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The fate of S-AGB stars

The fate of S-AGB stars. Why won’t my code converge?. or. Work in Progress! No answer yet!. Herbert Lau Pilar Gil Pons Carolyn Doherty Me. Overview. Numerical problem ?. Massive AGBs fail to converge when M env about 2 M ʘ. Physical problem ?. Overview. Numerical problem ?.

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The fate of S-AGB stars

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  1. The fate of S-AGB stars Why won’t my code converge? or

  2. Work in Progress!No answer yet! • Herbert Lau • Pilar Gil Pons • Carolyn Doherty • Me

  3. Overview Numericalproblem? MassiveAGBsfailto converge whenMenvabout 2 Mʘ Physicalproblem?

  4. Overview Numericalproblem? MassiveAGBsfailto converge whenMenvabout 2 Mʘ Physicalproblem? L>LEdd Extreme lowρbelow BCE

  5. Looking at the facts:when does it happen? The divergence happens for different evolution codes (EVOLV, MONSTAR) Can be delayed by increasing alpha (MLR)=> increasing mixing efficiency (Herwig et al., Althaus et al,…)

  6. Looking at the facts:when does it happen? Comparison: the last two TPs

  7. Pg < 0 • Code dies with negative gas pressure • T and P are dependent variables • So values chosen by matrix solution • Prad = 1/3aT4 then known • Pgas = P – Prad is known • Then the e.o.s. tells us r • So a Pg < 0 error means Prad provides all of P • ie b < 0 and L > LEdd • See Wood and Faulkner 1986!!

  8. Looking at the facts: contribution P/Prad in the hydrostatic case Convective case: then: then: with: with: Radiative case:

  9. Looking at the facts Conductivity radiation-to-convection ρ+Δρ P+ΔP T+ΔT ρe, Pe, Te U >> convectioninefficient U << convectionefficient ρ, P, T ρe, Pe, Te

  10. We have very inefficient convection here… After last TP

  11. What pushes L > LEdd ? Reduce LEdd by increasing <k> ???

  12. Hypothesis κ-peak • Petrovic et al (2006) • OPAL opacity tables display a peak due to presence of Fe, Ni at T aprox. 250000 K • This peak causes huge inflation and departure of hydrostatic equilibrium in WR stars • Could this be our case?

  13. Testing the κ-peak hypothesis:We do find the κ-peak

  14. Look at that radius!

  15. Testing the κ-peak hypothesis: • We smoothed out the peak • and the code keeps converging! • The star lost a further 0.5 Msun • Before it died again!.

  16. But there is another larger opacity peak…

  17. And another!

  18. Hypothesis κ-peak

  19. Multiple κ-peaks • We doubt the star can avoid its fate… • Deep envelope and low density mean a region of increasing opacity • The high luminosity drives dramatic expansion • In our hydrostatic case its supersonic! • 103 – 104 R per year! • What does a REAL star do? • We think the energy involved < binding energy of the envelope • But it might drive periodic, enhanced mass-loss at least?

  20. We need to sort this out!

  21. The general picture extreme low ρ-high T zone L>LEdd Energy released after K peak Loss of hydrostatic eq code converges If we avoid the K peak Another hydrodynamic problem...

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