White Dwarf Spectra and Atmospheres

1. White Dwarf Spectra and Atmospheres Tala Monroe A540 Stellar Atmospheres Apr. 6, 2005

2. Outline • History • Current Classification Scheme • Spectra • Atmospheres • Spectral Evolution • Future Work

3. History • 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

4. 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

5. 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

6. DB Spectra DA Spectra Rapid settling of elements heavier than H in high log g

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

8. 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

9. 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

10. 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

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

12. 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

13. 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

14. 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

15. ZZ Ceti Cooling Evolution

16. 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

17. 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

18. 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

21. DA Stars • 5,000-80,000K • Heavily broadened Balmer lines • Strongest near 12,000K at log g~8 (DA4) • No other features in optical spectrum • Rapid settling of elements heavier than H in high log g • Underabundances of elements by at least 1/100 • Higher dispersion revealed traces of helium in a few-DAO (HeII) and DAB (HeI)

22. DO Stars • Spectra dominated by He II • Teff >45,000 K • 2 subclasses • Cool (Teff~45-70,000K), very strong l4686, also He I features • Hot (Teff>80,000K), only l4686 • At Teff <30,000K, He II can no longer be detected, only see He I

23. DO Spectra

24. DB Stars • Classical DB stars have rich spectra of He I in optical, with nothing else • Coolest DB stars merge with He-rich DQ stars • Many DBs have H, metals (Ca II), and carbon (C I and C2)

25. DZ Stars & Spectra • He-rich stars to cool to show He I, below (DB5-9000K) still show metal features • Ca I, Ca II H and K, Mg I, Fe I, Na I

26. PG 1159 Stars • Features due to CNO ions, Teff>100,000K • Absence of H or He I features; He II, C IV, O VI • 3 groups • A: Cooler Teff~100,000K, He II, C IV, O VI • E: Teff~140,000K, emission cores, He II, C IV, O VI, some have N V (DOQZ1) • lgE: Low g central stars of planetary nuclei • Characteristic emission cores, narrower absorption features

27. DC Stars & Spectra • Featureless, no line deeper than 5% of continuum • Higher resolution reveals weak features • Many reclassified as DB or DA • True DCs remain, among coolest WDs, Teff < 11,000 K