What drives acoustic modes in magnetic, radiative stars, such as the rapidly oscillating Ap (roAp) stars? We propose that oscillations in the shape of the magnetic field can drive magnetosonic waves, which are observed as p modes. The force-free atmosphere minimizes magnetic energy. If the field is strong enough, the magnetic configuration can oscillate about its equilibrium state. Initial calculations showed that the wavenumbers, frequencies, and available energy are consistent with observations of roAp stars (Zita 1997). We present the results of new calculations and proposed tests of our model. We also discuss the resultant < υ × b > dynamo, which may shed light on observations in B stars.

So far, the effect of relativistic electrons has not been included in the Mihalas-Däppen-Hummer (Hummer & Mihalas 1988; Mihalas, Däppen, & Hummer 1988; Däppen et al. 1988; hereinafter MHD) equation of state, although degeneracy was taken into account. Following the findings about the detectability of the relativistic effect in helioseismological data of the solar center (Elliot & Kosovichev 1998; hereinafter EK98), we have upgraded the MHD equation of state to include relativistic degenerate electrons. Our numerical calculation confirms the result of EK98.

We consider the effect of the solar cycle on g-mode parametric resonance, and hence on the limiting amplitude of the overstable solar g1(ℓ = 1) mode. We find no change in the expected limiting amplitude from that found by Dziembowski (1983), who ignored cycle variations.

Detailed dynamical models and nucleosynthesis yields of very massive stars from early pre-MS through very late stages have been computed. The recent reduction of mass loss rates for the WR stages can have important consequences for both the evolution, surface composition and stability. Depending on mass and metallicity, in the present models instabilities occur during the accretion phase (pre-ZAMS), LBV stage and very late stages (WC).

We review the current status of our long-term monitoring project on slowly pulsating B stars that we started in the course of 1996 and that was recently completed as far as the first part of our plan is concerned. In total, we have selected 17 southern and 8 northern stars. The idea is to fully exploit our current data in the near future and to select the most interesting targets for further very-long-term follow-up monitoring. A first conclusion is that half of the southern targets turn out to be spectroscopic binaries. Some of these have circular orbits and periods of the same order of magnitude as the intrinsic pulsation period(s) of the primary. The eccentric binaries have periods ranging from 12 to 460 d. For most stars the photometric behaviour is dominated by the same frequency as the intrinsic spectroscopic variability. Multiperiodicity in the expected frequency range is found for almost all stars. Two objects, however, turn out to have only one dominant pulsation mode.

Our understanding of pulsations in β Cep, SPB and ζ Oph stars is discussed. We do not understand why there are no β Cep stars earlier than O9, though the models show that the instability strip extends to hotter stars. The main problem among the SPB stars is why rotation has a strong inhibiting effect on pulsation. The ζ Oph stars are an extension of the β Cep and SPB stars to higher spherical harmonic degrees. The pulsational stability of models of B stars to modes of high degree is discussed. Results are compared with two well observed ζ Oph stars. Although there is good agreement with ζ Oph itself, the periods of the high-degree modes in α Vir are incompatible with model predictions.

An overview of recent progress in the research on δ Scuti stars is given. Some intriguing results and open problems are pointed out, and some ideas for future investigations are provided.

Asteroseismology on δ Scuti stars has until now produced very few convincing results – if we aim at doing strong tests of details of stellar modelling. The main reason for the lack of success is probably that these stars often rotate, which split nonradial oscillation frequencies into many more frequencies. These many frequencies and the fact that the more evolved δ Scuti stars contain a strong chemical composition gradient at the edge of the convective core, produce a very complicated eigenfrequency spectrum. In contrast to this, we expect, in principle, seismological studies of δ Scuti stars to be a very simple task: One has to compare theoretical oscillations in model stars with the observed oscillations. However, in order to produce convincing asteroseismological results, we need to do three things: (1) Detect as many eigenfrequencies as possible at high precision, (2) identify the eigenmodes and (3) improve the theoretical models. By observing δ Scuti stars in open clusters using CCDs, we have a possibility to improve on (1) and (2) as well as providing an opening for an improvement in the theoretical models by doing accurate calibrations of the basic cluster properties. In this paper I shall describe some of the results from CCD studies of δ Scuti stars in open clusters and identify some future prospects for this technique.

The amplitude variability of δ Scuti stars is demonstrated by the behavior of the evolved δ Scuti star 4 CVn. The data on 4 CVn covering over 30 yr shows the amplitudes of almost all modes to be extremely variable with a time scales of ten years or longer. The most rapid decrease of amplitude in V is shown by the 7.38 d−1 mode of 4 CVn, which collapsed from 15 mmag in 1974 to 4 mmag in 1976, and 1 mmag in 1977. Between 1974–6 and 1977–8 phase jumps of almost half a cycle occurred. While a simple beating model of two close frequencies can be ruled out from the photometric data, the change is compatible with the growth of a new mode between 1976 and 1977.

In this contribution we describe a new class of pulsating stars, the prototype of which is the bright, early F-type dwarf, γ Doradus. These stars typically have between 1 and 5 periods ranging from 0.4 to 3 d with photometric amplitudes up to 0ṃ1 in Johnson V. The mechanism for these observed variations is high-order, low-degree, nonradial, gravity-mode pulsation. A recent theoretical development describing a proposed driving mechanism for these new variables is discussed.

For the purpose of observing β Cephei and SPB stars with optical telescopes on board future space missions, we have developed an automatic photometric search of stars falling inside the β Cephei and SPB instability strips. A list has been compiled which is available upon request from the authors ([email protected];[email protected]).

