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=4R2Te4 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 0.2mag0.6Gyr /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
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