Kjeldsen et al. (1995) reported a probable detection of solar-like, low-amplitude p-mode oscillations in η Bootes using equivalent width measurements from low-resolution spectra of hydrogen Balmer lines. However, this detection has not been confirmed so far. Indeed, there is no confirmed detection of p-mode oscillations in a solar-like star using the equivalent width technique or any other approach (Bedding 1998).

The frequency spectrum in Fig. 1 shows that at least five pulsation modes are excited in DD Lac. Three frequencies, f1, f4, and f3, form an equidistant triplet. In addition to the value of the central frequency, f4, the triplet can be characterized by the mean separation, S = (f3 − f1)/2, and the asymmetry, Δf = f4 − (f1 + f3)/2. Taking the values of the frequencies from a recent analysis of all available data (Pigulski 1994), we get S = 0.15544 ± 0.00021 and Δf = −0.00014 ± 0.00029 d−1.

The analytical expression between the line profile and its corresponding pulsation velocity field is derived by the assumption of Doppler Imaging (DI). Based on this approach, numerical experiments of the recovery of the one dimensional nonradial pulsation velocity distribution from the residual line profiles are presented.

The programme of long-term high-speed photometric survey about two hundred Normal and Peculiar A-type stars with 125cm RC telescope equipped by Two-Star Photometer is presented.

HR 2740 was a target for a photometric campaign carried out at La Silla (ESO) and Sutherland (SAAO) from 1997 January 14 to 1997 February 11 (Poretti et al., 1997). The campaign revealed that HR 2740 is one of the brightest γ Dor stars, a class of variable stars located near the cool border of the instability strip, and in which gravity pulsation modes are excited. Four frequencies were identified (f1=1.0434, f2=0.9951, f3=1.1088, f4=0.9019 c d−1), which together yield a satisfactory solution to the observed light curve. The frequency analysis was not simple, but thanks to the large coverage in longitude we could separate the effect of aliasing on the two terms f3=1.1088 c d−1 and f4=0.9019 c d−1, linked by the relationship f4 = 2 - f3. Moreover, only the long time baseline allowed us to resolve the two close terms f1=1.0434 c d−1 and f2=0.9951 c d−1. See Poretti et al. (1997) for a detailed discussion.

In two recent papers (Pardo & Poretti 1997; Poretti & Pardo 1997) we analyzed all the available photometry of galactic double-mode Cepheids (DMCs) with the aim of detecting in each case the importance of the harmonics and of the cross coupling terms. We found that no a priori fit can be reliably applied to the measurements of a DMC, but a careful frequency analysis must be done to evaluate the importance of each term. As a further application of this technique, we obtained very precise indications about the properties of the Fourier parameters. When discussing the generalized phase differences Gi,j we demonstrated that plotting them as a function of the order |i|+|j|, there are well-defined regions where they are confined: the second order terms have π < Gi,j < 3π/2; the third order terms have π/2 < Gi,j < π; the fourth order terms cluster around 2π.

Standard asymptotic descriptions of stellar p-modes use 1/ω as a small parameter; either the degree ℓ is kept constant in the asymptotic expansions (e.g. Tassoul 1980), or the parameter , which specifies the position of the inner turning point (e.g. Vorontsov 1991). At low degree, due to the strong effects of gravity perturbations, these expansions are known to produce rather poor results, even when developed to higher order (Roxburgh and Vorontsov 1994). Here, we employ an alternative asymptotic expansion, capable of providing much better accuracy at low degree-with as a small parameter instead of 1/ω, and test its diagnostic capabilities.

We investigate the observable spectroscopic characteristics of non-radial pulsation for stars with rotation rates large enough to resolve the stellar surface by Doppler imaging. We show that the intensity variations in time series of theoretical spectra, at each position in the line profile, cannot be described by a single sinusoid: at least one harmonic sinusoid needs to be included to describe the data. Across the line profile the relative amplitudes and phases of both these sinusoids vary independently.

