6,879 research outputs found
Evolution, Explosion and Nucleosynthesis of Core Collapse Supernovae
We present a new set of presupernova evolutions and explosive yields of
massive stars of initial solar composition (Y=0.285, Z=0.02) in the mass range
13-35 Msun. All the models have been computed with the latest version (4.97) of
the FRANEC code that now includes a nuclear network extending from neutrons to
Mo98. The explosive nucleosynthesis has been computed twice: a first one with
an hydro code and a second one following the simpler radiation dominated shock
approximation (RDA).Comment: 20 pages, 10 figures, 12 tables. Accepted for publication on Ap
Rotational Mixing in Magellanic Clouds B Stars - Theory versus Observation
We have used VLT FLAMES data to constrain the uncertain physics of rotational
mixing in stellar evolution models. We have simulated a population of single
stars and find two groups of observed stars that cannot be explained: (1) a
group of fast rotating stars which do not show evidence for rotational mixing
and (2) a group of slow rotators with strong N enrichment. Binary effects and
fossil magnetic fields may be considered to explain those two groups. We
suggest that the element boron could be used to distinguish between rotational
mixing and the binary scenario. Our single star population simulations quantify
the expected amount of boron in fast and slow rotators and allow a comparison
with measured nitrogen and boron abundances in B-stars.Comment: to appear in Comm. in Astroseismology - Contribution to the
Proceedings of the 38th LIAC, 200
The nature of B supergiants: clues from a steep drop in rotation rates at 22000 K. The possibility of Bi-stability braking
The location of B supergiants in the Hertzsprung-Russell diagram (HRD)
represents a long-standing problem in massive star evolution. Here we propose
their nature may be revealed utilising their rotational properties, and we
highlight a steep drop in massive star rotation rates at an effective
temperature of 22000 K. We discuss two potential explanations for it. On the
one hand, the feature might be due to the end of the main sequence, which could
potentially constrain the core overshooting parameter. On the other hand, the
feature might be the result of enhanced mass loss at the predicted location of
the bi-stability jump. We term this effect "bi-stability breaking" and discuss
its potential consequences for the evolution of massive stars.Comment: Accepted by A&A Letters (4 pages, 5 figures); typos correcte
On the origin of microturbulence in hot stars
We present results from the first extensive study of convection zones in the
envelopes of hot massive stars, which are caused by opacity peaks associated
with iron and helium ionization. These convective regions can be located very
close to the stellar surface. Recent observations of microturbulence in massive
stars from the VLT-Flames survey are in good agreement with our predictions
concerning the occurrence and the strength of sub-surface convection in hot
stars. We argue further that convection close to the surface may trigger
clumping at the base of the stellar wind of massive stars.Comment: to appear in Comm. in Astroseismology - Proceedings of the 38th
LIAC/HELAS-ESTA/BAG, 200
Magnetic field control of photon echo in the electron-trion system: Shuffling of coherences between optically accessible and inaccessible states
We report on magnetic field induced oscillations of the photon echo signal
from negatively charged excitons in a CdTe/(Cd,Mg)Te semiconductor quantum
well. The oscillatory signal is due to Larmor precession of the electron spin
about a transverse magnetic field and depends sensitively on the polarization
configuration of the exciting and refocusing pulses. The echo amplitude can be
fully tuned from maximum down to zero depending on the time delay between the
two pulses and the magnetic field strength. The results are explained in terms
of the optical Bloch equations accounting for the spin level structure of
electron and trion.Comment: 8 pages, 2 figure
Effective Temperatures of a Driven System Near Jamming
Fluctuations in a model of a sheared, zero-temperature foam are studied
numerically. Five different quantities that reduce to the true temperature in
an equilibrium thermal system are calculated. All five have the same shear-rate
dependence, and three have the same value. Near the onset of jamming, the
relaxation time is the same function of these three temperatures in the sheared
system as of the true temperature in an unsheared system. These results imply
that statistical mechanics is useful for the system and provide strong support
for the concept of jamming.Comment: 4 pages, 4 postscript figure
Quantum and Thermal Depinning of a String from a Linear Defect
The problem of a massive elastic string depinning from a linear defect under
the action of a small driving force is considered. To exponential accuracy the
decay rate is calculated with the help of the instanton method; then,
fluctuations of the quasiclassical solution are taken into account to determine
the preexponential factor. The decay rate exhibits a kind of first order
transition from quantum tunneling to thermal activation with vanishing
crossover region. The model may be applied to describe nucleation in
2-dimensional first order quantum phase transitions.Comment: Revtex. 11 pages + 4 PS figures. Accepted for publication in PR
Steady-State Cracks in Viscoelastic Lattice Models
We study the steady-state motion of mode III cracks propagating on a lattice
exhibiting viscoelastic dynamics. The introduction of a Kelvin viscosity
allows for a direct comparison between lattice results and continuum
treatments. Utilizing both numerical and analytical (Wiener-Hopf) techniques,
we explore this comparison as a function of the driving displacement
and the number of transverse sites . At any , the continuum theory misses
the lattice-trapping phenomenon; this is well-known, but the introduction of
introduces some new twists. More importantly, for large even at
large , the standard two-dimensional elastodynamics approach completely
misses the -dependent velocity selection, as this selection disappears
completely in the leading order naive continuum limit of the lattice problem.Comment: 27 pages, 8 figure
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