1. Mass Transfer in Double White Dwarf Binaries Juhan Frank Theoretical Astrophysics Group at Louisiana State University JPL 65th Birthday Conference Trzebieszowice October 2007

2. People at LSU Joel Tohline Patrick Motl (Postdoc) Wes Even (graduate student) Dominic Marcello (graduate student) Vayujeet Gokhale (graduate student) Mario D’Souza (graduate student) Juhan Frank JPL 65th Birthday Conference Trzebieszowice October 2007

3. Motivation DWDs are guaranteed sources of gravitational radiation for LISA DWD channel for SN Ia: structure & evolution of merged object DWD channel for R CrB stars Origin of massive WD with peculiar abundance patterns DWD channel for origin of AM CVn stars and the status of RX J0806 and V407 Vul Merger or Survival? Common Envelope? JPL 65th Birthday Conference Trzebieszowice October 2007

4. With apologies to Sidney Harris JPL 65th Birthday Conference Trzebieszowice October 2007

5. Chandra: RX J0806 PR JPL 65th Birthday Conference Trzebieszowice October 2007

6. AM Canis Venaticorum Nelemans (2005) ASP Conf 330, eds. Hameury & Lasota JPL 65th Birthday Conference Trzebieszowice October 2007 Discovery: Smak (1967); DWD model: Paczyński (1967)

7. The Twin-Degenerate Interacting Binary G61-29 (aka GP Com) Nather, Robinson & Stover 1981 ApJ 244, 269 JPL 65th Birthday Conference Trzebieszowice October 2007

8. Direct Impact Accretion JPL 65th Birthday Conference Trzebieszowice October 2007

9. Origin of AM CVn Binaries DWD driven to contact by gravitational radiation (e.g. Nather et al 1981; Tutukov & Yungelson 1996; Nelemans et al 2001) He star transfers mass to WD until He burning ceases and donor becomes semi- degenerate (e.g. Tutukov & Fedorova 1989; Iben & Tutukov 1991) CV with evolved donor (e.g. Podsiadlowski et al 2003) JPL 65th Birthday Conference Trzebieszowice October 2007

10. Mass Transfer Stability and Direct Impact Limit JPL 65th Birthday Conference Trzebieszowice October 2007 Mass transfer under direct impact accretion is less stable than through a disk: stability requires q<0.23 (Marsh, Nelemans & Steeghs 2004; Gokhale et al 2006). Using Paczynski’s Roche lobe formula for donor, and the polytropic mass-radius relationship R 2 /R 1 =q-1/3, Plavec & Kratochvil formula for b 1, and taking stream periastron = 0.5r circ, yields q>0.154 for direct impact to occur With orbit integrations and Eggleton’s formula, and still polytropic mass-radius relationship, yields q>0.143 for direct impact to occur Maybe lower if accretor is warm

11. q=0.5, driven initially JPL 65th Birthday Conference Trzebieszowice October 2007

12. Orange: driving Green: mass transfer Blue; solid-accretor, dashed-donor

13. q=0.4, driven at 2x10 -3 P -1 JPL 65th Birthday Conference Trzebieszowice October 2007

14. Mass Transfer Rate JPL 65th Birthday Conference Trzebieszowice October 2007

15. Binary Mass Ratio q=M donor /M accretor JPL 65th Birthday Conference Trzebieszowice October 2007

16. Binary Separation JPL 65th Birthday Conference Trzebieszowice October 2007

17. Orbital Angular Momentum JPL 65th Birthday Conference Trzebieszowice October 2007

18. Accretor Spin Angular Momentum JPL 65th Birthday Conference Trzebieszowice October 2007

19. JPL 65th Birthday Conference Trzebieszowice October 2007

20. Concluding Remarks The available simulations suggest that in some cases mass transfer stabilizes at q>0.23 (e.g around q=0.39 in one case). For that simulation we estimate effective q stable = 0.7 (~2/3?) Surface perturbations on the accretor suggest that generalized r- modes (Racine, Phinney & Arras 2007) play a role but results inconclusive. Disentangling effects of stream impact, accretion belt, and tidal coupling requires more simulations and diagnostics. Near future: aim to carry out high resolution 3-D simulations with more realistic EOS for ~10 2 orbits. Later: radiation forces and nuclear reactions. Understand nature of tidal coupling between spins and orbit and under which conditions can the dreaded CE phase be avoided. JPL 65th Birthday Conference Trzebieszowice October 2007