THE EFFECT OF 12 C(α,γ) 16 O 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 Sirius B very compact: M 1M sun R R earth 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=4 R 2 T e 4

WDs are objects extremely compact Central density ~ 10 6 – 10 7 g/cm 3 Surface gravity ~ cm/s 2 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 M sun He WDs M <0.5 M sun 2) 0.5 M sun M < 8 M sun C/O WDs M sun 3) 8 M sun M 10 M sun O/Ne WDs M sun M 10 M sun

Low mass star evolution

C/O White Dwarfs 98% C/O core 2% He buffer 0.01% H envelope e - highly degenerateisothermal He/H envelope C/O core energy reservoir C/O ions main energy reservoir e - non-degenerate no conduction thermal 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 M 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 3α 12 C( α,γ ) 16 O 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 Bare Schwarzschild Method Semiconvective Model High Overshoot Model

C/O core: 12 C(α,γ) 16 O reaction rate Kunz et al. 2002: at 300 keV Δt ~ 6% N A = 1.25( cm 3 mol -1 sec -1 )±30% Caughlan et al Caughlan & Fowler 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 distance uncertainty Prada Moroni & Straniero 2004 /0. 1mag

0.2mag 0.6Gyr Effect of C/O profiles on WD luminosity function Δ age ~ 5% Prada Moroni & Straniero 2004 /0. 1mag

Castellani et al WD isochrones

Theoretical luminosity functions 0.3 mag 1 Gyr 0.1 mag 1 Gyr While the TO luminosity Much less sensitive to distance uncertainty Prada Moroni & Straniero 2004 /0. 1mag

Model atmosphere Evolution at constant radius For T eff < 5000 °KH2H2 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: 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 uncertainty due 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