From Supernovae to Neutron Stars

Germán Lugones IAG – Universidade de São Paulo

Classification of Supernovae

Type Ia Ib Ic II

Spectrum No HydrogenSilicon

No HydrogenNo SiliconHelium

No HydrogenNo Silicon

No Helium

Hydrogen

PhysicalMechanism

Nuclearexplosion oflow mass star

Core collapse of evolved massive star ( may have lost its hydrogen or even heliumduring red-giant evolution)

LightCurve

Reproducible Large Variations

Neutrinos Insignificant ~100 times the visible energy

Compactremnant

None Neutron Star (typically appears as a pulsar)Sometimes Quark star?Sometimes Black hole ?

Observed Total : ~ 2000 as of today (nowadays ~ 200 / year )

Supernova Paradigm

1) Progenitor Star with M 8 MSUN onion structure all stable elements up to iron have been synthesized

2) Collapse of the iron core when it reaches Mchan

+ Iron desintegration + Neutronization

3) Shock wave formation at nuclear saturation density:

Collapsing matter rebounds on uncompressible nuclear matter

Shock wave expected to eject the mantle (0.1-0.2 c).

4) In the center: just born Neutron Star

- hot

- trapped neutrinos

Sketch of the post collapse stellar core during the neutrino heating and shock Revival

• Progenitor star with degenerate iron core:

109 g cm-3

• T 1010 K

• MFe 1.4 MSUN

• RFe 6000 km

• Proto-Neutron star:

• nuclear saturation density 3 x 1014 g cm-3

• T 30 MeV (1 MeV = 1.16 x 1010 K )

• MPNS 1.4 –1.7 MSUN due to accretion

• RPNS 10 -15 km

Core collapse supernova Energetics

Liberated gravitational binding energy of neutron star:• EB G M2 / RNS 3 x 1053 erg 17 % MSUN c2

This shows up as• 99 % neutrinos• 1% Kinetic energy of explosion (1% of this into cosmic rays)• 0.01 % Photons (outshine host galaxy)

Neutrino Luminosity • L 3 x 1053 erg / 3 sec 3 x 1019 LSUN While it lasts,

outshines the photon luminosity of the entire visible universe.

1-D Simulations

Revival of a stalled Supernova shock by neutrino heating Radial trajectories of equal mass shells

- Wilson, Proc. Univ. Illinois, Meeting on Numerical Astrophysics (1982) - Bethe & Wilson, ApJ 295 (1985) 14

Shock formation

Proto Neutron Star

Accretion onto the PNS

Supernova ejecta

Hot bubble

Shock propagation

Neutrino sphere

Rampp & Janka, ApJ 539 (2000) L33

1- D Failed Explosions

Mezzacappa et al., PRL 86 (2001) 1935

Spherically symmetric simulations, Newtonian and General Relativistic, with the most advanced treatment of neutrino transport do not produce explosions.

1-D simulations give failed explosions:

In general it is needed an increase in a factor 2 of the neutrino luminosity from the Proto Neutron Star at early times.

Why do explosions fail:

1) convection may be important.

2) there is physics still missing in the models (e.g. current simulations have an oversimplified description of neutrino interactions with nucleons in the nuclear medium of the neutron star).

3) Late phases transitions in the proto neutron star?

2-D Simulations

1600 kmProto Neutron Star

Shock Wave at 1400 km

Results of 2-D simulations:

Convection between the PNS and the shock wave helps shock revival but it is not sufficient.

Next steps: 2-D and 3-D simulations with state-of-the-art neutrino transport in progress.

Proto-Neutron Star Evolution

First stage of PNS evolution:

Deleptonization Neutrino diffusion deleptonizes the core on time scales of ~ 50 sec.

R2 /(c) (20 km)2 /(c 1 m) 1 sec   Temperature increases due to “Joule Heating”.

Neutrino Transparency After ~ 50 seconds the star becomes finally transparent to neutrinos because ~ R. At the end of deleptonization the PNS is still hot. The star has zero net neutrino number and so thermally produced neutrino pairs dominate the emission.

Black hole formation during neutron star birth?

Accretion onto the PNS occurs during the first few seconds of evolution. If mass increases beyond “MCh” black hole.

BH formation at the end of deleptonization: If MCh(cold, deleptonized PNS) < MCh(hot, lepton rich PNS)

Strange quark matter could be produced at the end of deleptonization. BH if MCh(SQM star) < MCh(cold, deleptonized PNS)

If the PNS collapses to form a black hole the - emission is believed to cease abruptly.

