Mass Transfer in Double White Dwarf Binaries

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



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?



With apologies to Sidney Harris



Chandra: RX J0806 PR



AM Canis Venaticorum

Nelemans (2005) ASP Conf 330, eds. Hameury & LasotaJPL 65th Birthday Conference


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


Direct Impact Accretion



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)



Mass Transfer Stability andDirect Impact Limit



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



Orange: driving Green: mass transferBlue; solid-accretor, dashed-donor

q=0.4, driven at 2x10-3 P-1



Mass Transfer Rate



Binary Mass Ratio q=Mdonor/Maccretor



Binary Separation



Orbital Angular Momentum



Accretor Spin Angular Momentum



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.



Mass Transfer in Double White Dwarf Binaries

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