Brown 2007) This is at the frontier of (brute force) molecular dynamics

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)

Richer et al completeness

Conclusions from NGC 6397 Confirm that crystallization occurs Confirm that Debye cooling occurs We can measure the Gamma of 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 = We find the first empirical evidence that Van Horn’s 1968 prediction is correct: Crystallization is a first order phase transition

The End Thank you

Some Recommended Reading Reviews: Fontaine, Brassard & Bergeron 2001, PASP 113:409 Hansen & Liebert 2003, ARA&A 41:465 Winget & Kepler 2008, ARA&A New: Richer et al. 2008, AJ, 135, 2141 Hansen et al. 2007, ApJ, 671,380 Winget, Kepler, Campos, Montgomery, Girardi & Bergeron 2008 submitted to ApJ Classic: Van Horn 1968, ApJ 151:227

Conclusions from NGC 6397 He mixing combined with current CIA opacities may solve the mystery of the “blue hook” in NGC 6397 The H/He ratio is consistent with cosmic … accretion over dredge up?

Questions from NGC 6397  What will new CIA opacities look like?  Caution: Is mixing weak or strong?  Is this sensitive to metallicity?  How does this affect ages and cosmochronology?  Have we solved the 30 year old mystery of cool WDs?  Is the disk the same or different in the ratio of DAs to nonDAs?

He mixing combined with CIA opacities explains the mysterious “blue hook.”

Observational and theoretical futures for EoS constraints and other physics from white dwarfs More fields for NGC 6397 and other globular clusters More clusters: globular and rich, old, open clusters different white dwarf and masses and Z, C/O = C/O(Z)? SDSS => enormous increase in the disk and halo white dwarfs SDSS => more asteroseismology of high (near Chandra mass) and (He-core) low mass white dwarf stars

Observational and theoretical futures for EoS constraints and other physics (cont’d) Measurements of evolutionary changes allows study of particle physics aspects and general thermal properties Bayesian analysis of data with different classes of theoretical models for these large observational samples New opacity calculations for warm and cool white dwarfs Your list goes here ….

Some Recommended Reading (reprise) Reviews: Fontaine, Brassard & Bergeron 2001, PASP 113:409 Hansen & Liebert 2003, ARA&A 41:465 Winget & Kepler 2008, ARA&A in press New: Richer et al. 2008, AJ, 135, 2141 Hansen et al. 2007, ApJ, 671,380 Winget, Kepler, Campos, Montgomery, Girardi & Bergeron 2008 submitted to ApJL Classic: Van Horn 1968, ApJ 151:227

CMDs for a Range of Cluster Ages

Cool WDs Theory vs Observation

LHS1126 Weird DQ Cool WD with Spitzer Spitzer Space Telescope Data Cool WDs Theory vs Observation

4 Cool WDs with Spitzer

Fontaine 2001 models and Winget et al models 0.8 Msun