Transcript White Dwarfs

White Dwarfs
With contributions from
S. R. Kulkarni
T. Monroe
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 WhiteDwarf Stars”
• Wickramsinghe & Ferrario PASP (2000)
“Magnetism in Isolated & Binary White
Dwarfs”
References
•
•
•
•
Dreizler, S. 1999, RvMA, 12, 255D
Fontaine et al. 2001, PASP, 113, 409
Hansen, B. 2004, Physics Reports, 399, 1
Hansen, B & Liebert, J. 2003 ARA&A, 41,
465
• Hearnshaw, J.B. 1986, The Analysis of
Starlight.
• Koester, D. & Chanmugam, G. 1990, RPPh,
53, 837K
• Shipman, H. 1997, White Dwarfs, p. 165.
Kluwer
• Wesemael et al. 1993, PASP, 105, 761
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
History of White Dwarf Discovery
• Bessell (1844)-variability in proper motions of Sirius and
Procyondark companions
• Clark (1861) visually sighted Sirius B
• Schaeberle (1896) Lick Obs. announced Procyon’s companion
• 40 Eri (faint white and red stars)
– Class A0, Russell dismissed when 1st Russell diagram published
– Adams confirmed A-type
• Adams (1915)-Sirius B spectrum Type A0
• Eddington (1924) Mass-Luminosity Relationship
– Coined “white dwarfs” for 1st time
– Deduced mass and radius of Sirius B density=53,000x water
• Fowler (1926) WDs supported by electron degeneracy
pressure, not thermal gas pressure
• Chandrasekhar (early 1930s) worked out details of white
dwarf structure, predicted upper mass limit of 1.44 Msun, &
found mass-radius relation
Early Classifications
• Kuiper (mid-1930s, Lick Obs.) WDs found in
increasing numbers
– 1941 introduced 1st WD classification scheme
• w in front of spectral type and Con stars
• Luyten (1921) proper motion studies from
faint blue star surveys
– 1952 presented new scheme for 44 WDs
• D for true degeneracy, followed by A, B, C, or F
• Greenstein (1958) introduced new scheme
– 9 types
Current Classifications
Sion (et al. 1983)
• ~2200 WDs w/in ~500 pc of Sun
• D=degenerate
• Second Letter-primary spectroscopic signature in
optical
–
–
–
–
–
–
DA-Hydrogen lines (5000K<Teff<80000K)
DB-He I lines (Teff<30000K)
DC-Continuous spectrum (Teff<11,000K)
DZ-Metal lines (Mg, Ca, Fe)
DQ-Atomic/Molecular carbon features
DO-He II lines (Teff>45,000K)
• Additional letters indicate increasingly weaker or
secondary features, e.g. DAZ, DQAB
– P-polarized magnetic, H-non-polarized magnetic, V-variable
• Teff indicated by digit at end; 50,400/Teff, e.g. DA4.5
• New class Teff<4000K, IR absorption for CIA by H2
DA Spectra
Rapid settling of elements
heavier than H in high
gravity
DB Spectra
DQ
Stars &
Spectra
• Helium-rich stars, generally characterized by
C2-Swan bands
• Hotter DQs have C I
PG 1159 Spectra
• Features due to CNO ions,
Teff>100,000K
• Absence of H or He I
features; He II, C IV, O VI
ZZ
Ceti
Magnetic WDs
• About 5% of field
white dwarfs display
strong magnetism
• 3 classes of Hatmosphere MWDs
based on field
strength
• He-atmosphere
MWDs have unique
features
Basic Picture
• 75% DA, 25% non-DA
• Spectral classification provides info about
principal constituent, with some T info
• Progenitors: Post-AGB stars, central stars
of planetary nebulae (CSPN), hot
subdwarfs
• Expected structure-stratified object with
<M>~0.6Msun
– C-O core, He-rich envelope, H-rich shell
• O-Ne cores-most massive
– Atmosphere contains <10-14 M
• Many WDs have pure H or He atmospheres
• Thicknesses of H and He
Mechanisms in Atmosphere
•
•
•
•
•
•
•
Gravitational diffusion
Convection
Radiative levitation
Magnetism
Accretion
Wind-loss
T-sensitive  T determines chemical
abundances
Effects of Mechanisms
• Diffusion & Settling
– Gravitational separation leads to pure envelope of
lightest element t<108 yr
• But, observations show traces of heavier elements
– radiative levitation
– Cooler WDs result of recent accretion event
• Radiative Levitation T>40kK
– Radiative acceleration on heavy elements
• Convection for T<12kK
–
–
–
–
Convection zone forms and increases inward as star cools
For He envelopes, convection begins at high T
Mixing changes surface composition
Need to couple models of atmospheres and interiors
Statistics
• T>45kK DA far outnumber DO
– Ratio increases to about 30kK (diffusion)
• DB gap in 45k-30kK range
– Float up of H
• Always enough H to form atmosphere?
– Dredge up of He
• T<30kK He convection zone massive engulfs
outer H layer if thin
– 30kK-12kK 25% stars revert to DB spectral type
(edge of ZZ Ceti Strip)
– Convection zone increases as T decreases. At
T~11kK, numbers of DAs and non-DAs are ~equal
(ZZ Ceti Strip)
• ‘Non-DA gap’ for 5000-6000K dearth of He
atmospheres
Spectral Evolution
• Gapsindividual WDs undergo spectral evolution
– Compositions change, DADBDA, as T changes
• Evolution of convection zone? Accretion?
• Explanation of ‘non-DA gap’-opacity? Bergeron et
al.
– Low opacity of He I means small amounts of H dominates
opacity
– H- atomic energy levels destroyed when H added to
dense atmosphere-reduces H opacity contribution
– Must accrete a lot of H to make difference in
photospheric conditionsDA (fixes 6000K edge)
– Re-appearance of DBs at 5000K b/c convection zone
grows, H is diluted with additional He
– This fails! Destruction of H- bound level produces free e, which provide opacity
Cooling Evolution
CSPN
Hot DAZs (T>40kK)
Radiative leviation makes Z
No Z cooler than 35kK
ZZ Ceti w/ variable H layers
10-8…………………10-4 Msun
He-Rich DA
(0.01<He/H<20)
Some DC, DZ
ZZ Ceti
Pure DA
(He/H<0.01)
Cool DAs
Some w/ T<5kK
Model Atmospheres
•
•
•
•
Plane-parallel geometry
Hydrostatic equilibrium (mass loss rates)
NLTE
Stratisfied Atmospheres
– Parameters: degree of ionization, intensity of
radiation field
• Make radiative cross sections of each element depth
dependent
• Convection
– Parameters of Mixing Length theory
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
Field White Dwarfs
• Identified by large proper motion yet
faint object
• LHS (Luyten Half Second)
• NLTT (New Luyten 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)
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 average, 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
Zeeman (Landau)
Splitting
Future/Active Work
• Exact masses of H and He layers
– Thin or Thick Envelopes
• Explanations for DB-gap
• Explanations for ‘non-DA gap’
• DAs outnumber He-rich WDs, yet
progenitor PNN have ~equal numbers
of H- and He-rich stars. What rids
degenerates of He?
• Couple core & atmosphere models