THE EFFECT OF
12C(α,γ)16O
ON
WHITE DWARF
EVOLUTION
Pier Giorgio Prada Moroni
Dipartimento di Fisica - Università di Pisa
Osservatorio Astronomico di Teramo
WHITE DWARF:
AN ASTRONOMICAL OXYMORON?
1844 Bessel
Detection of an invisible star
1862 Clark
Observation of a very faint star: DWARF
1915 Adams
Spectrum of a hot star: WHITE
L=4R2Te4
Sirius B very compact:
M 1Msun R  Rearth
First WD discovered
WDs are objects extremely compact
Mass of the order the SUN
Size of the order the EARTH
Central density ~ 106 – 107 g/cm3
Surface gravity ~ 108 - 109 cm/s2
WDs are the most common endpoint of stellar evolution
M  10 Msun
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
Low mass star evolution
C/O White Dwarfs
98% C/O core
2% He buffer
0.01% H envelope
C/O core
He/H envelope
e- highly degenerate
isothermal
C/O ions main energy reservoir
e- non-degenerate
thermal insulator
no conduction
C/O WD evolution
1) Neutrino energy loss
2) C/O crystallization
3) Convective – coupling
4) Debye regime
Why are WDs important?
Cosmic fossils
record of stellar populations
Renzini et al. 1996: standard candles
Alcock et al. 2000: microlensing experiments
Sizeable fraction of Galactic dark matter
Smichdt 1959: WDs can be used as cosmic clocks
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)
Richer et al. 2002
Luminosity function of M4
12
10
14
De Marchi, Paresce, Straniero, Prada Moroni 2004
The galactic open cluster NGC2420
Von Hippel & Gilmore 2000
The WD luminosity is largely supplied by
its thermal energy content
The WD evolution is essentially a
cooling process
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 heat capacity is dominated by C/O ions
The amounts of C and O left in the core have a great
influence on the WD cooling rate
The larger O content
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
3α
12C(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

The kinetic energy does not
vanish in correspondence of
the classical boundaries
Overshooting in
the radiative zone

rad
=

ad
rad
>

ad
C/O core: convective core extension
Any additional mixing
occurring in the final part
of He-burning
Strongly reduces the C
abundance in the core
Breathing pulses
t ~ 3%
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
NA<σ,v >= 1.25(10-15cm3mol-1sec-1)±30%
Caughlan et al. 1985  1.9
Caughlan & Fowler 1988  0.8
Δt ~ 6%
Prada Moroni & Straniero 2002
Theoretical isochrones
Prada Moroni & Straniero 2004
Theoretical isochrones
Prada Moroni & Straniero 2004
Theoretical luminosity functions
While the
TO luminosity
/0. 1mag
0.3 mag  1 Gyr
0.1 mag  1 Gyr
Much less
sensitive to
distance
uncertainty
Prada Moroni & Straniero 2004
0.2mag0.6Gyr
/0. 1mag
Effect of C/O profiles on WD luminosity function
Δage~ 5%
Prada Moroni & Straniero 2004
WD isochrones
Castellani et al. 2002
Theoretical luminosity functions
While the
TO luminosity
/0. 1mag
0.3 mag  1 Gyr
0.1 mag  1 Gyr
Much less
sensitive to
distance
uncertainty
Prada Moroni & Straniero 2004
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 differences
in the cooling ages:
Δt > 4 Gyr
at the faint end
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
C/O core
The global
uncertainty
due to the
core
chemical
profiles is
t ~ 9%
Conductive opacity
t ~ 16%
Very tricky!
Potekhin 1999
Covers the whole
range of parameters
suitable for WDs
WDs of different masses
Scarica

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