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White Dwarfs

White Dwarfs. With contributions from S. R. Kulkarni T. Monroe. References. D. Koester, A&A Review (2002) “White Dwarfs: Recent Developments” Hansen & Liebert, Ann Rev A&A (2003) “Cool White Dwarfs” Wesemael et al. PASP (1993) “An Atlas of Optical Spectra of White-Dwarf Stars”

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White Dwarfs

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  1. White Dwarfs With contributions from S. R. Kulkarni T. Monroe

  2. References • D. Koester, A&A Review (2002) “White Dwarfs: Recent Developments” • Hansen & Liebert, Ann Rev A&A (2003) “Cool White Dwarfs” • Wesemael et al. PASP (1993) “An Atlas of Optical Spectra of White-Dwarf Stars” • Wickramsinghe & Ferrario PASP (2000) “Magnetism in Isolated & Binary White Dwarfs”

  3. References • Dreizler, S. 1999, RvMA, 12, 255D • Fontaine et al. 2001, PASP, 113, 409 • Hansen, B. 2004, Physics Reports, 399, 1 • Hansen, B & Liebert, J. 2003 ARA&A, 41, 465 • Hearnshaw, J.B. 1986, The Analysis of Starlight. • Koester, D. & Chanmugam, G. 1990, RPPh, 53, 837K • Shipman, H. 1997, White Dwarfs, p. 165. Kluwer • Wesemael et al. 1993, PASP, 105, 761

  4. How stars die • Stars above 8 Msun form neutron stars and black holes • Below 8 Msun the stars condense to O-Ne-Mg white dwarfs (high mass stars) or usually C-O white dwarfs • Single stars do not form He white dwarfs but can form in binary stars • We know of no channel to form H white dwarfs of some reasonable mass

  5. History of White Dwarf Discovery • Bessell (1844)-variability in proper motions of Sirius and Procyondark companions • Clark (1861) visually sighted Sirius B • Schaeberle (1896) Lick Obs. announced Procyon’s companion • 40 Eri (faint white and red stars) • Class A0, Russell dismissed when 1st Russell diagram published • Adams confirmed A-type • Adams (1915)-Sirius B spectrum Type A0 • Eddington (1924) Mass-Luminosity Relationship • Coined “white dwarfs” for 1st time • Deduced mass and radius of Sirius B density=53,000x water • Fowler (1926) WDs supported by electron degeneracy pressure, not thermal gas pressure • Chandrasekhar (early 1930s) worked out details of white dwarf structure, predicted upper mass limit of 1.44 Msun, & found mass-radius relation

  6. Early Classifications • Kuiper (mid-1930s, Lick Obs.) WDs found in increasing numbers • 1941 introduced 1st WD classification scheme • w in front of spectral type and Con stars • Luyten (1921) proper motion studies from faint blue star surveys • 1952 presented new scheme for 44 WDs • D for true degeneracy, followed by A, B, C, or F • Greenstein (1958) introduced new scheme • 9 types

  7. Current ClassificationsSion (et al. 1983) • ~2200 WDs w/in ~500 pc of Sun • D=degenerate • Second Letter-primary spectroscopic signature in optical • DA-Hydrogen lines (5000K<Teff<80000K) • DB-He I lines (Teff<30000K) • DC-Continuous spectrum (Teff<11,000K) • DZ-Metal lines (Mg, Ca, Fe) • DQ-Atomic/Molecular carbon features • DO-He II lines (Teff>45,000K) • Additional letters indicate increasingly weaker or secondary features, e.g. DAZ, DQAB • P-polarized magnetic, H-non-polarized magnetic, V-variable • Teff indicated by digit at end; 50,400/Teff, e.g. DA4.5 • New class Teff<4000K, IR absorption for CIA by H2

  8. DB Spectra DA Spectra Rapid settling of elements heavier than H in high gravity

  9. DQ Stars & Spectra • Helium-rich stars, generally characterized by C2-Swan bands • Hotter DQs have C I

  10. ZZ Ceti PG 1159 Spectra • Features due to CNO ions, Teff>100,000K • Absence of H or He I features; He II, C IV, O VI

  11. Magnetic WDs • About 5% of field white dwarfs display strong magnetism • 3 classes of H-atmosphere MWDs based on field strength • He-atmosphere MWDs have unique features

  12. Basic Picture • 75% DA, 25% non-DA • Spectral classification provides info about principal constituent, with some T info • Progenitors: Post-AGB stars, central stars of planetary nebulae (CSPN), hot subdwarfs • Expected structure-stratified object with <M>~0.6Msun • C-O core, He-rich envelope, H-rich shell • O-Ne cores-most massive • Atmosphere contains <10-14 M • Many WDs have pure H or He atmospheres • Thicknesses of H and He