The Neutrino Signal from Supernovae and Proto Neutron

Stars

Supernova Neutrinos: Numerical Neutrino Signal

Totani, Sato, Dalhed & Wilson, ApJ 496 (1998) 216

NC CC

Supernova Neutrinos: Observed Neutrino Signal of SN 1987 A

Some Conclusions

Conclusions about Core Collapse Supernovae

The neutrino-heating mechanism is the favored explanation for the explosion.

However, it is still controversial, (the status of the model is not satisfactory)

Still missing: multidimensional simulations with an accurate and reliable handling of neutrino transport and an up-to-date treatment of the input physics (e.g. a better understanding of the neutrino interactions in neutron star matter )

It cannot be excluded that the energy for supernova explosions is provided by some other mechanism,

- Delayed phase transitions in the proto neutron star?

What do we really know ?

neutrinos with the expected characteristics are emitted from collapsed stellar cores

these neutrinos carry away the gravitational binding energy of the nascent neutron star

neutrino heating and strong convection must occur behind the stalled shock. This helps the explosion.

analytic studies and numerical simulations find explosions for a suitable combination of conditions

What do we really not know ?

Why do the supposedly best and most advanced spherical models not produce explosions

Can one trust current multidimensional simulations with their greatly simplified and approximate treatment of neutrino transport?

How can we explain the large asphericities and anisotropies observed in many supernovae?

What is the reason for the kicks by which pulsars are accelerated to average velocities of several hundred km/s presumably during the supernova explosion?

You may complete the list…

Phase Transitions in Neutron Stars

Transition to Quark Matter:

1) Deconfinement at 0

Neutrons, protons, Hyperons deconfine into their constituent quarks

Timescale of strong interactions. ~ 10-23 sec

In a Proto Neutron Star: Tansition must occur after a deleptonization timescale (~ 10 sec after Neutron Star birth) Lugones & Benvenuto (1998) PRD 083100, Benvenuto & Lugones MNRAS (1999) 304, L25.

In an old neutron star the transition is expected to happen after mass accretion has increased the central density over the deconfinement transition density.

2) Weak Decay of quarks: Releases ~ 4x1053 ergs and increases the temperature up to ~ 50 MeV.

Timescale: Weak interactions ~ 10-8 sec

The energy released in neutrinos is sufficient to give a successful explosion as it increases by a factor of ~ 2 or more the luminosity of the central compact object.

Propagation of the transition:

1) Begins as a deflagration: laminar velocity Vlam = 104 cm/sec

2) Due to the Raleigh-Taylor instability the flame rapidly enters a turbulent regime which increases the effective surface of burning and accelerates the combustion front.

3) The magnetic field acts as a surface tension in the region where the magnetic field is perpendicular to the flame velocity (i. e. the equatorial direction) and quenches the growing of the RT modes. On the contrary, it has no effect when the B-field is parallel to the flame velocity (i. e. in the polar direction) . The ratio of the polar and equatorial velocities is given by :

BBBB

Ve

Vp

[Ghezzi, de Gouveia Dal Pino, Horvath, APJ 548, 193 (2001)]:

A potential mechanism for GRB

In a NS, large asymmetries can be produced even for moderate values of B !!!

Lugones, Ghezzi, de Gouveia Dal Pino, Horvath, APJL (2002)

astro-ph / 0207262

As a consequence of the asymmetry in the velocity of the flame, the actual geometry achieved by the strange quark matter “core” will resemble a cylinder orientated in the direction of the magnetic poles of the neutron star.

Since R / d = Vp/Ve ~ 10 we find that typical dimensions of the cylinder are R~10 km d ~ 1km

Neutrino-antineutrino pairs are emitted through the polar caps (Fe/Fp ~ 10-2 - 10-4) and annihilate into electron-positron pairs in a small region just above the poles.

The model explain naturally various features of observed GRB:

1)Beamed emission

2)Total energy emitted: 1051-1052 ergs

3) Timescale of the emission: compatible with short GRB ( 0.2 sec)

Other characteristics depend on the specific scenario in which the neutron star is being burned,

isolated neutron star

Low Mass X-ray Binaries

just born neutron stars in supernova explosions

Even after many years of progress and development, we are far from a systematic and detailed understanding of the core-collapse supernova mechanism and the exact nature of compact stars . However the subject is much richer, the numerical tools are much better, and many insights have been won.

In addition, there are hints at connections between some supernovae and some gamma ray bursts, stellar mass black hole formation, the production of exotic phases of dense matter in neutron stars…

Conclusions