Neutrino radiation hydrodynamics in hot and dense nuclear matter and the role of microphysics in simulations of massive stars

The main results of my doctoral studies
were obtained from core collapse simulations of massive stars
using a numerical model based on radiation hydrodynamics
and three-flavour Boltzmann neutrino transport
in spherical symmetry.
It was continuously further developed
with respect to the involved microphysics,
such as neutrino-matter interactions, a nuclear reaction
network for low temperatures and densities
and the equation of state (EoS) for hot and dense nuclear matter.
These improvements made it possible to extend the
simulation times from about $1$ second to more than
20 seconds of physical time and allowed a detailed
and for the first time consistent radiation hydrodynamics
investigation of the neutrino driven wind,
which develops during the early post-explosion phase
of massive stars due to the continued neutrino energy
deposition.
The neutrinos that diffuse out of the central object,
a protoneutron star (PNS), heat the material
on top of the PNS surface.
This heat is partly converted into kinetic energy
which drives a matter outflow, known as the neutrino driven wind.
Neutrino driven explosions of massive Fe-core progenitors
of 10 and 18 solar mass were modelled
using enhanced neutrino opacities.
This was necessary because the explosion
mechanism of such stars is a subject of active research
and by present standard knowledge only working
in multi-dimensional models.
In the case of a special progenitor star,
the less massive 8.8 solar mass O-Ne-Mg-core,
the explosion in spherical symmetry was found
even without enhanced opacities.
The obtained post-explosion evolution
is in qualitative agreement with
previous static steady-state and parametrized
dynamic wind models.
On the other hand, we find generally smaller
neutrino luminosities and mean energies,
the neutrino driven wind is proton-rich for more then 10 seconds
and the PNS properties and the contraction behaviour
differ from the assumptions made in previous wind studies.
Despite the moderately large entropies per baryon of about 100
and the fast expansion timescale, the conditions found are
unlikely to favour r-process nucleosynthesis.
In addition, we discuss the formation of
stellar mass black holes via PNS collapse.
The simulations are launched from several
massive progenitors of 40 and 50 solar mass.
In the absence of an earlier explosion,
the PNS collapses to a black hole
due to the continued mass accretion.
We analyse the electron-neutrino luminosity
dependencies and construct a simple approximation
for the electron-neutrino luminosity.
Furthermore, we analyse different (mu,tau)-neutrino
pair-reactions separately
and compare the differences during the post-bounce phase.
We also investigate the connection between the
increasing (mu,tau)-neutrino luminosity
and the PNS contraction during the accretion phase
before black hole formation.
Comparing the different post-bounce phases
of the progenitor models under investigation,
we find large differences in the emitted neutrino spectra.
These differences and the analysis of the electron-neutrino
luminosity indicate a strong progenitor model
dependency of the emitted neutrino signal.
Including an EoS for strange quark matter
based on the simple and widely used MIT bag model,
we are able to study the appearance of quark
matter in core collapse simulations.
The transition from hadronic matter to quark matter
is modelled via a Gibbs construction which results
in an extended mixed phase.
Assuming small bag constants, the phase transition
occurs during the early post-bounce phase
of massive progenitor stars at densities
near nuclear saturation which are found at the PNS centre.
The simulations are launched from
10, $13 and 15 solar mass stars,
where in the absence of an earlier explosion the PNSs
contract due to the continued mass accretion on
a timescale of 100 ms.
A direct consequence of the phase transition is the formation
of a strong second accretion shock at the phase boundary between
the mixed and the pure hadronic phases.
It even turns into a dynamic shock and overtakes the
first shock, which remained unaffected from the happenings
inside the PNS.
In other words, a new explosion mechanism is discovered,
where moderate explosion energies of 1 Bethe are obtained.
As soon as this second shock propagates over the
sphere of last scattering where neutrinos decouple from matter,
a second neutrino burst is released which may possibly be
detectable for a future Galactic event,
if a quark phase transition has taken place.