  13. Mechanisms in Atmosphere • Gravitational diffusion • Convection • Radiative levitation • Magnetism • Accretion • Wind-loss • T-sensitive  T determines chemical abundances

  14. Effects of Mechanisms • Diffusion & Settling • Gravitational separation leads to pure envelope of lightest element t<108 yr • But, observations show traces of heavier elements • radiative levitation • Cooler WDs result of recent accretion event • Radiative Levitation T>40kK • Radiative acceleration on heavy elements • Convection for T<12kK • Convection zone forms and increases inward as star cools • For He envelopes, convection begins at high T • Mixing changes surface composition • Need to couple models of atmospheres and interiors

  15. Statistics • T>45kK DA far outnumber DO • Ratio increases to about 30kK (diffusion) • DB gap in 45k-30kK range • Float up of H • Always enough H to form atmosphere? • Dredge up of He • T<30kK He convection zone massive engulfs outer H layer if thin • 30kK-12kK 25% stars revert to DB spectral type (edge of ZZ Ceti Strip) • Convection zone increases as T decreases. At T~11kK, numbers of DAs and non-DAs are ~equal (ZZ Ceti Strip) • ‘Non-DA gap’ for 5000-6000K dearth of He atmospheres

  16. Spectral Evolution • Gapsindividual WDs undergo spectral evolution • Compositions change, DADBDA, as T changes • Evolution of convection zone? Accretion? • Explanation of ‘non-DA gap’-opacity? Bergeron et al. • Low opacity of He I means small amounts of H dominates opacity • H- atomic energy levels destroyed when H added to dense atmosphere-reduces H opacity contribution • Must accrete a lot of H to make difference in photospheric conditionsDA (fixes 6000K edge) • Re-appearance of DBs at 5000K b/c convection zone grows, H is diluted with additional He • This fails! Destruction of H- bound level produces free e-, which provide opacity

  17. ZZ Ceti Cooling Evolution

  18. Model Atmospheres • Plane-parallel geometry • Hydrostatic equilibrium (mass loss rates) • NLTE • Stratisfied Atmospheres • Parameters: degree of ionization, intensity of radiation field • Make radiative cross sections of each element depth dependent • Convection • Parameters of Mixing Length theory

  19. White Dwarfs in Globular Clusters

  20. Cluster White Dwarf Spectroscopy

  21. White Dwarfs in Clusters • Chronometers: Use cooling models to derive the ages of globular clusters • Yardsticks: Compare nearby and cluster white dwarfs. • Forensics: Diagnose the long dead population of massive stars

  22. The Globular Cluster M4 • Fainter white dwarfs are seen in this nearby cluster -> age = 12.7 +/- 0.7 Gyr M4 formed at about z=6 Disk formed at about z=1.5 • dN/dM, differential mass spectrum dN/dM propto M-0.9

  23. White Dwarfs in Open Clusters Open Clusters have a wide range of ages (100 Myr to 9 Gyr, the age of the disk) • Use white dwarfs as chronometers • Derive initial-mass to final-mass mapping Key Result: MWD about 8 MSun This result is in agreement with stellar models

  24. Field White Dwarfs • Identified by large proper motion yet faint object • LHS (Luyten Half Second) • NLTT (New Luyten Two Tenths) • Blue Objects (found in quasar surveys) • Very Hot objects (found in X-ray surveys)

  25. Field White Dwarfs

  26. Old White Dwarfs • Microlensing observations indicate presence of 0.5 Msun objects in the halo • Old white white dwarfs expected in our disk, thick disk and halo • These old white dwarfs are paradoxically blue (cf cool brown dwarfs)

  27. Determination of Mass (Field Objects) • Spectroscopic Method: Line (Hydrogen) width is sensitive to pressure which is proportional to gravity g = GM/R2 • Photometric Method: Broad-band photometry fitted to black body yields Teff and angular size Combine with parallax to get radius R Use Mass-Radius relation to derive Mass

  28. Masses of White Dwarfs

  29. Magnetism in Isolated White Dwarfs • About 5% of field white dwarfs exhibit strong magnetism • On average, these white dwarfs have larger mass • Some rotate rapidly and some not at all • Magnetism thus influences the initial-final mapping relation • Or speculatively, some of these are the result of coalescence of white dwarfs

  30. Zeeman (Landau) Splitting

  31. Future/Active Work • Exact masses of H and He layers • Thin or Thick Envelopes • Explanations for DB-gap • Explanations for ‘non-DA gap’ • DAs outnumber He-rich WDs, yet progenitor PNN have ~equal numbers of H- and He-rich stars. What rids degenerates of He? • Couple core & atmosphere models

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