THE EFFECT OF 12C(α,γ)16O
ONWHITE DWARF
EVOLUTION
Pier Giorgio Prada MoroniDipartimento di Fisica - Università di Pisa
Osservatorio Astronomico di Teramo
WHITE DWARF:AN ASTRONOMICAL OXYMORON?
1844 Bessel
Sirius B very compact: M 1Msun R Rearth
First WD discovered
Detection of an invisible star
1862 Clark Observation of a very faint star: DWARF
1915 Adams Spectrum of a hot star: WHITE
L=4R2Te4
WDs are objects extremely compact
Central density ~ 106 – 107 g/cm3
Surface gravity ~ 108 - 109 cm/s2
Mass of the order the SUN
Size of the order the EARTH
WDs are the most common endpoint of stellar evolution
1) M < 0.5 Msun He WDs M <0.5 Msun
2) 0.5 Msun M < 8 Msun C/O WDs 0.5-1.1 Msun
3) 8 Msun M 10 Msun O/Ne WDs 1.1-1.4 Msun
M 10 Msun
Low mass star evolution
C/O White Dwarfs
98% C/O core
2% He buffer
0.01% H envelope
e- highly degenerate isothermal
He/H envelope
C/O coreC/O ions main energy reservoirenergy reservoir
e- non-degenerate no conduction
thermal insulatorthermal insulator
C/O WD evolution
1) Neutrino energy loss
2) C/O crystallization
3) Convective – coupling
4) Debye regime
Cosmic fossils record of stellar populations
Why are WDs important?
Renzini et al. 1996: standard candles
Alcock et al. 2000: microlensing experiments
Sizeable fraction of Galactic dark matter
WD cosmo-chronology has become actually feasible only
recently thanks to the improvement of telescopes
Discovery of WD cooling sequences in globular clusters
(Paresce et al. 1995, Richer et al. 1997, Hansen et al. 2002)
Observation of the faint end of the WD sequence in open
clusters (Von Hippel & Gilmore 2000)
Smichdt 1959: WDs can be used as cosmic clocks
Richer et al. 2002
Luminosity function of M4
1012
14
De Marchi, Paresce, Straniero, Prada Moroni 2004
The galactic open cluster NGC2420
Von Hippel & Gilmore 2000
The WD evolution is essentially a cooling process
The WD luminosity is largely supplied by its thermal energy content
The temperature decrease rate depends on
The energy stored in the C/O core
The energy transport through the
thin He/H envelope
Internal stratification: the C/O core
The amounts of C and O left in the core have a great influence on the WD cooling rate
The larger O content
The heat capacity is dominated by C/O ions
a smaller heat capacity
a faster WD cooling
Internal stratification: the C/O core
The core chemical profiles are determined by:
1) the competition of the two major nuclear reactions powering the He-burning
3α12C(α,γ)16O
2) the efficiency of the convective mixing during the He-burning phase, mainly in its final part
C/O core: convective core extension
Lack of a satisfactory convection theory
Uncertainty on the C/O profiles in the core
Schwarzschild criterion rad > ad
The kinetic energy does not vanish in correspondence of the classical boundaries
rad = ad
Overshooting in the radiative zone
C/O core: convective core extension
t ~ 3%
Any additional mixing occurring in the final part
of He-burning
Strongly reduces the C abundance in the core
Breathing pulses
Prada Moroni & Straniero 2002
Straniero et al. 2003
Bare Schwarzschild Method
Semiconvective Model
High Overshoot Model
C/O core: 12C(α,γ)16O reaction rate
Kunz et al. 2002: at 300 keV
Δt ~ 6%
NA<σ,v >= 1.25(10-15cm3mol-1sec-1)±30%
Caughlan et al. 1985 1.9Caughlan & Fowler 1988 0.8
Prada Moroni & Straniero 2002
Theoretical isochrones
Prada Moroni & Straniero 2004
Theoretical isochrones
Prada Moroni & Straniero 2004
Theoretical luminosity functions
0.3 mag 1 Gyr
0.1 mag 1 Gyr
While the TO luminosity
Much less sensitive to
distanceuncertainty
Prada Moroni & Straniero 2004
/0. 1
mag
0.2mag0.6Gyr
Effect of C/O profiles on WD luminosity function
Δage~ 5%
Prada Moroni & Straniero 2004
/0. 1
mag
Castellani et al. 2002
WD isochrones
Theoretical luminosity functions
0.3 mag 1 Gyr
0.1 mag 1 Gyr
While the TO luminosity
Much less sensitive to
distanceuncertainty
Prada Moroni & Straniero 2004
/0. 1
mag
Model atmosphere
Evolution at constant radius
For Teff < 5000 °K H2
Main opacity source in IR
Departure from black body
Blue hook
The old and cold WDs are BLUE, not red!
Present uncertainty in theoretical cooling ages
Sizeable differencesin the cooling ages:
Likely due to: 1) the input physics 2) pre-WD evolution
The origin of these uncertainties should be identified
before adopting WD as cosmic clocks
at the faint end
Δt > 4 Gyr
C/O core
t ~ 9%
The global uncertaintydue to the
core chemical profiles is
Conductive opacity
Very tricky!
Potekhin 1999
t ~ 16%
Covers the whole range of parameters
suitable for WDs
WDs of different masses