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

HYDEF07, September 19,

2007

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

The Formation of The Formation of Hydrogen Deficient Hydrogen Deficient

Stars Through Common Stars Through Common Envelope EvolutionEnvelope Evolution

By Steven DiehlTheoretical Astrophysics Group (T-6)

Los Alamos National Laboratory

Main Collaborators: Chris FryerFalk HerwigOrsola De Marco


2007


OverviewOverview

• 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


2007


Smooth Particle Smooth Particle HydrodynamicsHydrodynamics


2007


SPH - The Concept 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)


2007


SPH vs. Grid CodesSPH vs. Grid Codes

SPH Grid

Angular Momentum Conservation

++ -

Shock resolving -- ++

Ease of implementing complex physics

+ -

Ease of setup + -

Computational Speed ++ --


2007


Common Envelope Common Envelope Evolution (CE)Evolution (CE)


2007


CE - Conceptual PictureCE - 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


2007


CE: When does it start?CE: When does it start?• Thermal Pulses trigger radius peaks

• Companion get engulfed by the envelope

• CE evolution starts

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

De Marco et al. (2003)


2007


CE and H-Deficient StarsCE 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

QuickTime™ and aTIFF (Uncompressed) decompressor


Herwig 1999


2007


CE - Previous WorkCE - 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)


2007


CE - Number of time CE - Number of time stepssteps

• 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


2007


CE - Worst case: Planets/Brown CE - Worst case: Planets/Brown DwarfsDwarfs

• 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


2007


CE - ScalingCE - 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


2007


CE - Avoiding the worst CE - Avoiding the worst casecase

• 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


2007


CE - DISCLAIMERCE - 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


2007


CE - Test Run: 0.9RG, CE - Test Run: 0.9RG, 0.25WD0.25WD

• 0.9 solar mass Red Giant (RG) and 0.25 solar mass White Dwarf (WD) companion

QuickTime™ and aMotion JPEG OpenDML decompressor



2007


CE - Test Run: 0.9RG, CE - Test Run: 0.9RG, 0.05BD0.05BD

• Dynamics are always similar: Bow-shock structure around the companion, spirals in, spiral density wave transports angular momentum and mass outward

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2007


CE - Comparison R vs. TCE - 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


2007


CE - Comparison E vs TCE - 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


2007


CE - When Does it Stop?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


2007


CE - Comparison R vs RdMCE - 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


2007


CE - Open Questions for CE - Open Questions for RGRG

• 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”?


2007


CE - Open Questions for CE - Open Questions for CompanionsCompanions

• 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


2007


CE - Open Questions for CE - Open Questions for EjectaEjecta

• 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?


2007


SUMMARY AND OUTLOOKSUMMARY 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.

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