This exposition focuses on the still elusive excitation mechanism of the rapid oscillations in some Ap stars. We visit the domain of overstable magnetic convection which was conjectured as the origin of the observed high radial-order p modes. Mostly we dwell, however, on the classical k-mechanism. In roAp stars, the zone of partial hydrogen ionization is driving rather effectively. Furthermore, we argue that an atmospheric temperature inversion supports the κ-mechanism driving by preventing oscillation energy from leaking into the atmosphere.

We have been using a new method to search for solar-like oscillations that involves measuring temperature changes via their effect on the equivalent widths of the Balmer hydrogen lines. We observed α Cen A over six nights in 1995 with the 3.9-metre Anglo-Australian Telescope and the European Southern Observatory's 3.6-metre telescope in Chile. Oscillations were not detected, with an upper limit only slightly higher than the expected signal.

Photometric results from the HIPPARCOS mission are of interest to stellar seismology. HIPPARCOS provides a general description of the HR diagram in terms of stability, microvariability and variability. The satellite has detected systematically variables with amplitudes exceeding a threshold, strongly magnitude dependent but as small as a few mmag for bright stars. When the signal is periodic, periods and amplitudes are determined. Because of the peculiar time sampling of HIPPARCOS only first modes and associated amplitudes are generally obtained. This information is particularly relevant to seismology in the case of pulsating stars as δ Scuti stars, slowly pulsating B stars and β Cephei stars.

The photometric experiment on board of Hipparcos has discovered, among other types of variables, a large amount of new Slowly Pulsating B Stars. We have selected the fourteen brightest stars of this sample, together with five previously known Slowly Pulsating B Stars, for long-term spectroscopic and photometric monitoring. The selected stars have spectral types ranging from B 2 up to B 9 and are thus nicely spread across the instability strip. We here present the results of a preliminary analysis of our data and point out that our sample is unique in the sense that it allows us to perform seismology of massive early-type stars.

For 5 out of 28 known rapidly oscillating magnetic chemically peculiar (roAp) stars, the largest observed frequency seems to exceed the theoretical acoustic cutoff frequency, which is determined by the outermost stellar regions. We show that a better modelling of the atmosphere reconciles the theory with the observations for at least the roAp star α Cir.

Seismology from the ground has already provided many important results, as discussed for instance in Frandsen (1997) and in this symposium. The limitations of this approach are essentially the day-night alternance and the meteorological instabilities for photometry, the lack of photons and the broadening of spectral lines due to rotation for spectroscopy. Seismology from space is less limited, as photometry at a very high accuracy on long uninterrupted observing runs is possible.

α Circini is the brightest of the known rapidly oscillating Ap (roAp) stars. Previous observations of this star in photometry (Kurtz et al. 1994) have shown that it has one dominant pulsation mode, which is a pure oblique dipole mode (ℓ = 1) with a frequency of 2442 μHz (P = 6.825 min). Kurtz et al. (1994) measured the amplitude of the principal mode to be 2.55 mmag (Strömgren v).

Rapidly oscillating Ap (roAp) stars are cool magnetic Ap SrCrEu stars which pulsate in high-overtone (n ≫ l), low-degree (ℓ ≤ 3) p-modes, with periods from 6 to 15 minutes and typical amplitudes of a few millimagnitudes. 28 such stars are currently known. The roAp phenomenon is confined to a well-defined region of the Strömgren photometry parameter space. However, this region also contains other Ap stars, in which no pulsation could be detected, despite sometimes thorough searches. These apparently constant Ap stars (non-oscillating Ap stars, or noAp stars) appear remarkably similar to the roAp stars in many respects (e.g. colour indices, abundances, magnetic fields). Here we present recently found indications for differences between both groups.

Rapidly oscillating Ap stars generally pulsate in multiple modes, characterized by different frequencies. The amplitudes of these modes may furthermore be modulated with the rotation frequency of the star. For the two roAp stars whose magnetic fields have been sufficiently studied, the maximum pulsation amplitude coincides in phase with one of the extrema of the mean longitudinal magnetic field. Two interpretations of this property have been proposed: the oblique pulsator model, according to which the pulsation modes are aligned with the magnetic axis of the star, and the spotted pulsator model, which assumes that the pulsation modes are symmetric about the rotation axis of the star, and that pulsation amplitude modulation is due to the inhomogeneity of the stellar surface (which, itself, is related to the magnetic field geometry). At present, no definite choice between these two models can be made, though the oblique pulsator model is often preferred.

The edge of a convective region inside a star gives rise to a characteristic periodic signal in the frequencies of its global p-modes (e.g. [1], [4]), such that the frequencies ω are then essentially a smooth function of the mode order n plus a periodic component . Here the amplitude is , with A1 and A2 being values that depend weakly on frequency ω: A1 is always present in general, but A2 will be non-zero only if there is overshoot; is essentially the acoustical depth τ (i.e. the sound travel time) of the edge of the convection zone measured from the surface of the star; and Φ0 is a constant related to the phase of the eigenfunctions. To facilitate the comparison between different stars, we consider the amplitude evaluated at a fiducial frequency by defining . For the Sun, we chose as the reference frequency . If we take this value and scale it for other stars (using just a standard homology scaling for frequencies), we find .

Sharp variations of the structure of the star create a characteristic signal in its frequencies of oscillation (e.g. [3]). The zone of the second ionization of helium is such a localized feature of the structure whose properties depend mainly on the abundance of helium and the equation of state. Considering that such a signal should easily be detectable provided the frequencies are measured to rather better than 1μHz accuracy (the COROT project should measure oscillation frequencies with an accuracy of 0.1μHz), we present here a tool to study this aspect of stellar structure.

