Recent Progress and New Challenges in Isospin Physics with Heavy-Ion Reactions

A new, comprehensive set of low-temperature opacity data has been assembled. From this basic data set, Rosseland and Planck mean opacities have been computed for temperatures between 12,500 and 700 K. In addition to the usual continuous absorbers, atomic line absorption (with more than 8 million lines), molecular line absorption (with nearly 60 million lines), and grain absorption and scattering (by silicates, iron, carbon, and SiC) have been accounted for. The absorption due to lines is computed monochromatically and included in the mean with the opacity sampling technique. Grains are assumed to form in chemical equilibrium with the gas and to form into a continuous distribution of ellipsoids. Agreement of these opacities with other recent tabulations of opacities for temperatures above 5000 K is excellent. It is shown that opacities which neglect molecules become unreliable for temperatures below 5000 K. Triatomic molecules become important absorbers at 3200 K. Similarly, grains must be included in the computation for temperatures below 1700 K.

Low Temperature Rosseland Opacities.

▪ Abstract The information on the chemical compositions of stars encoded in their spectra plays a central role in contemporary astrophysics. Stellar element abundances are, however, not observed: to decipher the spectral fingerprints in terms of abundances requires realistic models for the stellar atmospheres and the line-formation processes. Still today, the vast majority of abundance analyses of late-type stars rely on one-dimensional (1D), hydrostatic model atmospheres and the assumption of local thermodynamic equilibrium (LTE). In this review possible systematic errors in studies of F-, G- and K-type stars introduced by these questionable approximations are discussed. Departures from LTE are commonplace and often quite severe, in particular for low surface gravities or metallicities, with minority species and low-excitation transitions being the most vulnerable. Recently, time-dependent, 3D, hydrodynamical model atmospheres have started to be employed for stellar abundance purposes, with large differe...

New light on stellar abundance analyses: Departures from LTE and homogeneity.

https://escholarship.org/content/qt7511z1pj/qt7511z1pj.pdf?t=p0uvj3

Quantum theory of nonequilibrium processes, I

We update the capabilities of the open-knowledge software instrument Modules for Experiments in Stellar Astrophysics (MESA). RSP is a new functionality in MESAstar that models the nonlinear radial stellar pulsations that characterize RR Lyrae, Cepheids, and other classes of variable stars. We significantly enhance numerical energy conservation capabilities, including during mass changes. For example, this enables calculations through the He flash that conserve energy to better than 0.001%. To improve the modeling of rotating stars in MESA, we introduce a new approach to modifying the pressure and temperature equations of stellar structure, as well as a formulation of the projection effects of gravity darkening. A new scheme for tracking convective boundaries yields reliable values of the convective core mass and allows the natural emergence of adiabatic semiconvection regions during both core hydrogen- and helium-burning phases. We quantify the parallel performance of MESA on current-generation multicore architectures and demonstrate improvements in the computational efficiency of radiative levitation. We report updates to the equation of state and nuclear reaction physics modules. We briefly discuss the current treatment of fallback in core-collapse supernova models and the thermodynamic evolution of supernova explosions. We close by discussing the new MESA Testhub software infrastructure to enhance source code development.

https://iopscience.iop.org/article/10.3847/1538-4365/ab2241/pdf

Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation

An iterative scheme has been developed for the solution of the non-LTE line radiation transfer problem. The method uses an approximate operator that is deliberately chosen to be local so that it can be easily extended to multidimensional geometry. The difference between the formal and approximate solutions is used as a driving term for the iterations. In one-dimensional, semiinfinite and free-standing slabs, the technique is found to be very fast, robust, and applicable to a large class of problems.

A rapidly convergent iterative solution of the non-LTE line radiation transfer problem

The Blazhko effect is a long-term, generally irregular modulation of the light curves that occurs in a sizeable number of RR Lyrae stars. The physical origin of the effect has been a puzzle ever since its discovery over a hundred years ago. We build here upon the recent observational and theoretical work of Szabo et al. on RRab stars who found with hydrodynamical simulations that the fundamental pulsation mode can get destabilized by a 9:2 resonant interaction with the 9th overtone. Alternating pulsation cycles arise, although these remain periodic, i.e., not modulated as in the observations. Here we use the amplitude equation formalism to study this nonlinear, resonant interaction between the two modes. We show that not only does the fundamental pulsation mode break up into a period-two cycle through the nonlinear, resonant interaction with the overtone, but that the amplitudes are modulated, and that in a broad range of parameters the modulations are irregular as in the observations. This irregular behavior is in fact chaotic and arises from a strange attractor in the dynamics.

https://iopscience.iop.org/article/10.1088/0004-637X/731/1/24/pdf

On the Blazhko Effect in RR Lyrae Stars

Etude hydrodynamique numerique d'une sequence de modeles, classes comme W Vir, RV Tau ou semi-regulieres

Period Doubling Bifurcations and Chaos in W Virginis Models

Le comportement pulsationnel non lineaire de plusieurs sequences de modeles de Cepheides est calcule avec un code hydrodynamique numerique. Ces sequences revelent des doublements de periode lorsque la temperature effective est changee. Les proprietes de base de la bifurcation du doublement de periode sont examinees dans le formalisme des equations d'amplitude. Un modele simple de 2 oscillateurs couples non lineaires qui simule le comportement des modeles stellaires compliques est egalement analyse.

Resonances and period doubling in the pulsations of stellar models

Les proprietes pulsationnelles lineaires et non lineaires des etoiles variables de type W Vir, RV Tau et semi-regulieres sont etudiees au travers de modeles stellaires radiatifs

J. Robert Buchler

Papers

A rapidly convergent iterative solution of the non-LTE line radiation transfer problem

On the Blazhko Effect in RR Lyrae Stars

Period Doubling Bifurcations and Chaos in W Virginis Models

Resonances and period doubling in the pulsations of stellar models

Regular and Irregular Nonlinear Pulsations in Population II Cepheid Models