Alfred Gautschy

Theological Seminary Bienenberg

Author Statistics

Papers

Citation

H-Index

i-10 index

Research Trends

Author Order

Document Type

Co-Authors

Hideyuki Saio

Tohoku University

E. A. Dorfi

University of Vienna

W. Glatzel

University of Göttingen

L. G. Althaus

Instituto de Astrofísica de La Plata

Aaron Dotter

Dartmouth College

R. Farmer

The Netherlands Cancer Institute

H.‐G. Ludwig

Heidelberg University

Bill Paxton

University of California, Santa Barbara

Anne Thoul

University of Liège

Michael Zhang

University of Chicago

Cooperative Institutions

University of Vienna

University of California, Santa Barbara

Center for Astrophysics Harvard & Smithsonian

Max Planck Institute for Astrophysics

Max Planck Society

Saint Mary's University

KU Leuven

Kavli Institute for Particle Astrophysics and Cosmology

University of British Columbia

University of Victoria

Author Statistics

Papers

Citation

H-Index

i-10 index

Research Field

Nonlinear Pulsations of Luminous Blue Variables

International Astronomical Union Colloquium (2000)

Ernst A. Dorfl M. U. Feuchtinger Alfred Gautschy

Abstract We present the results of various nonlinear radial pulsation calculations of LBVs with different luminosity to mass ratios. Depending on the stellar parameters, most of these objects undergo strongly non-adiabatic pulsations which can also modify their stellar atmospheres. In some cases part of the kinetic energy of the motions is transferred by shock waves into the atmospheric layers leading to an overall decrease or increase of the stellar radius. This results in different mean density accompanied by a change of the pulsational behavior which can be compared to the results of a linear stability analysis. We also find that regular pulsations can occur around a different equilibrium than the initial configuration which has been determined by stellar evolution calculations. In cases of even higher luminosity to mass ratios the linear stability analysis predicts several unstable modes and the nonlinear computations can exhibit rather irregular light curves. Due to the large non-adiabaticity of the pulsation the corresponding velocity curves still remain more regular, but show increasing amplitudes until the energy stored is released. The pulsations are far from equilibrium conditions and may also lead to a pulsationally driven mass loss for certain stellar parameters.

Stellar pulsation

10.1017/s0252921100057262

Cite

Citations (1)

Grasping at the hot end of ZZ Ceti variability

White Dwarfs (2008)

Alfred Gautschy H.‐G. Ludwig

10.1007/3-540-59157-5_234

Cite

Citations (0)

Soft X-ray Spectroscopy of the Hot DA White Dwarf LB 1919 and the PG 1159 Star PG 1520+525

K. Werner J. J. Drake T. Rauch Sonja Schuh Alfred Gautschy

Star (game theory)

White (mutation)

Source

Cite

Citations (0)

The theorem that was none - I. Early history

arXiv (Cornell University) (2015)

Alfred Gautschy

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.

Realization (probability)

10.48550/arxiv.1504.08188

Cite

Citations (0)

Deneb's variability: a hint of a deep-lying convection zone?

Astronomy and Astrophysics (2009)

Alfred Gautschy

During their post – main-sequence evolution, massive star models harbor a convection zone on top of their hydrogen burning shell. This physical property helps, under suitable circumstances, to destabilize low-degree nonradial modes that might explain the cyclic light variability of α Cygni variables. The results of first exploratory computations demonstrate that modes with periods ranging from about one dozen to several dozen days can be overstable in star models at about the position on the HR diagram where the prototype of the class, Deneb, is observed.

