arXiv subjects

Alfred Gautschy

Publications and source records attributed to Alfred Gautschy.

16 recordsLinked to original sources

arXiv2025

\`A Propos Strange Pulsations of Blue Massive Stars

The properties of radial nonlinear pulsations of massive blue stars are computed with the MESA software instrument in its dynamical mode. Pulsational instabilities could be computationally detected and followed if the evolutionary timestep was reduced to a fraction of the unfolding pulsation period. Stellar variability was recovered in regions on the HR plane that have been studied before and that are known to host LBVs and relatives. Mode properties are analyzed on the full stellar-evolution models, which are not in thermal equilibrium. Despite persistent numerical shortcomings, it appears possible to compute strange-mode - like pulsations of massive blue stars with MESA.

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arXiv2023

\`A Propos Nonlinear Pulsations of R CrB Variables

Helium-star models were dynamically evolved into the region of the HR Diagram where R CrB variables are found. The MESA stellar evolution code was able to pick up pulsational instabilities with cycle lengths that are compatible with the periods observed in pulsating R CrB variables. The properties of the computed pulsations hint at their being strange modes.

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arXiv2023

On an Early - Post-AGB Instability

Dynamical stellar-evolution modeling through the AGB phase reveals that radial pulsations with very fast-growing amplitudes develop if the luminosity to mass ratio of stars with tenuous envelopes exceeds a critical limit. An instability going nonlinear already after a few pulsation cycles might qualify as a source of the superwind - postulated to shed a substantial part of a star's envelope over a very short time - of hitherto persistently mysterious nature.

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arXiv2022

A Propos Crossing the Hertzsprung Gap

The evolution of intermediate-mass and massive stars speeds up considerably after they finish their hydrogen core-burning. Due to this accelerated evolution, the probability to observe stars during this episode is small. In suitable stellar aggregates, in particular star clusters of appropriate ages, the fast evolutionary phase causes a depopulated area~--~referred to as the Hertzsprung gap~--~in color-magnitude diagrams and derivatives therefrom. The explanation of the speed-up usually resorts to the star's Kelvin-Helmholtz timescale and the Sch\"onberg-Chandrasekhar instability is called upon. This exposition challenges this viewpoint with counterexamples and argues that a suitably defined nuclear timescale is enough to explain the fast evolution. A thermal instability, even though it develops in stars evolving through the Hertzsprung gap, is not a necessary condition to trigger the phenomenon.

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arXiv2019

Overtone Pulsations Revisited

The shape of light cures of fundamental-mode and of first-overtone pulsators, as observed in RR~Lyrae variables and Cepheids, differ characteristically. The stellar physical origin of the morphological differences is not well documented and the topic seems not to be part of the elementary curriculum of students of stellar variability. To ameliorate the situation, this exposition analyzes hydrodynamical simulations of radial pulsations computed with the newly available numerical instrument RSP in MESA. The stellar physical processes that affect the light curves are identified and contrasted to the explanation based on experiments with one-zone models. The encounter of first-overtone pulsators sporting light curves that mimic those of fundamental-mode variables serves as a warning that light-curve morphology alone is not a reliable path to correctly classify pulsating variables; the exposition closes with a short discussion of constraints these modes might impose on understanding the origin of anomalous Cepheids.

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arXiv2018

The theorem that was none - II. The profound 70s

This second pedagogical installment on the history of the Vogt-Russell theorem focuses on research in the 1970s on the existence and uniqueness of star models. An appendix presents stellar evolution models, computed with the MESA code, in different approximations to emphasize points made in the main text.

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arXiv2017

On Binary Channels to Anomalous Cepheids

Anomalous Cepheids are a rather rare family of pulsating variables preferably found in dwarf galaxies. Attempts to model these variable stars via single-star evolution scenarios still leave space for improvements to better grasp their origin. Focusing on the LMC with its rich population of Anomalous Cepheids to compare against we probe the role binary stars might play to understand the nature of Anomalous Cepheids. The evolution of donors and accretors undergoing Case-B mass transfer along the first red-giant branch as well as merger-like models were calculated. First results show that in binary scenarios a larger range of star masses and metallicities up to Z < 0.008, higher than deemed possible hitherto, enter and pass through the instability strip. If binary stars play a role in Anomalous Cepheid populations, mass donors, mass accretors, or even mergers are potential candidates to counteract constraints imposed by the single-star approach.

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arXiv2015

The theorem that was none - I. Early history

The early history of the Vogt-Russell theorem is retraced following its route starting at the realization of a correlation between mass and luminosity of binary and pulsating stars, through the embossing of this observation into a theorem, and finally to the emerging first signs of its failure to serve as a theorem in the strict mathematical sense of the word.

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arXiv2013

The Thermal Pulses of Very-Low-Mass Stars

Very-low-mass stars can develop secularly unstable hydrogen-burning shells late in their life. Since the thermal pulses that go along are driven at the bottoms of very shallow envelopes, the stars' luminosities and effective temperatures react strongly during a pulse cycle. Towards the end of the Galaxy's stelliferous era, the hydrogen-shell flashing very-low-mass single stars should inflict an intricate light-show performed by the large population of previously inconspicuous dim stars. Unfortunately, this natural spectacle will discharge too late for mankind to indulge in. Not all is hopeless, though: In the case of close binary-star evolution, hydrogen-shell flashes of mass-stripped, very-low mass binary components can develop in a fraction of a Hubble time. Therefore, the Galaxy should be able put forth a few candidates that are going to evolve through a H-shell flash in a humanity-compatible time frame.

