White Dwarfs

With contributions fromS. 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”• 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

DA Spectra

Rapid settling of elements heavier than H in high gravity

DB Spectra

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 Globular Clusters

Cluster White Dwarf Spectroscopy

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)

Field White Dwarfs

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

Masses of White Dwarfs

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

Zeeman (Landau)Splitting

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