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 $\eta$ 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 $\Delta$ and the number of transverse sites $N$. At any $N$, the continuum theory misses the lattice-trapping phenomenon; this is well-known, but the introduction of $\eta$ introduces some new twists. More importantly, for large $N$ even at large $\Delta$, the standard two-dimensional elastodynamics approach completely misses the $\eta$-dependent velocity selection, as this selection disappears completely in the leading order naive continuum limit of the lattice problem.Comment: 27 pages, 8 figure