Post on 31-Jan-2016
description
Mass Transfer in Double White Dwarf Binaries
Juhan FrankTheoretical Astrophysics Group at
Louisiana State University
JPL 65th Birthday Conference
TrzebieszowiceOctober 2007
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
TrzebieszowiceOctober 2007
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
TrzebieszowiceOctober 2007
With apologies to Sidney Harris
JPL 65th Birthday Conference
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Chandra: RX J0806 PR
JPL 65th Birthday Conference
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AM Canis Venaticorum
Nelemans (2005) ASP Conf 330, eds. Hameury & LasotaJPL 65th Birthday Conference
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Discovery: Smak (1967); DWD model: Paczyński (1967)
The Twin-Degenerate Interacting Binary G61-29 (aka GP Com)
Nather, Robinson & Stover 1981 ApJ 244, 269 JPL 65th Birthday Conference
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Direct Impact Accretion
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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)
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Mass Transfer Stability andDirect Impact Limit
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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 R2/R1=q-1/3, Plavec & Kratochvil formula for b1, and taking stream periastron = 0.5rcirc, 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
q=0.5, driven initially
JPL 65th Birthday Conference
TrzebieszowiceOctober 2007
Orange: driving Green: mass transferBlue; solid-accretor, dashed-donor
q=0.4, driven at 2x10-3 P-1
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Mass Transfer Rate
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Binary Mass Ratio q=Mdonor/Maccretor
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Binary Separation
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Orbital Angular Momentum
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Accretor Spin Angular Momentum
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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 qstable = 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 ~102 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
TrzebieszowiceOctober 2007