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Evolution and Accretion of Helium White Dwarfs: Insights from Classical Novae Studies

This study explores the evolution and characteristics of low-mass helium white dwarfs (WDs) influenced by slow nova outbursts. We analyze the effects of accretion of cosmic-material mixes onto WDs with masses less than 0.48 solar masses, focusing on thermodynamic cycles, outburst intervals, and the relationship between accretion rates and core temperature variations. Using advanced hydrodynamic simulations, we examine the timescales of thermodynamic diffusion and the role of chemical diffusion in driving the evolution of these stellar remnants.

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Evolution and Accretion of Helium White Dwarfs: Insights from Classical Novae Studies

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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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