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Classical Novae on a Helium White Dwarf

Classical Novae on a Helium White Dwarf. Irit Idan (Technion) Lars Bildsten ((KITP, UCSB) Ken Shen (UCSB). Introduction. The evolution of a low mass star on the RG branch can be halted due to the filling of the RL - low mass (M<0.48 M  ) He core

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Classical Novae on a Helium White Dwarf

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  1. Classical Novae on a Helium White Dwarf Irit Idan (Technion) Lars Bildsten ((KITP, UCSB) Ken Shen (UCSB)

  2. Introduction • The evolution of a low mass star on the RG branch can be halted due to the filling of the RL - low mass (M<0.48 M) He core • Howell et al 2001 - 20% of CVs with Porb<2 hr - He WDs • Tight orbits -> contact leading to accretion of cosmic-mix material onto a pure He WD at a very low accretion rate 10-11 Myr-1

  3. Shara, Prialnik, Kovetz (1993) – accretion onto M=0.4M,Tc=107K He WD at accretion rate of 10-9Myr-1 for 10 cycles of nova outburst. • extremely slow nova. • mild outbursts. • time between outburst 106yr, Macc<10-3M • decreasing core temperature. • high luminosity - over 1000yr for L>L . • mass of the WD increase slowly.

  4. Goals • Study Mign and the Mej, evolution and the time scales on He WDs - accretion rate scenario • Abundances – No source of C/O from the WD. The Tmax in a hydrostatic flash on a low mass He WD is  2-3 x 108 K (Sugimoto & Fujimoto 1978). • Can the high temperatures (>2-3x108K) at the base of the burning H layer can ignite the underlying Helium WD and make it a low-mass Helium-burning star ?

  5. Method Study the accretion onto a small, cold (Tc=6E6K) He WD (98% He and 2% N) both analytically and numerically. Using the Prialnik and Kovetz code - hydrodynamic, Lagrangian stellar evolution code. • OPAL opacities • extended nuclear reactions network • mass loss algorithm • diffusion

  6. Timescale for thermal diffusion into the core - Analytic estimate For low mass and cold WD – the time between outbursts 108 yr significant thermal coupling between the accreting envelope and the core. Timescale for heat transport between r0 and r in non-convective regions (Henyey, L.,&L'Ecuyer, J. 1969)

  7. Numerical Estimate - Thermal Diffusion Time The timescale for the coupling  time between outburst for low accretion rates. Ethermal(Env) << Ethemal(Core)

  8. Multicycle Evolution Code - constant accretion rate

  9. Chemical diffusion during accumulation For and The diffusion timescale of H into He core is

  10. Maximum Temperatures For fixed core mass, envelope mass, and composition, there is a unique maximum base temperature for the fully convective envelope.

  11. Maximum Temperatures - Multicycle Md=0.2M Md=0.05M

  12. The average outburst parameters Mv = -4.5 SS phase1200 year

  13. Abundances

  14. Conclusions • Study Mign and the time scales on He WDs - good agreement between analytical results and multicycle calculations. • Extremely slow nova • Large ejected mass and low metalicity. • Time between outbursts - 108yr • Core temperature depend on the accretion rate. • High luminosity - over 1000yr for L>L -SS • The Tmax - 108K.

  15. But- Ethermal(Envelope) << Ethemal(Core) Ideal gas liquid ions The ratio

  16. Chemical diffusion during accumulation Time between outbursts for accretion rate of 10-11 Myr-1 is 108 yr Diffusion is important.

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