Mass transfer dynamics in white dwarf binary systems

Transcription

1 Mass transfer dynamics in white dwarf binary systems In collaboration with: Stephan Rosswog & Marcus Brüggen EUROWD10, Tübingen, 2010

2 Numbers on Galactic WDs total number of WDs: Napiwotzki (2009) Double White Dwarfs: 53% He-He DWDs 25% CO-CO DWDs 20% He-CO DWDs 1% ONeMg-He/CO DWDs Napiwotzki et al. (2007) 50% contact DWDs within t Hubble 9% AM CVn Nelemans et al. (2001) Nelemans et al. (2005)

3 Evolution of white dwarf binary systems that undergo mass transfer Mergers or semi-detached binary systems? Mergers Type Ia supernova Accretion induced collapse to a neutron star Extreme He star, R Coronae Borealis (RCrB) Semi-detached binaries AM Canum Venaticorum (AM CVn) stars

4 Mass transfer process RL R accretor R donor = Rdonor RL, mass transfer sets in M a R accretor CM L 1 RL R donor M d R donor a Ṙ donor = Ṙdonor RL, mass transfer is dynamically stable R min R accretor CM M d R donor M a a

5 Mass transfer process. Disc vs. direct impact accretion R min < R accretor, direct impact accretion R min > R accretor, disk accretion

6 Analytical estimates. Marsh et al. (2004) J orb = J GR + J MT + J tid.torque Guaranteed stable and unstable mass transfer regimes disk accretion stabilize mass transfer direct impact accretion destabilize mass transfer If Ṁ Ṁ Edd both stable and unstable binaries merge (Han & Webbink (1999)) Reproduced after Marsh et al. (2004)

7 Numerical model 3D smoothed particle hydrodynamics (SPH) code (Rosswog et al. (2008)) equation of state: HELMHOLTZ EOS (Center for Astrophysical Thermonuclear Flashes at the University of Chicago. (Timmes & Swesty (2000))) nuclear burning: QSE-reduced alpha network (Hix et al., 1998) gravitational forces: binary tree (Benz et al. (1990))

8 Numerical model. Initial conditions Numerically resolvable mass transfer: M num.res. 1 particle orbit 1.2 M 106 yr npart! Mtot 1.5 M!3/ cm!3/2 amt Constructing accurate initial conditions

9 Numerical results Performed simulations Run Masses a Composition MT T max ρ max L lost #Particles #Orbits No. [M ] [R 1 + R 2 ] [10 8 K] [10 7 g cm 3 ] [%L tot ] [10 3 ] He-C/O He-C/O He-ONeMg He-C/O C/O-C/O He-He He-He C/O-C/O Lower number of particles He-C/O He-C/O He-C/O He-C/O a MT is the separation at the moment when the relaxation in the corotating frame ends L lost is the angular momentum lost from the system #orbits = t end /P 0, where P 0 is the initial orbital period at the onset of mass tranfer and t end is the moment of disruption of the donor.

10 Numerical results: direct impact, unstable MT regime q = 0.66 (0.6 & 0.9M ), He/CO-CO merger.

11 Numerical results: direct impact, unstable MT regime q = 0.66 { WD masses: 0.6 & 0.9 M composition: He/CO-CO The mass transfer is NOT over in 2 orbits, but on 29

12 Numerical results: surface detonation of sub-chandrasekhar WDs What happens with the accreted Helium of the 0.6 & 0.9M system? Kelvin-Helmholtz instabilities trigger thermonuclear explosions of the He envelope Guillochon, Dan, Ramirez-Ruiz & Rosswog, ApJL, 709, L64, 2010

13 Numerical results. 0.2 & 0.8 M : stable or unstable mass transfer? q = 0.25 (0.2 & 0.8M ).

14 Numerical results. 0.2 & 0.8 M : stable or unstable MT? WD masses: 0.2 & 0.8 M q = 0.25 composition: He-donor and a C/O accretor no. of orbits: 85

15 Conclusions revision of earlier results (Benz et al. 1990): (numerical) mass transfer is NOT over in two orbital periods; instead it continues for several dozens of orbits unstable mass transfer of Helium can lead to surface detonations: Is this the ignition mechanism for a type Ia supernova? At Ṁ Ṁ Edd ALL close double white dwarfs with 0.25 q 1 merge

16 Thank you!