The structure and fate of white dwarf
merger remnants
Marius Dan
Hamburger Sternwarte, Universitat Hamburg
In collaboration with:
M. Bruggen (Universitat Hamburg) E. Ramirez-Ruiz (UC Santa Cruz)S. Rosswog (Stockholm University) J. Guillochon (Harvard, CfA)P. Podsiadlowski (University of Oxford)
March 25th, 2014
White dwarfs (WDs) in binaries
I ∼ 1010 WDs in Galaxy (Napiwotzki 2009)
I ∼ 108 Galactic double WDs (DWDs) (Nelemans+2001)
I 50% DWDs evolve into contact within Hubble time
Mass transfer phase
Roche model
I In a non-inertial frame, gravitational andcentrifugal forces are described by apotential
I Roche lobes: a critical equipotentialintersects itself at L1
I When WD fills its Roche lobe masstransfer sets in through L1
I Two regimes of mass transfer: disk anddirect impact
Mass transfer is...
I stable if donor has same size as itsRoche lobe −→ DWDs become AMCVn stars
I unstable if donor overflows its Rochelobe −→ binary merger
-10 -5 0 5 10
-10
-5
0
5
10
A large parameter space study
Numerical methods:(Benz+ 1990, Hix+ 1998, Timmes & Sweesty 2000, Rosswog+ 2008)
I Smoothed Particle HydrodynamicsI Helmholtz equation of stateI Quasi-equilibrium reduced α-networkI self-gravity via a binary tree
Initial conditions (Dan et al. 2011):
I binary carefully relaxed in corotating frame
I synchronized, cold and isothermal stars
-1 10-2
-8
-6
-4
-2
0
x [109 cm]
-p
m (
x, y, z)
| %
(x, y, z)
% t = 0
-pm
Parameter space:
I mass range between 0.2 and 1.2 M�
I different chemical compositions
I 225 systems
CO - CO
CO - He+CO
CO - HeHe+CO He - He
HeONe - He
ONe - CO
ONe
He+CO He+CO
-
-
He+CO-
CO-donors
He-donors
(Dan et al., 2012)
Mass transfer phase
I Marsh, Nelemans & Steeghs (2004):
I unstable for q & 2/3, (q = Mdon/Macc)
I stable for q . 1/5
I uncertain for 1/5 < q < 2/3, requires strong
spin-orbit coupling
I Mass transfer is long-lived
I Norbs increases with decreasing mass ratio
I Systems with CO donors are in unstable,direct impact regime of mass transfer:16− 28 orbits
I System within uncertain region merge after30− 60 orbits −→ consistent with a weakaccretor–orbit coupling
I Systems with q . 1/5 do not merge after75 – 90 orbits
I Norbs is an increasing function of resolution−→ conservative lower limit
NorbsCO donors
(Dan et al., 2012)
Structure of merger remnants at tmerger + 3P0
I cold core: fraction of former accretor
I hot envelope: mixed material Macc/Mdon
core
Keplerian disk
envelope
I Keplerian disk: mixed material Macc/Mdon, mainly Mdon
I tidal tail: fraction of former donor
(Dan et al, 2014)
Remnants’ morphology as a function of stars’ spins
I Corotating systems lead to hot spots incore’s outer layers while non-rotatingdeep inside core
I ρmax(corot.) at core’s centreρmax(non-rot.) off core’s centre
I ρmax(corot.) > ρmax(non-rot.)
I corotating systems lead to faster rotators
0 0.2 0.4 0.6 0.8 1
(Dan et al, 2014)
Dynamical burning and possible detonations
I τnuc = u/εnuc
I τdyn = (Gρ)−1/2
I τnuc/τdyn . 1 expansion tooslow to quench burning
−→ hydrodynamical burningand possibly a detonation
2.5 3 3.5 4 4.5 5 5.5 6 6.57.6
7.8
8
8.2
8.4
8.6
8.8
9
9.2
9.4
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
I Tmax − ρmax increase with Mtot and with numerical resolution
I He – detonations possible for Mtot & 1M�
I CO – detonations possible for Mtot & 2M�
I Schwab et al. (2012): τnuc/τdyn decreases by a factor of 10 duringviscous evolution
Ejected mass
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
4
8
12
16
20
× 10−3
0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
−1.5
−1
−0.5
0
0.5
1
I Mesc increases with decreasing q from ∼10−4 to 3.4× 10−2 M� (filled circles for
τnuc/τdyn ≤ 10)
I Lesc up to 12% of Ltot (larger symbols for τnuc/τdyn ≤ 10)
−→ Systems with substantial burning unbind more mass and angular momentum
Possible outcomes of white dwarf mergers
I He – He: extreme He stars (He-sdO andHe-sdB)
I He – CO: R Coronae Borealis (RCB) andextreme He stars; some sub-luminous TypeIa Supernovae (SN)
I CO – CO:• Mtot ∼ MCh, SN Ia or Accretion
Induced Collapse (AIC) to a Neutron Star(NS)
• Mtot < MCh, massive CO or ONeWDs; some sub-luminous SNe Ia
• Mtot > MCh, AIC to NS;super-Chandra SN Ia
I ONe – He: RCB and eHe stars; some donot merge (AM CVn stars)
I ONe – CO: AIC to a NS
I ONe – ONe: small mass black hole or willthey explode (Oxygen detonation)?
e.g., Webbink 1984, Iben & Tutukov 1984, Iben 1990,Saio & Nomoto 1985, Segretain+ 1997, Saio & Jeffery 2000,Yoon+ 2007, Han+ 2002, Justham+ 2010, Sim+ 2010Fryer+ 2010, Shen+ 2010,Pakmor+ 2010, Solheim 2010,Waldman+ 2011, Clayton 2012
CO-detonationssingle-det.
(1991bg-like SN)
SN Ia~10 yr5
He-rich sdO HeCO~10 yr7
sdB HeCO~10 yr8
7
R Coronae Borealis~10 yr CO
single-det.interm. phase RCB
(non-standard Ia later?)
M =1.4tot
double-det.(SN Ia)
double-det. supra-Ch. SN
AIC~10 yr
4
(interm. phase RCB)
(Dan et al., 2014)
Summary
I Mass transfer is long lived, duration increases with decreasing mass ratio:
I Systems in direct impact regime merge within ∼ 60 orbits;
I Disk accreting systems do not merger after 75− 90 orbits (AM CVn?).
I WD-WD mergers leave behind remnants consisting of a cold core, a hotenvelope, a Keplerian disk and a tidal tail
I Large fraction of systems can explode (i.e., τnuc/τdyn . 10)
I He mass transferring systems with Mtot & 1M�
I CO mass transferring systems with Mtot & 2M�
I WDs’ spin state could be decisive for the question where the ignition is
triggered:
I tidally locked stars produce hot spots in core’s outer layers
I irrotational systems produce hot spots deep inside core
I For most systems, Mesc and Lesc tend to increase with decreasing mass ratio.
Profiles of all 225 merger remnants are available at
http://tinyurl.com/nb83zev
poz. x−directionneg. x−directionpoz. y−directionneg. y−directionpoz. z−directionneg. z−direction
poz. x−directionneg. x−directionpoz. y−directionneg. y−directionpoz. z−directionneg. z−direction
poz. x−directionneg. x−directionpoz. y−directionneg. y−directionpoz. z−directionneg. z−direction
Stars’ mixing
Mixing between binary components increases with mass ratio q
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Chemical mixing between binary components
0
0.2
0.4
0.6
0.8
1
0
0
0.2
0.4
0.6
0
0.2
0.4
0.6
0
0.2
0.4
0.6
(Dan et al, 2014)