Time series spectra of the F5IV star Procyon (α CMi) were obtained at the Kitt Peak National Observatory during a 35-night observing run in January-February 1997. The observations were obtained as part of an international collaboration to detect and study acoustic p-mode oscillations in solar-type stars. Spectra covered the wavelength range from 4000 to 5300 Å, with a resolving power of approximately 3500 (1.3 Å resolution). The sampling rate was one observation per minute, and the typical S/N ratio per pixel after averaging along columns is in excess of 1000. We obtained 12,888 spectra. A sample spectrum is shown in Figure 1

Non-radial pulsations in isolated white dwarfs have been known for 25 years and it has been shown that the hydrogen-rich (DA) white dwarfs have a high probability of pulsating if they lie in the instability strip with effective temperature between 11500 and 13200 K - the ZZ Ceti stars (e.g. Kepler and Nelan 1993). Analysing techniques developed for such stars allow derivation of masses, luminosities, rotation periods, hydrogen surface layer masses and other properties (e.g. Kepler and Bradley 1995). A number of binary systems are known in which the primary is a white dwarf; dominant in this class are the cataclysmic variable (CV) stars. Until now no CV primary has been found to have non-radial pulsations.

We review recent observational developments which provide important asteroseismological tools. Extensive multisite campaigns of individual δ Scuti stars show that 24 or more pulsation modes with ℓ = 0 to 2 can be detected photometrically. Spectroscopically, also about 30 modes have been detected, and these can be identified with modes of ℓ values up to 20. Since each technique favors the detection of specific types of modes, hundreds or thousands of modes must be excited in δ Scuti stars.

We examine the quantities which can be matched between observations and theoretical models specifically computed for each star. Recent progress in tbe mode identification of multiple pulsation modes is illustrated by presenting an application of the phase shift method for the two stars, FG Vir and 4 CVn, recently measured by the Delta Scuti Network.

The purpose of asteroseismology is not only to derive the internal structure of individual stars from their observed oscillation frequencies, but also to test and extend our understanding of the physics of matter under the extremes of temperature, density, and pressure found in stellar interiors. In this review, I hope to point out what we can learn about the Sun by studying δ Scuti stars, as well as what we can learn about stars more massive or evolved than the Sun. I will discuss some of the difficulties in theoretical approaches to asteroseismology for δ Scuti stars, using FG Vir, δ Scuti, and CD −24° 7599 as examples.

Over two dozen early F-type variable stars have been identified which constitute a new class of pulsating stars. These stars typically have periods between 0.5 and 3 days with V-band variability of several hundredths of a magnitude. Given the time scale of the variability, the pulsations would have to be non-radial gravity-mode pulsations. The pulsational nature of some of these stars has been proven by means of coordinated multi-longitude photometric campaigns, radial velocity (RV) variations and line profile (LP) variations, indicating low degree spherical harmonics (ℓ = 3 or less). Evidence is that these stars are younger than 300 Myr; one would surmise that a rapid onset of convection in their outer photospheric layers puts an end to the pulsations. Further observation and modeling of these stars is important for our understanding of stellar evolution, the search for g-modes in the Sun, and is even relevant to the interpretation of radial velocity variations of solar-type stars (e.g. 51 Peg) in the search for extrasolar planets.

Improved observing and data analysis strategies have initiated a considerable expansion of the empirical knowledge about the pulsations of OB stars. Possible correlations between physical parameters and associated pulsation characteristics are becoming more clearly perceivable. This starts to include the asteroseismologically fundamental areas of g-modes and rapid rotation. The β Cephei instability strip continues to be the only locus where radial pulsations occur (but apparently not in all stars located in that strip). Except for spectral types B8/B9 near the main sequence, where pulsations are hardly detected even at low amplitudes, any major group of stars in the Galaxy that are obviously not candidate pulsators still remains to be identified. However, the incidence and amplitudes of OB star pulsations decrease steeply with metallicity. The behaviour of high-luminosity stars is less often dominated by very few modes. In broad-lined stars the moving-bump phenomenon is more common than low-order line-profile variability. But its relation to nonradial pulsation is not clear. The beating of low-ℓ nonradial pulsation modes that have identical angular mode indices may be the clockwork of the outbursts of at least some Be stars. The physics of this episodic mass loss process remains to be identified.

Progress in understanding B-type pulsators has been reviewed at several recent meetings (e.g. Dziembowski, 1993, 1995; Moskalik, 1995). Not much happened afterwards. Owing to further improvement in the stellar opacity calculations (Iglesias and Rogers 1996), the theoretical pulsation-instability domain in the upper Main Sequence is now more precisely determined. In the next section I will compare the predicted domain with the positions of various B-type pulsators in the H-R diagram.

The Edinburgh-Cape Blue Object Survey (Stobie et al. 1997a) is a southern hemisphere survey to discover hot, blue stellar objects brighter than B=18 at galactic latitudes more than 30° from the galactic plane. The main categories of object detected are hot subdwarfs, white dwarfs, blue horizontal branch stars, apparently normal B stars, cataclysmic variables and (stellar-like) active galactic nuclei. Over 50% of the EC Survey comprises hot subdwarfs.

The recent discovery of pulsating subdwarf B (sdB) stars at the South African Astronomical Observatory (Kilkenny et al. 1997; Koen et al. 1997; Stobie et al. 1997; O'Donoghue et al. 1997) has opened a brand new avenue in the field of asteroseismology. The study of these pulsators (dubbed EC14026 stars, after the prototype) offers the exciting possibility of exploiting the full power of asteroseismology to investigate the sdB phase of stellar evolution, one of the last frontiers in our general understanding of the history of stars.