White Dwarfs

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White Dwarfs
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References

• D. Koester, AA Review (2002)
• White Dwarfs Recent Developments
• Hansen Liebert, Ann Rev AA (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

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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 other than Brown Dwarfs

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White Dwarfs in Globular Clusters
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Cluster White Dwarf Spectroscopy
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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

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The Globular Cluster M4

• Fainter white dwarfs are seen in this nearby
  cluster
• -gt age 12.7 /- 0.7 Gyr
• M4 formed at about z6
• Disk formed at about z1.5
• dN/dM, differential mass spectrum
• dN/dM propto M-0.9

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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

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Open Cluster M67
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M67
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Age of M67
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Field White Dwarfs

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

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Field White Dwarfs
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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)

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Spectroscopic Classification

• DA, strong Hydrogen lines
• DB, strong He I lines
• DO, strong He II lines
• DC, no strong lines (continuous) spectrum
• DZ, strong metal lines (excluding carbon)
• DQ, strong carbon lines
• Multiple families shown in decreasing order e.g.
  DAB, DQAB, DAZ

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Spectroscopic Features A few comments

• Strong gravity of white dwarfs result in rapid
  settling of elements e.g. Hydrogen always rises
  to the top and can mask other elements
• Given the above white dwarf atmosphere modeling
  is generally considered to be more tractable than
  for other stars
• If trace elements are seen as in DZ white dwarfs
  then they must be of recent origin (e.g.
  accretion from the ISM, comets etc)

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DQZ T7740K log(g)8.0 Mass from Orbit
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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

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Masses of White Dwarfs
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Magnetism in Isolated White Dwarfs

• About 5 of field white dwarfs exhibit strong
  magnetism
• On an averge 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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Magnetism in White Dwarfs
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Zeeman (Landau) Splitting
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