white dwarfs
DESCRIPTION
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) - PowerPoint PPT PresentationTRANSCRIPT
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White Dwarfs
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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”
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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
-> 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
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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 linesMultiple 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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DQZT=7740Klog(g)=8.0Mass from Orbit
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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
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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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