With contributions from
S. R. Kulkarni
T. Monroe

• 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”

• 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

• 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

• 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 1 st Russell diagram published
– Adams confirmed A-type
• Adams (1915)-Sirius B spectrum  Type A0
• Eddington (1924) Mass-Luminosity Relationship
– Coined “white dwarfs” for 1 st 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 M found mass-radius relation sun
, &

• Kuiper (mid-1930s, Lick Obs.) WDs found in increasing numbers
– 1941 introduced 1 st 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

Sion (et al. 1983)
• ~2200 WDs w/in ~500 pc of Sun
• D=degenerate
• Second Letter-primary spectroscopic signature in optical
– DA-Hydrogen lines (5000K<T eff
<80000K)
– DB-He I lines (T eff
<30000K)
– DC-Continuous spectrum (T
– DO-He II lines (T eff eff
<11,000K)
– DZ-Metal lines (Mg, Ca, Fe)
– DQ-Atomic/Molecular carbon features
>45,000K)
• Additional letters indicate increasingly weaker or secondary features, e.g. DAZ, DQAB
• T
– P-polarized magnetic, H-non-polarized magnetic, V-variable eff indicated by digit at end; 50,400/T
• New class T eff
, e.g. DA4.5
eff
<4000K, IR absorption for CIA by H
2

Rapid settling of elements heavier than H in high gravity

• Helium-rich stars, generally characterized by
C
2
-Swan bands
• Hotter DQs have C I

• Features due to CNO ions,
T eff
>100,000K
• Absence of H or He I features; He II, C IV, O VI
ZZ
Ceti

• About 5% of field white dwarfs display strong magnetism
• 3 classes of Hatmosphere MWDs based on field strength
• He-atmosphere
MWDs have unique features

• 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>~0.6M
sun
– 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

• Gravitational diffusion
• Convection
• Radiative levitation
• Magnetism
• Accretion
• Wind-loss
• T-sensitive  T determines chemical abundances

• Diffusion & Settling
– Gravitational separation leads to pure envelope of lightest element t<10 8 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

• 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

• 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

CSPN
Hot DAZs (T>40kK)
Radiative leviation makes Z
No Z cooler than 35kK
ZZ Ceti w/ variable H layers
10 -8 …………………10 -4 M sun
He-Rich DA
(0.01<He/H<20)
Pure DA
(He/H<0.01)
Some DC, DZ
Cool DAs
Some w/ T<5kK
ZZ Ceti

• 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 Globular Clusters

Cluster White Dwarf Spectroscopy

• 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

• 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

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: M
WD about 8 M
Sun
This result is in agreement with stellar models

• 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)

Field 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)

• Spectroscopic Method:
Line (Hydrogen) width is sensitive to pressure which is proportional to gravity g = GM/R 2
• 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

Masses of 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

Zeeman (Landau)
Splitting

• 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