arXiv subjects

J. Robert Buchler

Publications and source records attributed to J. Robert Buchler.

30 records · Page 2Linked to original sources

arXiv2000

Double-Mode Stellar Pulsations

The status of the hydrodynamical modelling of nonlinear multi-mode stellar pulsations is discussed. The hydrodynamical modelling of steady double-mode (DM) pulsations has been a long-standing quest that is finally being concluded. Recent progress has been made thanks to the introduction of turbulent convection in the numerical hydrodynamical codes which provide detailed results for individual models. An overview of the modal selection problem in the HR diagram can be obtained in the form of bifurcation diagrams with the help of simple nonresonant amplitude equations that capture the DM phenomenon.

astro-ph↗

arXiv2000

Nonlinear Analysis of Irregular Variables

The Fourier spectral techniques that are common in Astronomy for analyzing periodic or multi-periodic light-curves lose their usefulness when they are applied to unsteady light-curves. We review some of the novel techniques that have been developed for analyzing irregular stellar light or radial velocity variations, and we describe what useful physical and astronomical information can be gained from their use.

astro-ph↗

arXiv1999

Nonlinear Pulsations of Convective Stellar Models

We review the numerical modelling of the nonlinear pulsations of classical variable stars with hydrocodes that include the effects of turbulent convection. Despite their simplicity these turbulent convective recipes appear to remove many of the difficulties that radiative codes faced. In particular, the numerical modelling of double mode pulsations has become possible.

astro-ph↗

arXiv1999

Turbulent Convection in the Classical Variable Stars

We give a status report of convective Cepheid and RR Lyrae model pulsations. Some striking successes can be reported, despite the use of a rather simple treatment of turbulent convection with a 1D time-dependent diffusion equation for the turbulent energy. It is now possible to obtain stable double-mode (beat) pulsations in both Cepheid and RR Lyrae models with astrophysical parameters, i.e. periods and amplitude ratios, that are in agreement with observations. The turbulent convective models, however, have difficulties giving global agreement with the observations. In particular, the Magellanic Cloud Cepheids, that have been observed in connection with the microlensing projects have imposed novel observational constraints because of the low metallicity of the MCs.

astro-ph↗

arXiv1999

Coherent Transport of Angular Momentum - The Ranque-Hilsch Tube as a Paradigm

The mechanism for efficient and coherent angular momentum transport remains one of the unsolved puzzles in astrophysics despite the enormous efforts that have been made. We suggest that important new insight could be gained in this problem through an experimental and theoretical study of a laboratory device (Ranque-Hilsch tube) that displays a similar enhanced angular momentum transfer which cannot be explained by a simple turbulent model. There is already good experimental evidence to suggest that the cause of this enhancement is the formation of aligned vortices that swirl around the symmetry axes very much like virtual paddle blades.

astro-ph↗

arXiv1999

Turbulent Convection in Pulsating Stars

We review recent results of stellar pulsation modelling that show that even very simple one-dimensional models for time dependent turbulent energy diffusion and convection provide a substantial improvement over purely radiative models.

astro-ph↗

arXiv1997

Stellar Arrhythmias

The light curves of the two large amplitude variable stars, R Scuti and AC Herculis, display irregular pulsation cycles with 'periods' of 75 and 35 days, respectively. We review the evidence that the observed time-series are generated by low dimensional chaotic dynamics. In particular, a global flow reconstruction technique indicates that the minimum embedding dimensions are 4 and 3 for R Sct and AC Her, respectively, whereas the fractal dimensions are inferred to be approximately 3.1 and 2.05 - 2.45, respectively. It thus appears that the dimensions of the dynamics themselves are also 4 and 3.

chao-dyn↗

arXiv1997

On the Deficiency of 8--10 Day Galactic Cepheids

The Galactic Cepheid period histogram has a strong dip between 8 and 10 days that has defied an explanation based on evolutionary and linear pulsation studies. We show here that this deficiency is caused by the instability of the nonlinear fundamental pulsation cycle in this period range. The strong metallicity dependence of this instability is consistent with the absence of a corresponding minimum in the Magellanic Cloud data. Our results also suggest that the Galactic Cepheids must have a large spread in metallicity.

astro-ph↗

arXiv1997

Nonlinear Pulsations

We review some of the recent advances in nonlinear pulsation theory, but also insist on some of the major extant shortcomings.

astro-ph↗

arXiv1997

An Adaptive Code For Radial Stellar Pulsations

We describe an implicit 1--D adaptive mesh hydrodynamics code that is specially tailored for radial stellar pulsations. In the Lagrangean limit the code reduces to the well tested Fraley scheme. The code has the useful feature that unwanted, long lasting transients can be avoided by smoothly switching on the adaptive mesh features starting from the Lagrangean code. Thus, a limit cycle pulsation that can readily be computed with the relaxation method of Stellingwerf will converge in a few tens of pulsation cycles when put into the adaptive mesh code. The code has been checked with two shock problems, viz Noh and Sedov, for which analytical solutions are known, and it has been found to be both accurate and stable. Superior results were obtained through the solution of the total energy (gravitational + kinetic + internal) equation rather than that of the internal energy only.

astro-ph↗

arXiv1997

Classical Cepheids -- a Review

We briefly review the evolutionary status of the classical Cepheid variables, their structure, and the properties of the linear modes. Then we discuss the current status of the nonlinear hydrodynamical modelling, including modern adaptive mesh techniques, and the sensitivity to opacity. We stress the importance of resonances among the normal modes of pulsation because they can be used to impose severe constraints on the models and on the input physics. We show that recent hydrodynamics computations provide an explanation for the hitherto unexplained dip in the histogram of Galactic Cepheids around 8--10 days. The EROS and MACHO observations of classical Cepheids in the Large and Small Magellanic Clouds have opened up a new exciting perspective on these variable stars. This new observational basis is very important, not only because of the sheer number of new Cepheids, but especially because the composition of the MCs is known to differ from our Galaxy's, thus extending the range of astrophysical parameters. These new data have been found not to be fully compatible with our current Cepheid modelling and are forcing a review of our assumptions and input physics.

astro-ph↗

arXiv1997

Search for Low-Dimensional Chaos in Observational Data

We introduce the reader to the "global flow reconstruction method". The purpose of the method is to see if a given temporal sequence has been generated by a low dimensional dynamics, and to determine that dimension and other properties of the dynamics. The method is shown to work very well on the well known Roessler system. An application of the method to the observational data of the light curve of the irregular variable star R Scuti is presented. This light curve is shown to have been generated by a 4 dimensional dynamics. From the functional form of the flow we find that the Lyapunov dimension is close to 3.1.

chao-dyn↗