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arXiv2012

Helium Ignition in the Cores of Low-Mass Stars

In stars with $M_\ast \lesssim 2 M_\odot$, nuclear burning of helium starts under degenerate conditions and, depending on the efficiency of neutrino cooling, more or less off-center. The behavior of the centers of low-mass stars undergoing core helium ignition on the $\logρ- \log T$ plane is not thoroughly explained in the textbooks on stellar evolution and the appropriate discussions remain scattered throughout the primary research literature. Therefore, in the following exposition we collect the available knowledge, we make use of computational data obtained with the open-source star-modeling package MESA, and we compare them with the results in the existing literature. The line of presentation follows essentially that of Thomas (1967) who was the first who outlined correctly the stellar behavior during the off-center helium flashes that lead to central helium burning. The exposition does not contain novel research results; it is intended to be a pedagogically oriented, edifying compilation of pertinent physical aspects which help to \emph{understand} the nature of the stars.

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arXiv1999

Any Recent Progress in the Theory of Pulsating Stars?

To answer the topical question we survey synoptically the recent literature in pulsation theory. We restrict the topics to research on roAp stars, EC 14026 variables, strange modes, luminous blue variables, pulsation-rotation coupling, and pulsations in compact objects seen from the classical, as well as the relativistic viewpoint.

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arXiv1999

An Attempt to Pin Down the Instability Domain of Long-Period Variables

The period-luminosity relation of Miras and semiregular variables in the Large Magellanic Cloud and in the Galaxy is used to locate their instability domain in the Hertzsprung-Russell diagram. We take advantage of the considerable width in luminosity of the relation at given period to assign masses to the observed long-period variables using stellar models on the the asymptotic giant branch and nonadiabatic pulsation computations. We study the sensitivity to chemical abundance of the position of the instability region on the Hertzsprung-Russell diagram. The mass function of the long-period variables along the AGB is discussed for Galactic and LMC variables. Finally, we contribute to the dispute on the pulsation mode of Miras and lend support to the view that, for most of the Mira variables, the pulsation in the fundamental mode is more likely.

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arXiv1999

On the Robustness of the Cepheids' PL Relation

The Cepheids' P-L relation got itself talked about again in the recent past due to claims of observed metallicity dependences of at least its zero point. The divergences of the observational results obtained by the different authors are large, however. Therefore, to date the situation concerning the magnitude and the reliability of the effect is rather uncertain on the observational side. Along the theoretical avenue, some progress towards more realistic and in particular more consistent Cepheid modeling has been achieved recently. Upper limits on the reaction of the P-L relation on assumptions in evolution computations, pulsation stability analyses, and mappings onto photometric passbands can be estimated now quite reliably. This informal review discusses some of the progress (relying mostly on an extensive comparison by Sandage et al. 1998) and it emphasizes modeling problems which still have to be overcome to finalize the theoretical foundation of the Cepheids' P-L relation.

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arXiv1998

How to drive roAp stars

Rapidly oscillating Ap stars consitute a unique class of pulsators to study nonradial oscillations under some - even for stars - unusual physical conditions. These stars are chemically peculiar, they have strong magnetic fields, and they often pulsate in several high-order acoustic modes simultaneously. We discuss here an excitation mechanism for short-period oscillation modes based on the classical kappa mechanism. We particularly stress the conditions that must be fulfilled for successful driving. Specifically, we discuss the roles of chemical peculiarity and strong magnetic field on the oscillation modes and what separates these pulsators from delta Scuti and Am-type stars.

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arXiv1996

A fresh look into pulsating PG1159 stars

To improve the understanding of the pulsational instabilities of some of the PG1159-type pre-white dwarfs we performed detailed stability analyses on helium star evolutionary models. With the canonical chemical compositions, pulsational instabilities were encountered for all stellar masses considered (0.57, 0.63, 0.7 M_sol). The blue boundary agrees satisfactorily with observations. The red edge, on the other hand, is found to extend lower temperatures than presently admitted by observers. The dependence of the pulsational instabilities on helium or even hydrogen abundance in the stellar envelopes is less stringent than emphasized hitherto. In particular, we found pulsationally unstable g-modes for model stellar envelopes which are representative for HS2324+3944, a newly discovered PG1159-star with a hydrogen abundance X of approximately 0.2 and being pulsating.

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arXiv1995

Overtures to the pulsational instability of ZZ Ceti variables

Results of nonradial, nonadiabatic pulsation calculations on hydrogen-rich white dwarf models are presented. In contrast to earlier attempts, the modeling builds on hydrodynamically simulated convective surface layers supplemented with standard interior models. Based on our stellar models and despite of various simple attempts to couple convection and pulsation we could not reproduce theoretically the presently adopted location of the observed blue edge of the ZZ Ceti variables. When the convective efficiency is high enough we found a sensitive dependence of the stability properties of the g-modes on the pulsational treatment of shear within the convection zone.

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