Mass ejection from neutron-star mergers in numerical relativity - PowerPoint PPT Presentation

Mass ejection � from neutron-star mergers � in numerical relativity Masaru Shibata 1 Center for Gravitational Physics, Yukawa Institute for Theoretical Physics, Kyoto U. 2 Max Planck Institute for Gravitational Physics at Potsdam

Outline I. Brief introduction II. Scenarios for NS mergers (NS-NS & BH-NS) III. Dynamical mass ejection IV. MHD/viscous ejection from merger remnants V. Summary

I Introduction: Why mass ejection from NS binaries is important ? 1. Electromagnetic counterparts of NS merger: Key for confirming gravitational-wave detection (talks by Tanaka, Metzger, and others this week) 2. Possible site of r-process nucleosynthesis (talk by Lattimer) Jet − ISM Shock (Afterglow) Optical (hours − days) Radio (weeks − years) Ejecta − ISM Shock Radio (years) θ obs GRB (t ~ 0.1 − 1 s) Kilonova Optical (t ~ 1 day) θ j Merger Ejecta Tidal Tail & Disk Wind v ~ 0.1 − 0.3 c BH Metzger & Berger 2012

In this talk, I focus on • Ejecta mass M eject • Electron fraction Y e (=[p]/[nucleon]) Abundance pattern Light curve 20 3rd peak 2nd peak 1st peak i band 21 200 Mpc Observed magnitude 22 1m 23 24 4m 25 8m 26 27 0 5 10 15 20 Days after the merger Korobkin et al. 2012 Tanaka & Hotokezaka 2013

II A Typical scenarios for NS-NS merger • Constraints from radio-telescope observations : 1. Approximately 2-solar-mass NSs exist (Demorest ea 2010, Antoniadis ea 2013) à Constraint to equation of state (EOS) for NS 2. Typical total mass of compact binary neutron stars à ~ 2.73±0.15 solar mass (by Pulsar timing obs.)

Compact NS-NS system in our galaxy Orbital period Eccentricity Each mass lifetime PSR P (day) e M ( M sun ) M 1 M 2 T GW 1. B1913+16 0.323 0.617 2.828 1.441 1.387 3.0 2. B1534+12 0.421 0.274 2.678 1.333 1.345 27 3. B2127+11C 0.335 0.681 2.71 1.35 1.36 2.2 4. J0737-3039 0.102 0.088 2.58 1.34 1.25 0.86 5. J1756-2251 0.32 0.18 2.57 1.34 1.23 17 6. J1906+746 0.166 0.085 2.61 1.29 1.32 3.1 7. J1913+1102 0.206 0.090 2.875 1.65 1.24 ~5 8. J1757-1854 0.184 0.606 2.74 1.35 1.39 ~0.75 × 10 8 yrs Ø Total Mass of NS in compact NS-NS is likely to be in a narrow range, m ≈ 2.73±0.15 M sun

II A Typical scenarios for NS-NS merger • Constraints from radio-telescope observations : 1. Approximately 2-solar-mass NSs exist (Demorest ea 2010, Antoniadis ea 2013) à equation of state (EOS) for NS has to be stiff 2. Typical total mass of compact binary neutron stars à ~ 2.73±0.15 solar mass (by Pulsar timing obs.) • Numerical relativity simulations have shown that merger results typically in high-mass neutron stars (not BH) (Shibata et al. 2005, 2006.. recently many works….)

Possible outcomes of NS-NS mergers & Prediction by numerical relativity Likely typical cases M thr > ~2.8 M sun for M tot = 2.6—2.8 M sun Depends on EOS

Mass ejection history for MNS formation Time after merger 0 10 100 1000 ms Dynamical ejection (Sec. III) (determined by dynamical timescale of NS) MHD/viscous ejection (Sec. IV) (by viscous timescale of remnant MNS/torus) Neutrino irradiation (for neutrino emission timescale) (minor effects in mass but major effect in Y e )

II B Scenarios for BH-NS merger • Almost no observational constraints à Wide parameter space has to be explored • Fate = two possibilities: 1. Tidal disruption of NS 2. Simple plunge of NS into BH (no disruption)

Condition for tidal disruption ( ) < BH tidal force ( ) For tidal disruption, Self gravity of NS ⇓ 3 M NS 2 α R NS 3 ( ) M BH α R NS ⎛ ⎞ ⎛ ⎞ ⎛ ⎞ M NS ) ⇒ 1 ≤ M BH ( α > 1 2 < ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ r 3 r M BH M NS ( ) α R NS ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ISCO r α R NS • For tidal disruption v Large NS Radius or B NS H v Small BH mass or v High corotation spin R NS ISCO is necessary B NS H

For tidal disruption of plausible BH-NS with M NS =1.35 M sun , R NS ~ 12 km, & M BH > 5 M sun High BH spin is necessary > ~ 0.5 Foucart et al. ( ‘ 13,14,…); Kyutoku et al. (‘15) 3 7 M NS 2 3 3 ⎛ ⎞ ⎛ ⎞ ⎛ ⎞ 1 ≤ 0.1 6 M BH R NS ⎛ ⎞ α ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ r M BH 6 M NS 1.7 ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ISCO ( M BH ≤ r ISCO ≤ 9 M BH )

Mass ejection history for BH-NS � (in the case of tidal disruption of NS) � & NS-NS à prompt collapse to BH Time after merger 0 10 100 1000 ms Dynamical ejection (Sec. III) (determined by dynamical timescale of system) Long-term MHD/viscous ejection (Sec. IV) (by viscous timescale of disk/torus) (Fernandez-Metzger ’13, ‘14, Just+ ‘15…) Neutrino irradiation (would be minor)

