ASTEROSEISMOLOGY OF LATE STAGES OF STELLAR EVOLUTION

Gérard VauclairObservatoire 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

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

• 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 profilein 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

• 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