SPH simulation of double white dwarfs merger
for investigating progenitor models of SNe Ia
Yushi Sato(1), Naohito Nakasato(2), Ataru Tanikawa(3),Keiichi Maeda(4), Izumi Hachisu(1), Ken'ichi Nomoto(5)(1)University of Tokyo, (2)University of Aizu, (3)RIKEN,(4)Kyoto University, (5)Kavli-IPMU

2014/7/8 @ RIKEN

Today's my talk is organized as ...
Introduction↓Methods↓
Results↓
Discussion↓
SummaryIntroduction

Type Ia Supernovae・Explosion of WD in binary system
・Cosmological standard candle
SN 2014J・Major souces of iron group elementsImage Credit: NASA, ESA, A. Goobar (StockholmUniversity), and the Hubble Heritage Team
(STScI/AURA)

Very important for studying evolution ofthe Universe!

Models of progenitor

orIllustration: NASA/CXC/M Weiss

Illustration: NASA/CXC/M Weiss

Single Degenerate ?

Double Degenerate ?

(Whelan&Iben, 1973, ApJ, 186, 1007-1014)(Webbink, 1984, ApJ, 277, 355-360)

Observational approaches
・Direct detection of companion before/after explosion→ No certain detection (DD model)(e.g. Schaefer&Pagnotta, 2012 Nature, 481, 164-166)
・Detection of interaction between ejecta and companion→ No certain detection (DD model)(e.g. Kasen, 2010, ApJ, 708, 1025-1031)
・Detection of signatures of CSM→ Detected in some SNe Ia (SD model)(e.g. Foley et al. 2012, ApJ, 752, 101)

Imply the contribution from both models

Accretion Induced Collapse

(e.g.

CO

Saio & Nomoto, 1985, A&A, 150, L21)

COC ignite

C ignition
Quasi-stationary phase
Neutrino cooling < Nuclear reaction

ONeMg

M > MChNSSPH simulation of WDs merger
・e.g. Yoon et al. 2007, MNRAS, 380, 933-948(0.9+0.6Msun, 2×105)
Long-lived merger ・・・ explode after merger andaccretion

・e.g. Pakmor et al. 2010, Nature, 463, 61-64(0.9+0.9Msun, ～2×106)
Violent merger ・・・ explode at merger→ Imply possibility SNe Ia from WDs mergers

・Long-lived merger (LLM) scenario

COIa

COQuasi-stationary phase

・Violent merger (VM) scenario

COIa

CO
Violently accrete (merger phase)

The conditions for SNe Ia from WDs merger
LLM scenario ・・・
Carbon burning does not ignite on surface in quasi-stationaryphase ＋ Total mass exceeds Chandrasekhar mass (Mch)※Here, MCh = 1.38Msun

VM scenario ・・・
Carbon burning dynamically occurs at merger phase

What's mass range satisfying the above conditions?
Implementing SPH simulation of CO WDs merger over wide mass range

Methods

Smoothed Particle Hydrodynamics (SPH)・mesh-free Lagrangian method
・express fluid as a set of discrete elements(SPH particles)
・approximate fluid's physical quantity as
superposition of SPH particle'sh (smoothing length)・・・size of particle
W (kernel function)・・・distribution of particle

Simulation setup
Code : Nakasato et al. 2012, arXiv1206.1199
Gravitational calclation : Oct tree method
EOS : Timmes & Swesty, 2000, ApJS, 126, 501
Nuclear reaction : Not included
Artificial viscosity : Monaghan, 1992, ARA&A, 30, 543-574
Initial condition : Dan et al. 2011, ApJ, 737, 89
Mass range : 0.5～1.1Msun(0.1Msun)
Composition : C 50%, O 50%
Particle number : 10k, 50k, 100k, 500k(/Msun) (k=1,024)→ for confirming numerical convergenceResultsExample of simulation
Mass ・・・ 1.1+0.9 Msun
Particle number ・・・ 500k/Msun
Color ・・・ temperature(109K)

orbital plane

meridional plane

Structure in merger phase (VM scenario)

Hot spot

Structure in quasi-stationary phase(LLM scenario)

Keplerian Disk

Hot Envelope

Cold Core

Criteria for Carbon burning in each scenarios

in merger phase (for VM scenario)
Carbon burning time scale ＜ Dynamical time scale(Nomoto, 1982b, ApJ, 257, 780-792)

in quasi-stationary phase (for LLM scenario)
Carbon burning time scale ＜ Neutrino cooling time scale(Yoon et al. 2007, MNRAS, 380, 933-948)

Particle number 500k/Msun
Judgement of nuclear burning in merger phase

VM scenario

Particle number 500k/Msun
Judgement of nuclear burning in quasi-stationary phase

LLM scenario

Mtot > MChDiscussion

Mass range of double CO WDs for SNe Ia

VM scenario
M1 > 0.9Msun, M2 > 0.8Msun

LLM scenario
M1 < 0.9Msun, M2 < 0.8Msun

Contribution of double WDs mergers (DD model)
to Galactic SNe Ia
SNe Ia rate of our Galaxy (SBc galaxy)～ 0.00704/yr (Li et al. 2011, MNRAS, 412, 1473)
Merger rate of double WDs in our Galaxy～ 0.00896/yr
Maoz, 2012,
7% (Badenesof all &Galactic
SNeApJ,Ia 749, L11)
Mass distribution of single WDs from SDSS
(Kleinman et al. 2013, ApJS, 204, 5)
Assuming flat distribution of mass-ratio (q = M2/M1)
Merger rate of systems which can be progenitor of SNe Ia～ 0.000493/yr

Numerical convergence(maximum temperatures in merger phase)

Seem Not to converge ・・・ ?

Numerical convergence(maximum temperatures in quasi-stationary phase)

Seem to converge ・・・ ?

SummarySummary・Implement SPH simulation of double CO WDs merger・Investigate the mass range for SNe Ia
and estimate contribution to Galactic SNe Ia
・Mass range is ・・・
VM scenario M1 > 0.9Msun and M2 > 0.8Msun
LLM scenario M1 < 0.9Msun and M2 < 0.8Msun & M1+M2 > MCh・Contribution from WDs mergers (DD model) is
significantly small in our Galaxy→ Other models (e.g. SD model) are more important・Numerical convergence should be studied more carefully