486 research outputs found
Magnetic fields in cosmological simulations of disk galaxies
Observationally, magnetic fields reach equipartition with thermal energy and
cosmic rays in the interstellar medium of disk galaxies such as the Milky Way.
However, thus far cosmological simulations of the formation and evolution of
galaxies have usually neglected magnetic fields. We employ the moving-mesh code
\textsc{Arepo} to follow for the first time the formation and evolution of a
Milky Way-like disk galaxy in its full cosmological context while taking into
account magnetic fields. We find that a prescribed tiny magnetic seed field
grows exponentially by a small-scale dynamo until it saturates around
with a magnetic energy of about of the kinetic energy in the center of
the galaxy's main progenitor halo. By , a well-defined gaseous disk forms
in which the magnetic field is further amplified by differential rotation,
until it saturates at an average field strength of \sim 6 \mug in the disk
plane. In this phase, the magnetic field is transformed from a chaotic
small-scale field to an ordered large-scale field coherent on scales comparable
to the disk radius. The final magnetic field strength, its radial profile and
the stellar structure of the disk compare well with observational data. A minor
merger temporarily increases the magnetic field strength by about a factor of
two, before it quickly decays back to its saturation value. Our results are
highly insensitive to the initial seed field strength and suggest that the
large-scale magnetic field in spiral galaxies can be explained as a result of
the cosmic structure formation process.Comment: 5 pages, 4 figures, accepted to ApJ
Helium-ignited violent mergers as a unified model for normal and rapidly declining Type Ia Supernovae
The progenitors of Type Ia Supernovae (SNe Ia) are still unknown, despite
significant progress during the last years in theory and observations. Violent
mergers of two carbon--oxygen (CO) white dwarfs (WDs) are one candidate
suggested to be responsible for at least a significant fraction of normal SNe
Ia. Here, we simulate the merger of two CO WDs using a moving-mesh code that
allows for the inclusion of thin helium (He) shells (0.01\,\msun) on top of the
WDs, at an unprecedented numerical resolution. The accretion of He onto the
primary WD leads to the formation of a detonation in its He shell. This
detonation propagates around the CO WD and sends a converging shock wave into
its core, known to robustly trigger a second detonation, as in the well-known
double-detonation scenario for
He-accreting CO WDs. However, in contrast to that scenario where a massive He
shell is required to form a detonation through thermal instability, here the He
detonation is ignited dynamically. Accordingly the required He-shell mass is
significantly smaller, and hence its burning products are unlikely to affect
the optical display of the explosion. We show that this scenario, which works
for CO primary WDs with CO- as well as He-WD companions, has the potential to
explain the different brightness distributions, delay times and relative rates
of normal and fast declining SNe Ia. Finally, we discuss extensions to our
unified merger model needed to obtain a comprehensive picture of the full
observed diversity of SNe Ia.Comment: accepted for publication by ApJL, significant changes to first
version, including addition of merger simulatio
Stellar GADGET: A smooth particle hydrodynamics code for stellar astrophysics and its application to Type Ia supernovae from white dwarf mergers
Mergers of two carbon-oxygen white dwarfs have long been suspected to be
progenitors of Type Ia Supernovae. Here we present our modifications to the
cosmological smoothed particle hydrodynamics code Gadget to apply it to stellar
physics including but not limited to mergers of white dwarfs. We demonstrate a
new method to map a one-dimensional profile of an object in hydrostatic
equilibrium to a stable particle distribution. We use the code to study the
effect of initial conditions and resolution on the properties of the merger of
two white dwarfs. We compare mergers with approximate and exact binary initial
conditions and find that exact binary initial conditions lead to a much more
stable binary system but there is no difference in the properties of the actual
merger. In contrast, we find that resolution is a critical issue for
simulations of white dwarf mergers. Carbon burning hotspots which may lead to a
detonation in the so-called violent merger scenario emerge only in simulations
with sufficient resolution but independent of the type of binary initial
conditions. We conclude that simulations of white dwarf mergers which attempt
to investigate their potential for Type Ia supernovae should be carried out
with at least 10^6 particles.Comment: 11 pages, 6 figures, accepted for publication in MNRA
Oxygen emission in remnants of thermonuclear supernovae as a probe for their progenitor system
Recent progress in numerical simulations of thermonuclear supernova
explosions brings up a unique opportunity in studying the progenitors of Type
Ia supernovae. Coupling state-of-the-art explosion models with detailed
hydrodynamical simulations of the supernova remnant evolution and the most
up-to-date atomic data for X-ray emission calculations makes it possible to
create realistic synthetic X-ray spectra for the supernova remnant phase.
Comparing such spectra with high quality observations of supernova remnants
could allow to constrain the explosion mechanism and the progenitor of the
supernova. The present study focuses in particular on the oxygen emission line
properties in young supernova remnants, since different explosion scenarios
predict a different amount and distribution of this element. Analysis of the
soft X-ray spectra from supernova remnants in the Large Magellanic Cloud and
confrontation with remnant models for different explosion scenarios suggests
that SNR 0509-67.5 could originate from a delayed detonation explosion and SNR
0519-69.0 from an oxygen-rich merger.Comment: 8 pages, 4 figures, MNRAS accepte
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