561 research outputs found
Boltzmann equations for neutrinos with flavor mixings
With a view of applications to the simulations of supernova explosion and
proto neutron star cooling, we derive the Boltzmann equations for the neutrino
transport with the flavor mixing based on the real time formalism of the
nonequilibrium field theory and the gradient expansion of the Green function.
The relativistic kinematics is properly taken into account. The advection terms
are derived in the mean field approximation for the neutrino self-energy whiles
the collision terms are obtained in the Born approximation. The resulting
equations take the familiar form of the Boltzmann equation with corrections due
to the mixing both in the advection part and in the collision part. These
corrections are essentially the same as those derived by Sirera et al. for the
advection terms and those by Raffelt et al. for the collision terms,
respectively, though the formalism employed here is different from theirs. The
derived equations will be easily implemented in numerical codes employed in the
simulations of supernova explosions and proto neutron star cooling.Comment: 14 pages, 2 figures, REVTeX, to be published in Physical Review
Linear analysis on the growth of non-spherical perturbations in supersonic accretion flows
We analyzed the growth of non-spherical perturbations in supersonic accretion
flows. We have in mind the application to the post-bounce phase of
core-collapse supernovae (CCSNe). Such non-spherical perturbations have been
suggested by a series of papers by Arnett, who has numerically investigated
violent convections in the outer layers of pre-collapse stars. Moreover, Couch
& Ott (2013) demonstrated in their numerical simulations that such
perturbations may lead to a successful supernova even for a progenitor that
fails to explode without the fluctuations. This study investigated the linear
growth of perturbations during the infall onto a stalled shock wave. The
linearized equations are solved as an initial and boundary value problem with
the use of Laplace transform. The background is a Bondi accretion flow whose
parameters are chosen to mimic the 15 progenitor model by
Woosley & Heger (2007), which is supposed to be a typical progenitor of CCSNe.
We found that the perturbations that are given at a large radius grow as they
flow down to the shock radius; the density perturbations can be amplified by a
factor of 30, for example. We analytically showed that the growth rate is
proportional to , the index of the spherical harmonics. We also found that
the perturbations oscillate in time with frequencies that are similar to those
of the standing accretion shock instability. This may have an implication for
shock revival in CCSNe, which will be investigated in our forthcoming paper in
more detail.Comment: 11 pages, 9 figures, accepted for publication in The Astrophysical
Journa
The Evolution and Impacts of Magnetorotational Instability in Magnetized Core-Collapse Supernovae
We carried out 2D-axisymmetric MHD simulations of core-collapse supernovae
for rapidly-rotating magnetized progenitors. By changing both the strength of
the magnetic field and the spatial resolution, the evolution of the
magnetorotational instability (MRI) and its impacts upon the dynamics are
investigated. We found that the MRI greatly amplifies the seed magnetic fields
in the regime where not the Alfv\'en mode but the buoyant mode plays a primary
role in the exponential growth phase. The MRI indeed has a powerful impact on
the supernova dynamics. It makes the shock expansion faster and the explosion
more energetic, with some models being accompanied by the collimated-jet
formations. These effects, however, are not made by the magnetic pressure
except for the collimated-jet formations. The angular momentum transfer induced
by the MRI causes the expansion of the heating region, by which the accreting
matter gain an additional time to be heated by neutrinos. The MRI also drifts
low- matter from the deep inside of the core to the heating region, which
makes the net neutrino heating rate larger by the reduction of the cooling due
to the electron capture. These two effects enhance the efficiency of the
neutrino heating, which is found to be the key to boost the explosion. Indeed
we found that our models explode far more weakly when the net neutrino heating
is switched off. The contribution of the neutrino heating to the explosion
energy could reach 60\% even in the case of strongest magnetic field in the
current simulations.Comment: 17 pages, 18 figures, ApJ in pres
Possible Signatures of Ejecta-Companion Interaction in iPTF 13bvn
We investigate the possible effects of the supernova ejecta hitting the
companion star in iPTF 13bvn, focusing on the observable features when it
becomes visible. iPTF 13bvn is a type Ib supernova that may become the first
case that its progenitor is identified as a binary by near future observations.