III Dynamical mass ejection • First, for NS-NS • Then, for BH-NS

Neutrino-radiation hydro for dynamical ejecta Stiff EOS (DD2, R ~13.2 km): 1.30-1.40 M sun Total mass ~ 0.001—0.01 M sun Rest-mass density depending on EOS & each mass Orbital plane Neutrino luminosity (mass ratio 0.8—1.0) ν e ν e ν others x-z plane Ejection occurs primarily toward equatorial plan Sekiguchi et al. 2016 15

Summary for dynamical ejecta mass in NR Ejecta mass depends significantly on NS EOS & mass Nearly equal Unequal mass: Small total mass m 1 / m 2 < 0.9 mass system ( M tot ~ 2.7 M sun ) ( M tot ~ 2.7 M sun ) (< 2.6 M sun ) HMNS à BH HMNS à BH MNS (long lived) Soft EOS M eje ~10 -2 M sun M eje ~10 -3 M sun M eje ~10 -2 M sun ( R =11-12 km) MNS (long lived) MNS (long lived) MNS (long lived) Stiff EOS M eje ~10 -2.5 M sun M eje ~10 -3 M sun M eje ~10 -3 M sun ( R =13-15km) e.g. Foucart et al. ’16 Ø Typical average velocity: 0.15—0.25 c See also the talk slide by Bauswein on June 5th

High temperature ⇒ γγ → e − + e + , n + e + → p + ν e Neutrino-radiation hydrodynamics simulation SFHo ( R ~11.9 km): 1.30-1.40 M sun Neutrino irradiation ⇒ n + ν → p + e − Y e =[p]/[nucleon] Electron fraction (x-y) Neutrino luminosity ν e ν e ν others Electron fraction (x-z) Green = neutron rich 17 Sekiguchi et al. (2016)

Electron fraction profile: Broad t - t M-6 [ms] Sekiguchi et al. 2015 PRD 1.35-1.35 solar case 10 0 SFHo Original values DD2 Fraction of mass of NS 10 -1 TM1 10 -2 10 -3 0 0.1 0.2 0.3 0.4 0.5 Ye Ø Average depends on EOS but typically peak at 0.2—0.3 Ø Broad distribution irrespective of EOS Ø Similar results by Radice+16, Lehner+15,16 ……..

Good agreement with solar abundance pattern -2 10 solar r-abundance -3 TM1 (1.35+1.35) 10 DD2 (1.35+1.35) SFHo (1.35+1.35) -4 10 abundance lanthanide -5 10 -6 Y e ~ 0.4 10 κ ~ 0.1cm 2 /g Y e ~ 0.25 Y e < 0.15 -7 10 κ ~ 1cm 2 /g κ ~ 10 cm 2 /g Wanajo, Sekiguchi unpublished -8 10 0 50 100 150 200 250 mass number

Neutrino-radiation hydrodynamics simulation Y e SFHo ( R ~11.9 km): 1.25-1.55 M sun More neutron-rich except for disk surrounding BH Green = neutron rich Shibata et al. (2017)

Electron fraction distribution: Broad irrespective of EOS and mass à Good for producing a variety of r-elements 10 0 Asymmetric binary SFHo 135-135 Soft EOS 133-137 130-140 10 -1 Mass fraction 125-145 Mass fraction 10 -2 10 -3 10 -4 10 0 DD2 135-135 130-140 Stiff EOS 125-145 10 -1 Mass fraction 10 -2 10 -3 10 -4 0 0.1 0.2 0.3 0.4 0.5 Electron fraction (Ye) Sekiguchi+ ‘16 Electron fraction Y e See also Radice ‘16 , ‘17

Neutrino irradiation: subdominant effect for mass ejection but important for Y e Ejecta mass Electron fraction Heating on Heating on Heating off Heating off Sekiguchi+ 2015 Neutrino irradiation from MNS increases Ø the ejecta mass increases by ~ 0.001 solar mass Ø Average value of Y e increases by ~ 0.03 in 30 ms See also, Perego et al. 2014; Goriely et al. 2015; Martin et al. 2015; Foucart et al. 2016

Note on massive NS-NS merger à Direct BH formation May be the fate for PSR J1913+1102 of total mass 2.875 M sun Ø For this case, mass ejection is possible only at a merger phase of short timescale • Nearly equal-mass : negligible mass ejection < 0.001 solar mass (e.g., Shibata + ‘06, Hotokezaka et al ‘13) • Asymmetric case : Mass increases with the degree of asymmetry; could be ~ 0.01 solar mass for q ~0.75 • Y e would be always low (almost no heating & no neutrino irradiation) See also talk slide by Bauswein on June 5th

Note on ejecta velocity: presence of high v � HB-135-135 10 -2 N182 V ave ~ 0.2c but N150 10 -3 N130 N110 M eje [ M sun ] 10 -4 Effect of shock heating 10 -5 10 -6 10 -7 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 v / c

BH-NS merger (SFHo EOS: density) M BH =5.4M sun , M NS =1.35M sun , a BH =0.75 Mass ejection occurs by tidal force of BH Kyutoku et al. 2018; Also many pioneer works by F. Foucart et al.

BH-NS with NS mass 1.35M sun (Q=5 & 7) Data: Kyutoku et al. 2015 10 -1 M NS =1.35 solar mass GW170817 Higher spin M *ejecta (solar mass) 10 -2 Dim EM signals? Soft EOS Stiff EOS results in 10 -3 < ~0.01 M sun high mass > 0.01 M sun M BH =9.45, a=0.75 M BH =9.45, a=0.50 M BH =6.75, a=0.50 10 -4 10 11 12 13 14 15 Radius of 1.35 solar mass NS R 1.35 (km) High BH spin is important for mass ejection