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 P orb <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=0.4 M ,T c =10 7 K He WD at accretion rate of 10 -9 M  yr -1 for 10 cycles of nova outburst. extremely slow nova. mild outbursts. time between outburst 10 6 yr, M acc <10 -3 M  decreasing core temperature. high luminosity - over 1000yr for L>L . mass of the WD increase slowly.

4. Goals Study M ign and the M ej, 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 10 8 K (Sugimoto & Fujimoto 1978). Can the high temperatures (>2-3x10 8 K) 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 10 8 yr significant thermal coupling between the accreting envelope and the core. Timescale for heat transport between r 0 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

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

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

12. Maximum Temperatures - Multicycle M d =0.2M  M d =0.05M 

13. The average outburst parameters M v = -4.5 SS phase1200 year

15. Abundances ElementSolarEjecta He WDEjecta CO WD H0.70.650.622 He0.280.330.258 Z0.02 0.12 C 12 3.90(-3)2.80(-4) depletion C 13 4.30(-5)9.09(-5) N 14 1.00(-3)9.00(-3) enhancement N 15 3.60(-6)3.00(-7) O 16 9.40(-3)1.05(-2) enhancement O 17 3.50(-6)1.36(-3)

16. Conclusions Study M ign 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 - 10 8 yr Core temperature depend on the accretion rate. High luminosity - over 1000yr for L>L  -SS The T max - 10 8 K.

18. But- E thermal (Envelope) << E themal (Core) The ratio Ideal gas liquid ions

19. Chemical diffusion during accumulation Time between outbursts for accretion rate of 10 -11 M  yr -1 is 10 8 yr Diffusion is important.