* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Slide 1
Cygnus (constellation) wikipedia , lookup
Cassiopeia (constellation) wikipedia , lookup
Modified Newtonian dynamics wikipedia , lookup
Definition of planet wikipedia , lookup
Perseus (constellation) wikipedia , lookup
Cosmic distance ladder wikipedia , lookup
Nebular hypothesis wikipedia , lookup
Stellar classification wikipedia , lookup
Brown dwarf wikipedia , lookup
Aquarius (constellation) wikipedia , lookup
Future of an expanding universe wikipedia , lookup
Timeline of astronomy wikipedia , lookup
Accretion disk wikipedia , lookup
Star formation wikipedia , lookup
Astronomical spectroscopy wikipedia , lookup
Corvus (constellation) wikipedia , lookup
Stellar kinematics wikipedia , lookup
Open cluster wikipedia , lookup
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 White Dwarf Stars: Eddington’s “Impossible” Star •1844: Bessel notices “wobble” in Sirius’ position •1862: Alvan Clark directly observes a faint companion Sirius B 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 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! … 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 Cosmoshronology •Observations: finding the coolest white dwarf stars in a population –Thin disk –Open clusters –Thick disk –Halo 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… Hot pre-white dwarf model cool white dwarf model Various physical processes thought to occur in WDs as they cool The DB “Gap” 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 portion of Going after the cool WDs:left Mukremin Kilic …. the reduced proper motion diagram from SDSS Data Release 2. HET Spectra of Cool White Dwarf Stars 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. 2008 models 0.5 Msun 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 HST Observations Hansen et al. 2007 point sources only Fixing the WD evolutionary tracks in the CMD by simultaneously fitting the main sequence and the WDs gives Z, (m-M) and E Data: proper motion screened sample from Richer et al. 2008, AJ, 135,2131 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. 2008 (proper motion) Hansen et. Al. 2007 Richer et al. 2008 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)= 230 - 260 (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. 2008 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 = 230 - 260 • 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