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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”
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
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
History of White Dwarf Discovery
• Bessell (1844)-variability in proper motions of Sirius and Procyondark 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
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
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
DQ Stars & Spectra
• Helium-rich stars, generally characterized by C2-Swan bands
• Hotter DQs have C I
PG 1159 Spectra
• Features due to CNO ions, Teff>100,000K
• Absence of H or He I features; He II, C IV, O VI
ZZ Ceti
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
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
Mechanisms in Atmosphere
• Gravitational diffusion• Convection• Radiative levitation• Magnetism• Accretion• Wind-loss• T-sensitive T determines
chemical abundances
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
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
Spectral Evolution
• Gapsindividual WDs undergo spectral evolution– Compositions change, DADBDA, 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
conditionsDA (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
ZZ Ceti
Cooling Evolution
CSPNCSPN
Hot DAZs (T>40kK)Hot DAZs (T>40kK)Radiative leviation makes ZRadiative leviation makes Z
No Z cooler than 35kKNo Z cooler than 35kK
ZZ Ceti w/ variable H layersZZ Ceti w/ variable H layers1010-8-8…………………10…………………10-4-4 M Msunsun
He-Rich DAHe-Rich DA(0.01<He/H<20)(0.01<He/H<20)
Pure DAPure DA(He/H<0.01)(He/H<0.01)
Some DC, DZSome DC, DZ Cool DAsCool DAsSome w/ T<5kKSome w/ T<5kK
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
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
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
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
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)
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)
Determination of Mass (Field Objects)
• Spectroscopic Method:Line (Hydrogen) width is sensitive to
pressure which is proportional to gravityg = GM/R2
• Photometric Method:Broad-band photometry fitted to black
body yields Teff and angular sizeCombine with parallax to get radius RUse Mass-Radius relation to derive
Mass
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
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