We discuss the oscillation features of β Cephei, which is a magnetic star in which the magnetic axis seems to be oblique to the rotation axis. We interpret the observed equi-distant fine structure of the frequency spectrum as a manifestation of a magnetic effect on an eigenmode, which would be a radial mode in the absence of the magnetic field. Besides these frequency components, we interpret another peak in the frequency spectrum as an independent quadrupole mode. By this mode identification, we deduce the mass, the evolutionary stage, the rotational frequency, the magnetic field strength, and the geometrical configuration of β Cephei.

Herbig Ae stars are pre-main sequence of masses ~2M⊙. As they trace their course toward the ZAMS, they cross the classical instability strip. At this stage, their evolution has considerably slowed down. Their structure differs from that of stars evolving after the ZAMS (δ Scuti stars) in the very deep layers where nuclear burning has only recently started. But the outer layers for stars before and after the ZAMS are expected to be similar. As these layers drive the pulsation of δ Scuti stars, it is reasonable to expect that PMS stars in the instability strip also are destabilized by the κ mechanism with a similar range of unstable modes. Indeed, recently a Herbig Ae star has been reported as a variable (Kurtz and Marang 1995).

We have performed a linear pulsational stability survey of long period variable models. The dynamic and thermodynamic couplings between convection and oscillations are treated by using a statistical theory of nonlocal and time-dependent convection. The results show that the fundamental and all the low overtones are always pulsationally unstable for the low-temperature models when the coupling between convection and oscillations is ignored. When the coupling is considered, there is indeed a “Mira” pulsationally instability region outside of the Cepheid instability strip on the H-R diagram. The coolest models near the Hayashi track are pulsationally stable. Towards high temperature the fundamental mode first becomes unstable, and then the first overtone. Some of the 2nd −4th overtones may become unstable for the hotter models. All the modes higher than 4th (n > 4) are pulsationally stable. The position and the width of such an instability region on the H-R diagram critically depends on the mass, luminosity and metal abundance of the star.

Using a nonlocal time-dependent theory of convection, we have calculated the linear non-adiabatic oscillations of the Horizontal Branch (HB) stars, with both the dynamic and thermodynamic coupling between convection and oscillations been carefully treated. Turbulent pressure and turbulent viscosity have been included consistently in our equations of non-adiabatic pulsation. When the coupling between convection and oscillations is ignored, for all models with Te ≤ 7350K, the fundamental through the second overtone are pulsationally unstable; while for Te ≤ 6200K all the models are unstable up to (at least) the 9th overtone. When the coupling between convection and oscillations is included, the RR Lyrae instability strip is very well predicted. Within the strip most models are pulsationally unstable only for the fundamental and the first few overtones. Turbulent viscosity is an important damping mechanism. Being exclusively distinct from the luminous red variables (long period variables), the HB stars to the right of the RR Lyrae strip are pulsationally stable for the fundamental and low-order overtones, but become unstable for some of the high-order overtones. This may provide a valuable clue for the short period, low amplitude red variables found outside the red edge of the RR Lyrae strip on the H-R diagram of globular clusters. Moreover, we present a new radiation modulated excitation mechanism functioning in radiation flux gradient regions. The effects of nonlocal convection and the dynamic coupling between convection and oscillations are discussed. The spatial oscillations of the thermal variables in the pulsational calculations have been effectively suppressed.

The themes woven into this review are: (1) the solar outer atmosphere is highly inhomogeneous; (2) anomalies in the behavior of particular wavelength intervals can signal key inconsistencies in an accepted model; and (3) the observations still hold surprises, particularly in the thermal infrared.

This paper reviews the properties of the solar oscillation power spectrum at frequencies near and above the acoustic cut-off frequency for the atmosphere. This region of the spectrum contains over 50% of the total peak structure observed and is a source of information not only on the outer layers of the sun and the origin of solar oscillations, but also on the physics of the solar core and chromosphere.

