1. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - The Formation of Hydrogen Deficient Stars Through Common Envelope Evolution By Steven Diehl Theoretical Astrophysics Group (T-6) Los Alamos National Laboratory Main Collaborators: Chris Fryer Falk Herwig Orsola De Marco

2. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - Overview The SPH technique: a brief introduction Common Envelope Evolution –Conceptual Picture –Why do we care? –Preliminary SPH simulations –Some Results [Double Degenerate Mergers -> Chris Fryers talk on Friday] Summary and Outlook

3. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - Smooth Particle Hydrodynamics

4. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - SPH - The Concept SPH = Smooth Particle Hydrodynamics Lagrangian Techique: –Fluid/Gas properties are carried by SPH particles: Temperature, mass, density, composition, velocity, … –Intrinsically adaptive, particles follow fluid flow –Every particle represents a gas blob Each point in the gas flow is the result of a superposition of many SPH particles (usually around 64-128)

5. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - SPH vs. Grid Codes SPHGrid Angular Momentum Conservation++- Shock resolving--++ Ease of implementing complex physics +- Ease of setup+- Computational Speed++--

6. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - Common Envelope Evolution (CE)

7. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - CE - Conceptual Picture Low-mass companion enters the atmosphere of a red giant or asymptotic giant branch star Companion spirals in and transfers orbital energy and angular momentum into the envelope Parts or all of the envelope is removed The companion either stops in a tight orbit or even merges into the core

8. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - CE: When does it start? Thermal Pulses trigger radius peaks Companion get engulfed by the envelope CE evolution starts De Marco et al. (2003)

9. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - CE and H-Deficient Stars After CE: only little mass from the H-envelope may be left a dredge-up event dilutes the remaining mass -> Hydrogen- deficiency Herwig 1999

10. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - CE - Previous Work SPH simulations by Rasio et al (1995) –Only one simulations available, between a 4M red giant and a 0.7M main sequence companion –Low resolution (50k particles), a factor of 2 lower than even our smallest test runs Nested grids by De Marco et al (2003) –Technique limited, unable to cover the huge dynamical ranges –They are now improving with AMR codes (Enzo)

11. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - CE - Number of time steps Time stepping: dt~h/cs Core size: <0.1 solar radii BD of 0.05 should spiral to around.6 solar radii, Number of particles required: >10000 within the last radius. -> required h of at least 0.01 Rsun Cs around the core is about 100Rsun/day -> dt is about 0.01/100=1/10000 day Need a few hundred days or years to complete -> Millions of time steps

12. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - CE - Worst case: Planets/Brown Dwarfs The lower the mass of the companion, the further it is expected to spiral inwards More resolution required inside a smaller Volume Numerically more challenging, as the sound speed rises fast close to the center Dynamical range: 0.01 - 100 solar radii for RG, for AGB stars it gets even worse

13. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - CE - Scaling Lets assume we increase the resolution by a factor of q: h=h/q if h=h/q then c s =qc s and dt h/c s dt/q 2 -> number of time-steps: N dt =q 2 N dt Number of particles (3d-Volume): h=h/q -> N=q 3 N Computing speed per timestep: µ NlogN µ (if all particles updated all the time) Total computing time: µ tot =µ*N dt q 5 logq 3 µ tot q=2 -> µ tot =66µ tot, q=10 -> µ tot =690775µ tot

14. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - CE - Avoiding the worst case Individual time-stepping of particles absolutely crucial, this avoids the NlogN scaling of the time step, only the system time step (all particles advanced) scales this way. Be smart on where to put the extra resolution, only add particles at center Even then, we probably have to regrid the center of the simulation at one point for very low-mass companions

15. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - CE - DISCLAIMER All the results you will see are to be considered VERY PRELIMINARY We have significantly modified the code and improved its performance. These are test runs and we are still in the debugging phase Do NOT take these results quantitatively literally, but rather use them to get an intuition on how the dynamics work out

16. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - CE - Test Run: 0.9RG, 0.25WD 0.9 solar mass Red Giant (RG) and 0.25 solar mass White Dwarf (WD) companion

17. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - CE - Test Run: 0.9RG, 0.05BD Dynamics are always similar: Bow- shock structure around the companion, spirals in, spiral density wave transports angular momentum and mass outward

18. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - CE - Comparison R vs. T Heavy companion spirals in faster, but then stalls Low-mass companion still keeps on going at the end of the simulation (as far as we have run it) Low-mass comp. are more likely to produce tight binaries or merge into the core 0.25M 0.05M

19. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - CE - Comparison E vs T Red: total thermal energy of the envelope Green: negative value of orbital energy Blue: orbital energy transferred into the envelope Energy is still transferred from low-mas companion, high-mass essentially stopped 0.25M 0.05M

20. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - CE - When Does it Stop? The evolution seems to seize when the energy released due to a decrease in orbit dR is larger than the energy to shed the envelope between R and R-dR -> I.e. you shed faster than you spiral in -> there is no more material to plow through -> you cant transfer the energy into the envelope anymore

21. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - CE - Comparison R vs RdM Plot is logR vs R*dM, I.e. area under the curve is proportional to the mass at that radius Colors: different times (dark=early) 0.25M: order of magnitude further out 0.25M 0.05M

22. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - CE - Open Questions for RG Does the evolution really stop at the end? Or does the RG recuperate and increase in size again? -> map the remnant into a stellar evolution code Is the remaining envelope mass below the critical mass to support the Giant solution? If it is, will the envelope expand again and start born-again CE?

23. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - CE - Open Questions for Companions Which companions merge into the core? What are the consequences for the composition of the envelope and nuclear burning? Do the companions accrete mass? Or do they rather lose mass? Can the companions survive at all? -> we will use different spots in the CE companion trajectory and do zoom-in study on the companion

24. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - CE - Open Questions for Ejecta Does all of the ejected envelope stay/become unbound? Does some of it fall back? When do the ejecta form dust, and would they be observable? Could this process explain some of the dust composition and morphology seen in some planetary nebula? Can the ejecta be crucial for forming a planetary nebula when for example a wind plows into it later on?

25. HYDEF07, September 19, 2007 Common Envelope Evolution - Steven Diehl, T-6 (LANL) - SUMMARY AND OUTLOOK We now have a tool to successfully model common envelope evolution with SPH The code is fast, robust and versatile CE simulations will provide valuable input for PN formation, stellar population synthesis models, dust formation, hydrogen deficient stars, etc.