Dozen

Hertzsprung–Russell diagram

Star (game theory)

10.3929/ethz-a-005793625

Cite

Citations (1)

Most Detects G- and P-Modes in the B Supergiant HD 163899 (B2Ib/II)

arXiv (Cornell University) (2006)

Hideyuki Saio R. Kuschnig Alfred Gautschy C. Cameron G. A. H. Walker

The {\it Microvariability and Oscillations of Stars (MOST)} satellite observed the B supergiant HD 163899 (B2 Ib/II) for 37 days as a guide star and detected 48 frequencies $\la$ 2.8 c d$^{-1}$ with amplitudes of a few milli-magnitudes (mmag) and less. The frequency range embraces g- and p-mode pulsations. It was generally thought that no g-modes are excited in less luminous B supergiants because strong radiative damping is expected in the core. Our theoretical models, however, show that such g-modes are excited in massive post-main-sequence stars, in accordance with these observations. The nonradial pulsations excited in models between $20M_\odot$ at $\log T_{\rm eff} \approx 4.41$ and $15M_\odot$ at $\log T_{\rm eff} \approx 4.36$ are roughly consistent with the observed frequency range. Excitation by the Fe-bump in opacity is possible because g-modes can be partially reflected at a convective zone associated with the hydrogen-burning shell, which significantly reduces radiative damping in the core. The {\it MOST} light curve of HD 163899 shows that such a reflection of g-modes actually occurs, and reveals the existence of a previously unrecognized type of variable, slowly pulsating B supergiants (SPBsg) distinct from $α$ Cyg variables. Such g-modes have great potential for asteroseismology.

Asteroseismology

Opacity

10.48550/arxiv.astro-ph/0606712

Cite

Citations (1)

The treatment of highly non-adiabatic, non-radial pulsations by application of the Riccati method to the example of hydrogen-deficient carbon stars

Monthly Notices of the Royal Astronomical Society (1992)

W. Glatzel Alfred Gautschy

The Riccati method, developed to solve the linear boundary-value problem of radial stellar pulsations, is generalized and shown to be applicable to the calculation of non-radial, non-adiabatic oscillations. Its advantages are demonstrated for the extreme example of a highly non-adiabatic helium star. The acoustic spectrum of helium stars is studied in detail. Mode-coupling instabilities occur in the non-radial spectrum for a wide range of the harmonic degree l, their appearance being similar to the radial case studied previously. With increasing l, the width of the instability strip in the HRD – extending up to |$\text {log}\enspace T_\text{eff} \approx 4.3 $| for low l – shrinks, and the growth rates of the instabilities decrease. Stabilization is reached between l = 10 and 100.

Star (game theory)

Asteroseismology

Radial velocity

Degree (music)

10.1093/mnras/256.2.209

Cite

Citations (24)

On the Robustness of the Cepheids' PL Relation

arXiv (Cornell University) (1999)

Alfred Gautschy

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.

Robustness

Source

Cite

Citations (0)

Soft X-ray Spectroscopy of the Hot DA White Dwarf LB1919 and the PG1159 Star PG1520+525

arXiv (Cornell University) (2006)

K. Werner J. J. Drake T. Rauch Sonja Schuh Alfred Gautschy

We have performed soft X-ray spectroscopy of two hot white dwarfs with the Chandra observatory using the Low Energy Transmission Grating. The first target is the hot DA white dwarf LB1919 (Teff=69000 K). This star is representative of a small group of hot DAs whose metallicities lie well below predictions from radiative levitation theory. The Chandra spectrum shows a rich absorption line spectrum which may allow to find the origin of the low-metallicity nature of these DAs. The second target is PG1520+525, a very hot non-pulsating PG1159 star. We find that it is hotter (Teff=150 000 K) than the pulsating prototype PG1159-035 (Teff=140 000) and conclude that both stars confine the blue edge of the GW Vir instability strip.

Instability strip

10.48550/arxiv.astro-ph/0610013

Cite

Citations (0)

OVERTURES TO THE PULSATIONAL INSTABILITY OF ZZ CETI VARIABLES

Astronomy and Astrophysics (1996)

Alfred Gautschy H.‐G. Ludwig B. Freytag

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.

Instability strip

Stellar pulsation

Convective instability

Source

Cite

Citations (3)