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The Formation of Hydrogen Deficient Stars Through Common Envelope Evolution

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. Overview. The SPH technique : a brief introduction

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The Formation of Hydrogen Deficient Stars Through Common Envelope Evolution

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  1. 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 Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

  2. 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 Fryer’s talk on Friday] • Summary and Outlook Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

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

  4. 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) Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

  5. SPH vs. Grid Codes Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

  6. Common Envelope Evolution (CE) Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

  7. 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 Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

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

  9. 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 Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

  10. 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) Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

  11. 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 Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

  12. 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 Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

  13. CE - Scaling • Let’s assume we increase the resolution by a factor of q: h’=h/q • if h’=h/q then cs’=qcs and dt’h’/cs’dt/q2-> number of time-steps: Ndt’=q2Ndt • Number of particles (3d-Volume): h’=h/q -> N’=q3N • Computing speed per timestep: µ’N’logN’ µ (if all particles updated all the time) • Total computing time: µ’tot=µ’*Ndt’q5 logq3µtot • q=2 -> µ’tot=66µtot, q=10 -> µ’tot=690775µtot Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

  14. 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 Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

  15. 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 Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

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

  17. 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 Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

  18. 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 Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

  19. 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 Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

  20. 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 can’t transfer the energy into the envelope anymore Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

  21. 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 Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

  22. 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”? Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

  23. 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 Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

  24. 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? Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

  25. 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. Common Envelope Evolution - Steven Diehl, T-6 (LANL) -

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