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