According to calculations by Bersten et al. (2014), the progenitor should have
a mass to reproduce the supernova light curve, and such
compact stars could only be produced via binary evolution. This is one of the
reasons that we expect the progenitor to be a binary, but it should be
confirmed by observing the remaining companion after the supernova. Their
evolutionary calculations suggest that the companion star will be an
overluminous OB star at the moment of supernova. With a combination of
hydrodynamical and evolutionary simulations, we find that the secondary star
will be heated by the supernova ejecta and expand to have larger luminosities
and lower surface effective temperatures. The star will look rather like a red
super giant, and this should be taken into account when searching for the
companion star in the supernova ejecta in future observations.Comment: few corrections made, published in Ap
Regular and non-regular solutions of the Riemann problem in ideal magnetohydrodynamics
We have built a code to numerically solve the Riemann problem in ideal
magnetohydrodynamics (MHD) for an arbitrary initial condition to investigate a
variety of solutions more thoroughly. The code can handle not only regular
solutions, in which no intermediate shocks are involved, but also all types of
non-regular solutions if any. As a first application, we explored the
neighborhood of the initial condition that was first picked up by Brio & Wu
(1988) and has been frequently employed in the literature as a standard problem
to validate numerical codes. Contrary to the conventional wisdom that there
will always be a regular solution, we found an initial condition, for which
there is no regular solution but a non-regular one. The latter solution has
only regular solutions in its neighborhood and actually sits on the boundary of
regular solutions. This implies that the regular solutions are not sufficient
to solve the ideal MHD Riemann problem and suggests that at least some types of
non-regular solutions are physical. We also demonstrate that the non-regular
solutions are not unique. In fact, we found for the Brio & Wu initial condition
that there are uncountably many non-regular solutions. This poses an intriguing
question: why a particular non-regular solution is always obtained in numerical
simulations? This has important ramifications to the discussion of which
intermediate shocks are really admissible.Comment: 32 pages, 12 figures, accepted for publication in Journal of Plasma
Physic
Systematic Analysis of the Effects of Mode Conversion on Thermal Radiation from Neutron Stars
In this paper, we systematically calculate the polarization in soft X-rays
emitted from magnetized neutron stars, which are expected to be observed by the
next-generation X-ray satellites. Magnetars are one of the targets for these
observations. This is because thermal radiation is normally observed in the
soft X-ray band, and it is thought to be linearly polarized because of
different opacities for two polarization modes of photons in the magnetized
atmosphere of neutron stars and the dielectric properties of the vacuum in
strong magnetic fields. In their previous study, Taverna et al. illustrated how
strong magnetic fields influence the behavior of the polarization observables
for radiation propagating in vacuo without addressing a precise, physical
emission model. In this paper, we pay attention to the conversion of photon
polarization modes that can occur in the presence of an atmospheric layer above
the neutron star surface, computing the polarization angle and fraction and
systematically changing the magnetic field strength, radii of the emission
region, temperature, mass, and radii of the neutron stars. We confirmed that if
plasma is present, the effects of mode conversion cannot be neglected when the
magnetic field is relatively weak, . Our results
indicate that strongly magnetized () neutron
stars are suitable to detect polarizations, but not-so-strongly magnetized () neutron stars will be the ones to confirm the mode
conversion.Comment: 34 pages, 29 figures, published in The Astrophysical Journa
Vacuum Polarization and Photon Propagation in an Electromagnetic Plane Wave
The QED vacuum polarization in external monochromatic plane-wave
electromagnetic fields is calculated with spatial and temporal variations of
the external fields being taken into account. We develop a perturbation theory
to calculate the induced electromagnetic current that appears in the Maxwell
equations, based on Schwinger's proper-time method, and combine it with the
so-called gradient expansion to handle the variation of external fields
perturbatively. The crossed field, i.e., the long wavelength limit of the
electromagnetic wave is first considered. The eigenmodes and the refractive
indices as the eigenvalues associated with the eigenmodes are computed
numerically for the probe photon propagating in some particular directions. In
so doing, no limitation is imposed on the field strength and the photon energy
unlike previous studies. It is shown that the real part of the refractive index
becomes less than unity for strong fields, the phenomenon that has been known
to occur for high-energy probe photons. We then evaluate numerically the
lowest-order corrections to the crossed-field resulting from the field
variations in space and time. It is demonstrated that the corrections occur
mainly in the imaginary part of the refractive index.Comment: 50 pages, 17 figures, accepted for publication in Progress of
Theoretical and Experimental Physic
Exact Riemann solver for ideal magnetohydrodynamics that can handle all types of intermediate shocks and switch-on/off waves
We have built a code to obtain the exact solutions of Riemann problems in
ideal magnetohydrodynamics (MHD) for an arbitrary initial condition. The code
can handle not only regular waves but also switch-on/off rarefactions and all
types of non-regular shocks: intermediate shocks and switch-on/off shocks.
