Transcript White Dwarfs - California Institute of Technology

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
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)
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)
DQZ
T=7740K
log(g)=8.0
Mass from Orbit
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
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