90,272 research outputs found
Thermonuclear supernova simulations with stochastic ignition
We apply an ad hoc model for dynamical ignition in three-dimensional
numerical simulations of thermonuclear supernovae assuming pure deflagrations.
The model makes use of the statistical description of temperature fluctuations
in the pre-supernova core proposed by Wunsch & Woosley (2004). Randomness in
time is implemented by means of a Poisson process. We are able to vary the
explosion energy and nucleosynthesis depending on the free parameter of the
model which controls the rapidity of the ignition process. However, beyond a
certain threshold, the strength of the explosion saturates and the outcome
appears to be robust with respect to number of ignitions. In the most energetic
explosions, we find about 0.75 solar masses of iron group elements. Other than
in simulations with simultaneous multi-spot ignition, the amount of unburned
carbon and oxygen at radial velocities of a few 1000 km/s tends to be reduced
for an ever increasing number of ignition events and, accordingly, more
pronounced layering results.Comment: 7 pages, 6 figures, accepted for publication in Astron. Astrophys.;
PDF version with full resolution figures available from
http://www.astro.uni-wuerzburg.de/~schmidt/Paper/StochIgnt_AA.pd
Thermonuclear explosions of rapidly rotating white dwarfs - I. Deflagrations
Context: Turbulent deflagrations of Chandrasekhar mass White Dwarfs are
commonly used to model Type Ia Supernova explosions. In this context, rapid
rotation of the progenitor star is plausible but has so far been neglected.
Aims: The aim of this work is to explore the influence of rapid rotation on the
deflagration scenario. Methods: We use three dimensional hydrodynamical
simulations to model turbulent deflagrations ignited within a variety of
rapidly rotating CO WDs obeying rotation laws suggested by accretion studies.
Results: We find that rotation has a significant impact on the explosion. The
flame develops a strong anisotropy with a preferred direction towards the
stellar poles, leaving great amounts of unburnt matter along the equatorial
plane. Conclusions: The large amount of unburnt matter is contrary to observed
spectral features of SNe Ia. Thus, rapid rotation of the progenitor star and
the deflagration scenario are incompatible in order to explain SNe Ia.Comment: 13 pages, 10 figures, accepted for publication by A&
Numerical dissipation and the bottleneck effect in simulations of compressible isotropic turbulence
The piece-wise parabolic method (PPM) is applied to simulations of forced
isotropic turbulence with Mach numbers . The equation of state
is dominated by the Fermi pressure of an electron-degenerate fluid. The
dissipation in these simulations is of purely numerical origin. For the
dimensionless mean rate of dissipation, we find values in agreement with known
results from mostly incompressible turbulence simulations. The calculation of a
Smagorinsky length corresponding to the rate of numerical dissipation supports
the notion of the PPM supplying an implicit subgrid scale model. In the
turbulence energy spectra of various flow realisations, we find the so-called
bottleneck phenomenon, i.e., a flattening of the spectrum function near the
wavenumber of maximal dissipation. The shape of the bottleneck peak in the
compensated spectrum functions is comparable to what is found in turbulence
simulations with hyperviscosity. Although the bottleneck effect reduces the
range of nearly inertial length scales considerably, we are able to estimate
the value of the Kolmogorov constant. For steady turbulence with a balance
between energy injection and dissipation, it appears that .
However, a smaller value is found in the case of transonic turbulence with a
large fraction of compressive components in the driving force. Moreover, we
discuss length scales related to the dissipation, in particular, an effective
numerical length scale , which can be regarded as the
characteristic smoothing length of the implicit filter associated with the PPM.Comment: 23 pages, 7 figures. Revised version accepted by Comp. Fluids. Not
all figures included due to size restriction. Complete PDF available at
http://www.astro.uni-wuerzburg.de/%7Eschmidt/Paper/NumDiss_CF.pd
A Study of the Structure of the Transition Metal Hydryls Final Report
Crystal and molecular structures of transition rhodium and lithium hydril
Thermonuclear explosions of rapidly rotating white dwarfs - II. Detonations
Context: Superluminous type Ia supernovae (SNe Ia) may be explained by
super-Chandrasekhar-mass explosions of rapidly rotating white dwarfs (WDs). In
a preceding paper, we showed that the deflagration scenario applied to rapidly
rotating WDs generates explosions that cannot explain the majority of SNe Ia.
Aims: Rotation of the progenitor star allows super-Chandrasekhar-mass WDs to
form that have a shallower density stratification. We use simple estimates of
the production of intermediate and iron group elements in pure detonations of
rapidly rotating WDs to assess their viability in explaining rare SNe Ia.
Methods: We numerically construct WDs in hydrostatic equilibrium that rotate
according to a variety of rotation laws. The explosion products are estimated
by considering the density stratification and by evaluating the result of
hydrodynamics simulations. Results: We show that a significant amount of
intermediate mass elements is produced for theoretically motivated rotation
laws, even for prompt detonations of WDs. Conclusions: Rapidly rotating WDs
that detonate may provide an explanation of rare superluminous SNe Ia in terms
of both burning species and explosion kinematics.Comment: 7 pages, 5 figures, accepted for publication by A&
Measurement of spark probability of GEM detector for CBM muon chamber (MUCH)
The stability of triple GEM detector setups in an environment of high
energetic showers is studied. To this end the spark probability in a shower
environment is compared to the spark probability in a pion beam.Comment: 5 pages, 10 figure
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