ASTEROSEISMOLOGY OF LATE STAGES
OF STELLAR EVOLUTION
Gérard Vauclair
Observatoire Midi-Pyrénées, Toulouse
Second COROT-BRASIL Workshop, November 5, 2005
Overview
• 1- Pulsators in late stages of stellar evolution: their
location in the HR diagram
• 2- the precursors to the white dwarf cooling
sequence:
• a) the AGB-PN channel: PNNV and PG1159
• b) the EBHB-sdB channel: sdBV
• 3- the white dwarf pulsators: DBV, DAV
• 4- What can be done with COROT?
PNNV and PG1159 (GW Vir) pulsators
• PNNV: central stars PG1159 spectral type
He, C, O (+H in ‘hybrid-PG1159)
PN, ongoing mass-loss
• PG1159 pulsators: no PN , ongoing mass-loss
• Teff: [170 kK – 80 kK]; log g:[6 -8]
• Periods: ~3000 s - ~400 s, g-modes
• Instability: K-mechanism C,O
• Pulsators and non-pulsators mixed
• Structure: C/O core, He/C/O enveloppe
• Gravitational settling : He vs C/O?
• Evolution from PG1159 to DB
Seismic diagnostic in white dwarfs
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g-modes : f< Braunt-Vaisala and < Lamb
In wd Braunt-Vaisala decreases towards interior:
homogeneous composition : uniform period spacing
total mass determination
stratification :deviation from uniform period spacing,
mode trapping
reflexion of waves with nodes at transition zones
fractional mass above transition zones
rotational splitting = rotation period
Subdwarf B pulsators
• Two classes:
• Short period variables (spv or EC14026)
periods [~80 s - ~600 s]; p-modes
• Long period variables (lpv or ‘Betsy stars’)
periods: hours; g-modes
Instability: Fe accumulation by diffusion
opacity bump = K-mechanism
works for both spv and lpv
Pulsators and non-pulsators mixed
Charpinet et al.
Sample of sdB light curves from CFHT (Fontaine, Charpinet)
White dwarf pulsators: the DBVs
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DBV: Helium white dwarfs; 8 pulsators
Teff: 25kK– 20 kK (depends on H:He)
Instability: K-mechanism of He
Diffusion equilibrium not reached
Layered composition: He/He-C-O in envelope
He-C-O/C-O envelope – core
C/O in the core
Signature of core chemical composition and profile
in the period distribution?
White dwarf pulsators: the DAVs (ZZ Ceti)
• DAV: H envelope: ~100 pulsators (~60 from
SLOAN)
• Periods: 70 s – 1500 s
• Diffusion equilibrium almost achieved:
C/O core, He layer, H envelope
Instability: K-mechanism H in hot DAV +
Convective driving in cool DAV
Instability strip: Teff [~12500 K - ~11000 K]; pure?
Core composition, M, H mass fraction, rotation
DA models, cosmochronology
Bergeron, Fontaine, Brassard
ZZ Ceti instability strip
sdB and COROT
• Two long period sdBV for short runs
• KPD 0716+0258
• KPD 0629-0016
White dwarfs with COROT
• WD catalogues: 3000, 80% DA, 4% DAV
• Only 15 with V<16 in GC
• “” 13 “” “” in GAC
• None is a pulsator
• WD (V<16) surface density (high b)
• ~2 x 10**-3/sq.deg (PG, SDSS)
WD density in Gal. plane? x 10, x 15?
A few ZZ Ceti (V<16) expected in COROT
exo fields
White Dwarf Rotation
• Angular Momentum evolution: the end
• rotation periods distribution: constraint on
angular momentum evolution
• from ground-based multisite astero:
rotation periods between ~0.5 and ~2 days
but: observational bias against slow rotators
COROT: potential slow rotators accessible
Amplitude variations
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Commun in all WD pulsators
non linear effects?
characteristic time scales?
COROT: continuous 150 days photometry
Conclusions
• Asteroseismology constraints on:
• Fundamental parameters of white dwarfs and their
progenitors ; internal structure and stratification
• p-modes and g-modes in sdBs: better understanding of the
HB
• g-modes in white dwarfs:constraints on physical processes
- in previous evolution (mass loss, angular momentum,
convection, overshooting..)
- ongoing along the cooling sequence (convection,
gravitational settling, crystallization…)
• better models for cosmochronology