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Models of Black-Hole Accretion DisksFormed After Neutron-Star Mergers

Oliver JustMax-Planck-Institut für Astrophysik

MPPC Meeting, Berlin, June 30th 2014

With: H.-Th. Janka, S. Goriely, A. Bauswein, R. Ardevol,M. Obergaulinger, N. Schwarz, C. Weinberger and others

June 30th, 2014 MPPC Meeting, Berlin 2

The Fate of Compact Binaries

● CBs emit gravitational waves (GW)!!!

→ orbitial distance decays→ merger inevitable

● bad news: No NS merger has been observed so far, only 10 NS-NS binaries known, 0 NS-BH binaries known

● good news: orbital decay is measured for the Hulse-Taylor pulsar and precisely confirms prediction by general relativity

● AdvLIGO, AdvVIRGO start taking data ~ 2017

● expected merger rate (Abadie 2010):~ 10^-4 … 10^-6 per year per galaxy

(Lattimer)

(Weisberg, Taylor, 2004)


Neutron-Star Mergers: Theoretical Picture

(Hyper-) Massive Neutron Star

Black Hole – Torus System

delayed collapse

NS-BH NS-NS

NS

NS/BH

prompt collapse

GW

Inspiral

Merger


Neutron-Star Mergers: Theoretical Picture

➔ massive outflows… dynamical ejecta… ν-driven ejecta… viscous+recombination ejecta

➔ short GRB… from BH-torus system… from magnetized (H)MNS

➔ GW signal… progenitor masses… nuclear EOS

(Hyper-) Massive Neutron Star

Black Hole – Torus System

delayed collapse

NS-BH NS-NS

NS

NS/BH

prompt collapse

GW

Inspiral

Merger


1) NS-mergers could be main/significant source of heavy elements in the universe!

origin of elements heavier than iron:

➔ rapid neutron capture process (r-process)

➔ BUT: astrophysical site not identified so far!

Why Study Outflows of NS-Mergers?

Observed solar r-process abundance


1) NS-mergers could be main/significant source of heavy elements in the universe!

main condition for r-process: many neutrons (low Ye)!

➔ until recently core-collapse supernovae seemed promising

but newest simulations give proton-rich outflow

➔ dynamical ejecta from NS-mergers yield robust solar-like r-process for the heaviest elements A > 130

➔ What about ejecta from the BH-torus remnants???



2) NS mergers could be observable (in optical and infrared)! ("Kilonova" / "Macronova" )

➔ radioactive decay heats material → causes electromagnetic transient

➔ possibly first Kilonova already measured

(Berger et. al. 2013)

theoretical lightcurve (Barnes & Kasen 2013)



Physics of Post-Merger BH-Torus

subrelativistic winds?(driven by neutrinos, magnetic fields, recombination?)

e

MM d ∼ 0.01−0.3 M⊙

ρ ∼ 1010−1012 g/cm3

T ∼ 1−10 MeVY e ∼ 0.05−0.3

M

e e−

e

BH

(r-process?) nucleosynthesis?

short GRB(at r∼1014−1016 cm)

B

ultra-relativistic

outflow

neutrino cooling

neutrino heating

(short after its formation)

Role of neutrinos:➔ … influence accretion dynamics!➔ … determine Ye in the outflows!➔ … possibly launch GRB jet!


Modeling the post-merger torus

Two main computational challenges:● Neutrino transport

(i.e. solving the generally 7-dimensional Boltzmann equation)● MRI driven turbulence

(convergence, topology, reconnection...)

➔approximations inevitable!!!

Our approach:

Elaborate neutrino transport + Simplified magnetic field effects

➔ Two-moment transport with algebraic Eddington factor

(a.k.a. "M1 scheme")

➔ 2D axisymmetry➔ presently alpha-viscosity➔ next step: mag. fields


Neutrino TransportOur approach:

➔ Two-moment scheme with algebraic Eddington factor (aka "M1 scheme")

← energy density

← momentum density

← pressure

evolutionequations

approximate algebraicclosure relations (e.g. "M1 closure")


Neutrino TransportDetails of the algorithm:

● energy-dependent (multi-group), fully multidimensional● O(v/c) effects advection, aberration and Doppler shift included● finite-volume discretization, similar to well-known hydrodynamics solvers● IMEX scheme for time integration → efficiently scalable● implemented most important neutrino-interaction channels● extensively tested in 1D and 2D (Just et al., to be submitted)

2D static and dynamic diffusion

2D shadow test


Setup of BH-Torus Models

● initial models: j-constant equilibrium tori

● axisymmetry

● angular momentum transport: Shakura & Sunyaev α-viscosity

● most dominant interactions included:✔ beta-processes

✔ neutrino-nucleon scattering

✔ neutrino-antineutrino annihilation

● pseudo-Newtonian gravitational potential (mimics the ISCO and BH spin)

● variation in Mtorus, MBH, α


Disk Properties 2 evolutionary phases: ➔ first few 100 ms: "Neutrino-dominated accretion flow" (NDAF)

➔ subsequently: "Convection-dominated accretion flow" (CDAF)

time = 50 ms

time = 2 s


Ejecta Properties

Typical properties:● total outflow mass Mout ~ 20% Mtorus● electron fraction Ye ~ 0.2 – 0.3● velocity v ~ 10^9 cm/s

Typical nucleosynthesis yields:● solar like for 90 < A < 140● solar deficient for A > 140


Combined Nucleosynthesis Yields

➔DISK ejecta (mainly A ~ 90 - 140)

➔DYNAMICAL ejecta (mainly A ~ 140 - 210)

➔DISK + DYNAMICAL ejecta

➔nicely recovers the full mass range A > 90

➔BH-torus ejecta could be significant sources

of heavy elements!


Summary➔ NS mergers could be significant "heavy element factories"➔ up to now, only prompt ejecta have been examined in

nucleosynthesis studies → only produce elements A>140➔ BH-torus remnants extremely hard to model (neutrinos, MHD...)➔ developed a multidimensional nu-transport code based on M1

to model BH-torus systems and CCSNe➔ analyzed BH-torus ejecta + dynamical ejecta and performed

nucleosynthesis calculations

➔ main result: BH-torus systems could be responsible for significant amounts of 90 < A < 140 elements in the universe

(more details in arXiv:1406.2687)


Outlook

➔ main deficiency: no magnetic fields yet!

… but, MHD models are in the pipeline!

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