Stellar pulsation and the RV Tauri phenomenon (original) (raw)

Progress and problems in massive star pulsation theory

Proceedings of the International Astronomical Union, 2015

Massive stars pulsate in various modes; radial and nonradial p-modes, g-modes, and strange modes including oscillatory convective (non-adiabatic g−) modes. Those modes are responsible for the light and velocity variations of β Cephei stars, slowly pulsating B (SPB) stars, and α Cyg variables. The instability mechanisms for these pulsations are discussed. We also discuss the relation between the evolution of massive stars and the excitation of strange modes, which are considered responsible for the pulsation in most of the α Cyg variables. The surface He and CNO abundances of hotter α Cyg variables seem to indicate that the Ledoux criterion of convection is better than the Schwarzschild criterion, although the latter is extensively used in stellar evolution computations.

Pulsation in Intermediate-Mass Stars

Frontiers in Astronomy and Space Sciences

A new perspective of pulsation in stars within the δ Scuti instability strip has recently emerged as a result of Kepler observations. The majority of stars within the instability strip do not pulsate and practically all δ Scuti stars contain low frequencies. Because γ Doradus stars co-exist with δ Sct stars in the same region of the instability strip, it follows that γ Dor stars are driven by the same mechanism as δ Sct stars. The difference must be due to different mode selection processes. The search for an unknown damping factor which is missing from the models will be essential for further progress. Maia variables and hot γ Dor stars are briefly discussed. Luminosities of roAp stars obtained from Gaia DR2 parallaxes and spectroscopic effective temperatures show that the roAp stars are slightly evolved with temperatures in the range 6,300-8,300 K, considerably cooler than predicted by the models. The roAp stars and stars with solar-like oscillations share the same mass-temperature-luminosity relation, but with frequencies which are about 50 percent higher. This suggests that roAp frequencies are determined by the critical acoustic frequency, but this frequency is larger than in standard models, perhaps as a result of a temperature inversion in the atmosphere.

An overview of stellar pulsation theory

Astrophysics and Space Science, 1993

In this paper I will give overview of the stellar pulsation theory. Starting with basic equations I will discuss modal properties of oscillations and excitation mechanisms. I also mention briefly the effects of rotation.

New Links Between Pulsation and Stellar History

Astrophysics and Space Science Proceedings, 2012

Cepheids, particularly about their formation and history. Three approaches are discussed, using space (Hubble and Chandra) and ground-based studies (radial velocities). First, we are conducting a survey of Cepheids with the Hubble Space Telescope Wide Field Camera 3 (WFC3) to identify possible resolved companions (for example Eta Aql) and thus provide constraints on star formation. Followup Xray observations (Chandra and XMM-Newton) can confirm whether possible low mass companions are young enough to be physical companions of Cepheids. In a related study of intermediate mass stars, Chandra X-ray observations of late B stars in Tr 16 have been used to determine the fraction which have X-ray active low mass companions. Finally, the Tennessee State Automatic Spectroscopic Telescope AST and the Moscow University group have obtained velocities of a number of Cepheids. As an example, the orbit of V350 Sgr has been redetermined, providing a new level of accuracy to the orbital velocity amplitude, which is needed for mass determination.

STELLAR PULSATIONS ACROSS THE HR DIAGRAM: Part II

Annual Review of Astronomy and Astrophysics, 1996

▪ Stars over essentially the whole mass domain can become pulsationally unstable during various stages of their evolution. They will appear as variable stars with characteristics that are of much diagnostic value to astronomers. The analysis of such observations provides a challenging and unique approach to study aspects of the internal constitution and evolutionary status of these objects that are not accessible otherwise. This review touches on most classes of known pulsating variable stars and tries to elucidate connections to stellar physical aspects. To aid future investigations, we stress questions and problems that we believe are yet to be resolved satisfactorily.

Photometric Behavior of Five Long-Period Pulsating Stars

arXiv (Cornell University), 2012

The variation of average brightness during the time and the variation of the amplitude of the Mira-type stars T Cep, X CrB, U UMi, U Cyg, BG Cyg is studied. For the research, the observations of the members of the French association of observers of variable stars (AFOEV) covering almost 100 years are used. All stars show cyclic variations of the specified parameters. For T Cep, U UMi and U Cyg, the values of quasi-period of the variations of average brightness are about 1400, 1025 and 1680 days, respectively. For the stars T Cep, U UMi and BG Cyg, the period of the brightness variations changes, but we used a mean one. The dependences «average brightness-JD» and «amplitude-JD» are plotted. Results are discussed.

A- and B-type star pulsations in the Kepler and CoRoT era: theoretical considerations

Cornell University - arXiv, 2013

Among A-type main-sequence variables, pulsations of δ Sct and γ Dor variables are driven in the He II ionization zone, while H-ionization zone and strong magnetic fields seem to play roles in the excitation of high-order pmodes in rapidly oscillating Ap (roAp) stars. Pulsations in B-type variables, β Cephei and slowly pulsating B (SPB) stars are excited by the κ-mechanism at the Fe-opacity bump at T ≈ 2 × 10 5 K. In addition, the strange-mode instability seems responsible for the excitation of pulsations in luminous AB-supergiants (α Cygni variables). We discuss excitation mechanisms for pulsations in A-and B-type variables stars.

Nonlinear model pulsations for long-period Cepheids. I. Galactic Cepheids

Astronomy and Astrophysics, 2000

Nonlinear pulsation models for long-period Cepheids in Galaxy were constructed and their theoretical light and velocity curves were compared with observations. Two different mass-luminosity (M − L) relations were assumed, one for canonical evolutionary models and the other for models with overshooting. A model sequence was constructed by varying the masses and correspondingly the luminosities for both relations. The values of the effective temperatures T e of the models were assumed to be 200 K smaller than those of theoretical blue edge for the same masses and luminosities. Each sequence consisted of about 50 models with pulsation periods from about 10 to 100 days. Nonlinear hydrodynamic simulations were performed to get limit cycles. When nonlinear pulsation settled into limit cycles, light and velocity curves were Fourier decomposed and compared with observational results. It is concluded that the models with the overshooting-type are globally in better agreement with observations than those with the standard relation, while there are discrepancies for higher order Fourier components in both cases. Two additional model sequences were constructed by changing the value of artificial viscosity coefficients and of T e. Decreasing the artificial viscosity can produce a slightly better agreement between models and observations for the higher order Fourier components, while the effects of different T e on the same components are small. The stability of the limit cycles are briefly discussed along with the lack of indications of modal resonance phenomena and the possible importance of the degree of nonadiabaticity.

Pulsational frequencies in theδScuti stars V624 Tauri and HD 23194

Astronomy & Astrophysics, 2002

The results of the tenth multi-site campaign of the STEPHI network are reported. The δ Scuti stars V624 Tau (HD 23156) and HD 23194, belonging to the Pleiades cluster, were observed photometrically for 34 days on three continents during 1999 November-December. An overall run of 343 hours of data was collected. Seven frequencies for V624 Tau and two frequencies for HD 23194 have been found above a 99% confidence level. These results greatly improve those found in previous studies with much less data. A preliminary comparison of observed and theoretical frequencies suggests that both stars may oscillate with radial and non-radial p modes of radial orders typical among δ Scuti stars.