The boundaries of the instability strips have been defined by observational β – c0 HR diagrams based on Stræmgren photometry (Sterken & Jerzykiewicz 1993 = SK93). The ZAMS has been set as the lower limit of both strips. Low and high temperature limits are set as those given by SK93 and the upper limit is fixed by the photometry β of Cep stars found in the literature. For the SPB stars we have followed the same procedure. We have found the following boundaries: β Cephei: 0.2295c0 + 2.6263 > β > 0.0394c0 + 2.5656; 0.17 > c0 > –0.1; SPB: 2.630 > β > 0.2642co + 2.6207; 0.8 > c0 > 0.17.

β Cephei stars are early B-type stars which show periodic variations of luminosity, radial velocity and line profiles. They have periods between 2 and 7 hr. Their variability is now believed to be caused by the classical κ-mechanism operating in the heavy-element ionization zones. Many of the β Cephei stars are multimode pulsators, and radial as well as nonradial pulsation may occur. Advances in the last few years in both the observational and theoretical studies of multiple pulsation modes of δ Scuti stars have reached the stage that realistic comparisons of the results and examination of predictions can be carried out (Breger et al. 1998; Stankov et al. 1999). We now want to apply those asteroseismological methods to the much hotter β Cephei stars.

We report on an ongoing study on the importance of the contributions of different photospheric variations to the line profile variations of hot pulsating stars.

For the β Cephei variables, we compare the periods derived from ground-based observations with those given in the Hipparcos catalogue. For the SPB stars, of which most have been discovered by Hipparcos, we examine the question of multiperiodicity.

The satellite Hipparcos led to the discovery of 267 new variable B-type stars. Some 100 of them were classified as candidate slowly pulsating B stars (SPBs) by Waelkens et al. (1998). Twelve of the brightest southern candidate SPBs were selected together with 5 confirmed SPBs to start a long-term follow-up study (Aerts et al. 1999). From 1996 up to 1998, numerous high-resolution, high signal-to-noise spectra were taken with the CAT/CES combination at La Silla in order to study the line profile variations of the Si II-doublet centered at 4130 Å.

We report our finding that at least 8 of 17 targets turn out to be spectroscopic binaries. We have found a large variety in the obtained orbits. HD 123515 and HD 140873 were known as single-lined spectroscopic binaries, but both turn out to be double-lined. All the others binaries are single-lined. For HD 140873 and HD 177863, we find orbits with large eccentricities of respectively e = 0.731 α 0.006 and e = 0.603 α 0.007. HD 69144, HD 92287 and HD 169978 are three binaries with circular orbits and very short orbital periods (a few days). Since their photometric measurements are dominated by (close to) sinusoidal variations with twice the orbital frequency, these stars are ellipsoidal variables. Their orbital periods are of the same order of magnitude as the expected periods of pulsation.

After removing the orbit, we find the same first frequency in the residual radial velocities as in the gathered photometric measurements for 6 stars. For HD 69144 and HD 169978 we did not yet succeed in deriving an intrinsic period, although HD 69144 has prominent line profile variations. For HD 169978, we have serious doubts about the SPB nature.

For a detailed description, we refer to De Cat et al. (1999).

HD 105382 is clearly a monoperiodic, very active λ Eri star with the frequency 0.7721 d−1. Its photometric behaviour and the moment variations are not typical for a standard pulsational model.

Z-bump opacities are known to excite radial pulsations in helium stars with 102 ≲ L/M ≲ 104 and Teff ∼ 25 000K. Well-studied cases are V652 Her and BX Cir. The absence of variations in the metalpoor star HD 144941 demonstrates the importance of Z-bump opacities to drive these pulsations. Several other helium-rich stars lie close to the theoretical blue-edge of the Z-bump instability strip. One of these, LS IV+6°2, is a known radial velocity variable. We report on recent (1999 June) observations of two helium stars close to the Z-bump instability boundary. Both appear to be variable.

Radial and nonradial pulsations are excited in low-mass helium stars with effective temperatures between 7000 and 35000 K. In the case of the most luminous stars, these are driven by strange-mode instabilities (Saio & Jeffery 1988). Less luminous helium stars are mostly non-variable, except around 25000 K. V652Her is the prototype radially pulsating helium star, with Teff ~ 25000 K and a pulsation period of 0ḍ08 (Hill et al. 1981). The pulsation was only understood to be driven by the K-mechanism with the introduction of Z-bump opacity (Saio 1993).

The importance of Z-bump instability in low-mass helium stars has been investigated further through a series of pulsation models (Jeffery & Saio 1999a). It is shown that Z-bump pulsations can persist to surprisingly high hydrogen abundances in low-mass stars, and may consequently be excited in several helium-rich hot subdwarfs with Teff ~ 25000 K. Within the Z-bump instability region, both radial and nonradial pulsations are excited, and multi-periodic variability is expected.

Pulsation properties are closely linked to the global dimensions of a pulsating star by the period – mean density relation. When spectroscopic measurements are also available, the twin constraints of period and surface gravity can place very tight limits on the stellar radius (Jeffery & Saio 1999b). These are in excellent agreement with independent measurements for V652 Her and place a lower limit on the mass of the recently discovered Z-bump pulsator BXCir (Kilkenny et al. 1999).

The behavior of the velocity projection factor was studied for three individual Lines, He ɪ 4438 Å, Si ɪɪɪ 4553 Å, and N ɪɪ 4442 Å, and for the complete spectrum in the radially pulsating helium rich star V652 Her. It is shown that the projection factor is itself a function of expansion velocity.

With new data on stellar opacity and equation of state, we computed the theoretical blue edges of the classical instability strip in the vicinity of the main sequence. The results were compared with observational data on δ Scuti variables using various diagrams (HR, log g – log Teff, Mv – (b – y)0 and so on). An example of these diagrams is shown in Fig. 1. Detailed results can be found in Pamyatnykh (2000).