Furthermore, the initial conditions with vanishing normal or transverse
magnetic fields can be handled although the code is partly based on the
algorithm proposed by Torrilhon (2002) (Torrilhon 2002 Exact Solver and
Uniqueness Condition for Riemann problems of Ideal Magnetohydrodynamics.
Research report 2002-06, Seminar for Applied Mathematics, ETH, Zurich), which
cannot deal with all types of non-regular waves nor the initial conditions
without normal or transverse magnetic fields. Our solver can find all the
solutions for a given Riemann problem and hence, as demonstrated in this paper,
one can investigate the structure of the solution space in detail. Therefore
the solver is a powerful instrument to solve the outstanding problem of the
existence and uniqueness of solutions of MHD Riemann problems. Moreover, the
solver may be applied to numerical MHD schemes like the Godunov scheme in the
future.Comment: 32 pages, 9 figures, accepted for publication in Journal of Plasma
Physic
A Hydrodynamical Study on the Conversion of Hadronic Matter to Quark Matter: I. Shock-Induced Conversion
We study transitions of hadronic matter (HM) to 3-flavor quark matter (3QM)
locally, regarding the conversion processes as combustion and describing them
hydrodynamically. Not only the jump condition on both sides of the conversion
front but the structures inside the front are also considered by taking into
account what happens during the conversion processes on the time scale of weak
interactions as well as equations of state (EOS's) in the mixed phase. Under
the assumption that HM is metastable with their free energies being larger than
those of 3QM but smaller than those of 2-flavor quark matter (2QM), we consider
the transition via 2QM triggered by a rapid density rise in a shock wave. Based
on the results, we discuss which combustion modes (strong/weak detonation) may
be realized. HM is described by an EOS based on the relativistic mean field
theory and 2, 3QM's are approximated by the MIT bag model. We demonstrate for a
wide range of bag constant and strong coupling constant in this combination of
EOS's that the combustion may occur in the so-called endothermic regime, in
which the Hugoniot curve for combustion runs below the one for the shock wave
in P-V plane, and which has no terrestrial counter part. We find that strong
detonation always occurs. Depending on the EOS of quark matter (QM) as well as
the density of HM and the Mach number of the detonation front, deconfinement
from HM to 2QM is either completed or not completed in the shock wave. In the
latter case, which is more likely if the EOS of QM ensures that deconfinement
occurs above the nuclear saturation density and that the maximum mass of cold
quark stars is larger than two solar mass, the conversion continues further via
the mixing state of HM and 3QM on the time scale of weak interactions.Comment: 44 pages, 18 figures, accepted for publication in PR
A Hydrodynamical Study on the Conversion of Hadronic Matter to Quark Matter: II. Diffusion-Induced Conversion
We study transitions of hadronic matter (HM) to 3-flavor quark matter (3QM),
regarding the conversion processes as combustion and describing them
hydrodynamically. Under the assumption that HM is metastable with their free
energies being larger than those of 3QM but smaller than those of 2-flavor
quark matter (2QM), we consider in this paper the conversion induced by
diffusions of seed 3QM. This is a sequel to our previous paper, in which the
shock-induced conversion was studied in the same frame work. We not only pay
attention to the jump condition on both sides of the conversion front but the
structures inside the front are also considered by taking into account what
happens during the conversion processes on the time scale of weak interactions.
We employ for HM the Shen's EOS, which is based on the relativistic mean field
theory, and the bag model-based EOS for QM just as in the previous paper. We
demonstrated in that paper that in this combination of EOS's the combustion
will occur for a wide range of the bag constant and strong coupling constant in
the so-called endothermic regime, in which the Hugoniot curve for combustion
runs below the initial state. We find that weak deflagration nearly always
occurs and that weak detonation is possible only when the diffusion constant is
(unrealistically) large and the critical strange fraction is small. The
velocities of the conversion front are ~ cm/s depending on the
initial temperature and density as well as the parameters in the QM EOS and
become particularly small when the final state is in the mixed phase. Finally
we study linear stability of the laminar weak-deflagration front and find that
it is unstable in the exothermic regime (Darrius-Landau instability) but stable
in the endothermic regime, which is quite contrary to the ordinary combustions.Comment: 36 pages, 21 figures, accepted for publication in PR
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