White Dwarfs

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”

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]

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

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Open Cluster M67

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)

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)

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

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)

DQZT=7740Klog(g)=8.0Mass from Orbit

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 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

Magnetism in White Dwarfs

Zeeman (Landau)Splitting