lecture i: introduction to white dwarfs s. r. kulkarni, e. ofek srk/ay125
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Lecture I: Introduction to White Dwarfs
S. R. Kulkarni, E. Ofek
http://astro.caltech.edu/~srk/ay125/
Lecture 1: White Dwarfs (Introduction)
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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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White Dwarfs – Physical understanding
1926: Fermi & Dirac - statistical physics of electron gas
1926: Fowler – electron degeneracy pressure can support
stellar mass object against its gravity
1939: Chandrasekhar – derived the equations for the
structure, mass-radius-core composition
relations.
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Interesting implications of degenerate electron gas
“inverted” mass radius relation
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Interesting implications of degenerate electron gas
Nuclear reactions are explosive
Novae
Type Ia supernovae
He flash in evolved stars
WD are condactive
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Observed properties
T=4,500 – 150,000 K
log g = 7 – 9 (cgs)
mass = 0.4 – 1.4 solar mass
mean radius = 0.012 solar radius
log L = -4.3 – 3 (in solar luminosity)
Upper limit on rotation : Vsini < 65 km/s
observed mass density 0.002 solar mass per cubic pc
formation rate: 4.5-7.5x10-13 pc-3 yr-1
Galactic scale height: 250-300 pc
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Spectroscopic classification
DA - only Balmer lines; no HeI or metals (75% of WD)
DB – HeI lines, no H or metals
DC – Continuous spectrum, no lines deeper than 5%
DO – strong HeII, HeI or H present
DZ – Metal lines only, no H or He
DQ – Carbon features
Temperature index in the range 0-9 in units of 5040K
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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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Determination of Mass (Field Objects)
• Spectroscopic Method:Line (Hydrogen) width is sensitive to pressure
which is proportional to gravityg = GM/R2
* gravitational redshift (M/R)• Photometric Method:
Broad-band photometry fitted to black body yields Teff and angular size
Combine with parallax to get radius RUse Mass-Radius relation to derive Mass
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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 – Supported by cluster observations
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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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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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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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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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Pulsating WD
ZZ Cet: DA Teff~12,000+/-1000K P~100-1200s
GD358: DB Teff~28,000+/-1000K P~100-1200s
GW Vir: DO Teff~130,000+/-20,000 P~200-2000s
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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
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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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