We investigate the influence of a magnetic field on the radiative damping of magnetoatmospheric waves, extending our previous work on the adiabatic modes of an isothermal stratified atmosphere with a uniform vertical magnetic field. Banerjee, Hasan & Christensen-Dalsgaard (1996, 1997) generalized this work to include radiative effects using Newton's law of cooling for a weak magnetic field. The present study examines the variation of the mode damping rate with increasing magnetic field strength. We find that a moderate field suppresses radiative damping.

Based on spectral observations with high spatial and temporal resolution, we discuss in this brief review the dynamic behaviour of the upper layers of the quiet solar atmosphere. A proper description of this behaviour is fundamental for the understanding (and simulation) of the temperature structure and the heating of the chromosphere, and the interpretation of such conspicuous phenomena like the chromospheric “grains”. The evidence is drawn from waveforms, power spectra, and phase spectra of velocity and brightness fluctuations in numerous Fraunhofer lines formed over an extended radial range of the atmosphere. A comparison with recent predictions of simulation calculations documents the strong need to extend the simulation codes to 3-D, and also to incorporate a realistic chromosphere/coronal transition layer, to better reproduce the observations.

The natural state of the Solar chromosphere is very dynamic. Any photospheric disturbance will grow and naturally form shocks over the twenty scale-heights in density between the photosphere and the corona. Observations in the resonance lines from singly ionized calcium and recently in the ultraviolet region of the spectrum observed with the Solar and Heliospheric Observatory satellite also show a very dynamic chromosphere. This dynamic picture is further supported by numerical simulations. Static and dynamic pictures of the chromosphere are fundamentally different. The simulations also show that time variations are crucial for our understanding of the chromosphere itself and for the spectral features formed there.

The analysis of ≈ 460 days of high quality data provided by the GOLF experiment on board SOHO, has unambiguously revealed the presence of signal in the power spectrum in the region 5.8 to 7.5 mHz, well above the p-modes cut-off frequency. In this contribution, the observed structure of these full-disk observations is presented. High frequency peaks (hereafter HFPs) well above the acoustic cutoff frequency of the solar p modes (≈ 5.5 mHz) have been already observed in intermediate and high spatial resolution oscillation data (Duvall et al. 1991). The different interpretations of the observed pattern (Kumar 1994 and references therein; Rast and Gough 1995) agree that it provides direct information on the location and some characteristics of the source of the acoustic oscillations (see also Vorontsov et al. 1997).

The Vacuum Tower Telescope (VTT) at Tenerife has been used together with a polarimeter in front of a two-dimensional imaging spectrometer with a Fabry-Perot interferometer. The spectrometer has been described by Bendlin et al. (1992) and Bendlin et al. (1995), the remaining instrumentation, calibration, and data reduction by Horn et al. (1996). Some parameters of the setup were: 384×286 pixel CCD-camera (0.2” per pixel); resolution for the images better than 0.8”; filtergrams in the photospheric spectral line Fe I 6173.4 Å with Δλ = 10.9 mÅ; 45 filtergrams for each scan; 127 scans within 114 min. Δt = 54 s. White-light images from the same field of view were taken strictly simultaneously to all narrow-band pictures to correct for image blurring and motion.

The present observations were obtained on July 20, 1994, and were focused on the main spot of the active region NOAA 7757, 30° NE from the disk center.

Extending our previous studies of the dynamics of solar granulation (Nesis et al., 1997) we investigated the relationship between granular flow and the emergence of turbulence in the deep photosphere. Our main goal is to explore if such a relationship exists, and if so, to define it quantitatively. To this end we take advantage of the excellent signal approximation property of wavelets. The material for the present work is a series of spectrograms of high spatial resolution covering a time span of 12 min. They were taken at the center of the solar disk with the German Vacuum Tower Telescope in Izaña (Tenerife, Spain) in 1994, and include several absorption lines of different strengths; for more details see Nesis et al. (1997). The spectrograms were digitized and processed with wavelet techniques and regression analysis, in order to investigate the granular convective flow, the associated turbulence, and their mutual connection.