Numerous infrared Spectroscopic observations were obtained of eight AGB field M giants that have multiple periods of light variability. For six of the eight giants we found radial-velocity periods that confirm the long-period light variability. Although we consider the possibility that the velocity variations result from orbital motion, we conclude that the long-period velocity changes in most, if not all of our sample stars, likely result from pulsation rather than duplicity.

We have (1) accumulated long-term (up to 15 years) photoelectric photometry of 76 bright pulsating red giants (PRGs) and related stars, with V amplitudes of 0.04 to ≥ 1 magnitude; (2) used a variety of techniques, including autocorrelation, to determine both “short” radial pulsation periods, and long secondary periods; (3) determined pulsation modes using the Q-value method; the modes range from fundamental to second overtone, independent of temperature; (4) tested wavelet analysis on small-amplitude PRGs; (5) made a special study of PRGs with amplitudes less than 0.1 magnitude; and (6) called further attention to the problem of the long secondary periods of these stars. Our studies provide a “model” for the analysis and interpretation of larger, fainter surveys such as MACHO and OGLE.

Red giant stars located on the AGB are intrinsically variable, which is usually associated with stellar pulsation. The complex light variation of semiregular variables (SRVs) has many alternative explanations. Besides the pulsation, other contributors to variability cannot be solidly excluded, such as time-dependent surface inhomogeneities, ellipsoidal deformation due to the presence of a close companion, or spots on a rotating star. The identification of ultra-short period M-giants with periods of a few days raised the possibility of very high-order overtone pulsation of these stars. Thus, it is very important to draw various constraints on the stellar pulsation in semiregular variables and to separate other physical causes. Here we present new empirical evidence for radial pulsation in SRVs through studying their period-gravity relation, following the ideas outlined by Fernie (1995, F95). F95 studied a log P versus log g relation for various types of radially pulsating variable stars. He found a well-defined linear relation, which might serve to test whether the pulsation is radial or nonradial and, in the former case, fundamental or overtone. The main goal of our study is to extend the relation into the domain of SRVs.

The use of aperiodic ordered sequences described by Lindenmayer systems in the study of the strings of periods for multiperiodic semiregular variables is proposed. Fibonacci sequences play a central role in the light curve analysis of UW Herculis.

Some numerical parameters of long-term changes of light curves in Long Period Variables (LPVs) are obtained. Results of analysis of the sample of 53 LPVs and classification of these stars are discussed.

I discuss single-channel differential observations of late B and early A supergiants and AGB S stars obtained with the Four College Automated Photoelectric Telescope. The suggestion of periods longer than several years often seen in the seasonal mean magnitudes of both kinds of stars requires confirmation. I used Program Period98 to derive the most important frequencies for Deneb and for three AGB S stars.

Solutions for the stellar winds of hot luminous stars are obtained by solving the magnetohydrodynamic (MHD) equations, combined with the theory of radiation-driven winds. The formalism is basically the same as developed by Rotstein & Giménez de Castro (1996). We have considered the interaction of radiation pressure with an open magnetic field . The magnetic field lines are pushed by the intense radiation field and adopt a purely radial configuration:

and the polar magnetic flux at the photosphere:

Rotstein & Giménez de Castro (1996) assume all the lines contributing to the radiation pressure are optically thick (α = 1) in order to decouple the MHD equations. Here the model has been improved to adopt a value of α = 0.6 which is a better approach for A-supergiants.

Wavelet analysis has been performed on the American Association of Variable Star Observers (AAVSO) visual estimates of the RV Tauri star U Mon from 1961-2000 using the AAVSO WWZ wavelet analysis program. Periods of 46.07, 92.40 and 2498 days are present but the 92 day period is not always detectable. A constant ratio between the amplitudes of the 46 and 92 day periods is found during one single long-term deep minimum but not another.

We discuss the stellar parameters (M, L, Teff) of a post-AGB star, HD 56126, deduced from observations and non-linear radiative pulsation models. We show that pure radiative pulsation models are in contrast with stellar evolution.

Based on a linear and non-linear study of radial pulsations in the envelopes of classical novae (Schenker, 1999), I discuss the results both from the point of view of pulsation theory as well as their consequences for current nova models. Starting from initially static envelope structures at various stages during the decline of a nova outburst, strong ‘running-wave’ instabilities have been found that rapidly grow into shocks. Improved analytical concepts give a new direction to the interpretation of such highly non-adiabatic radial pulsations. For direct observational confirmation, a search for short period variability in the UV and soft X-ray is suggested during the very late decline phase. Speculative consequences for mass-loss scenarios in novae due to these instabilities will need some more work in the future.

Quasi-Periodic Oscillations (QPOs) in Cataclysmic Variable stars (CVs) with periods in the range 100 – 2000 s have been known for 25 years (see Chapter 8 of Warner, 1995a). We distinguish here between QPOs and Dwarf Nova Oscillations (DNOs) – the latter being of shorter period (5 – 50 s) and greater coherence. QPOs appear as brightness modulations and typically have quite short coherence lengths (a few cycles).

QPOs are largely associated with discs transferring mass at a high rate – i.e., dwarf novae during outburst or nova-like variables; but the QPOs are not always present and many systems have not shown QPOs at all.

Generally it is thought that QPOs result from non-radial oscillations of the accretion discs surrounding the white dwarf component in CVs (Carroll et al., 1985; Collins et al., 2000). They could, e.g., be quite coherent oscillations of individual annuli in a disc, each with a different period. Alternatively, a particular oscillation mode may be excited for only a few cycles and then be re-excited at an arbitrary phase. Most theoretical proposals have sought to make the QPOs self-luminous pulsations of the disc, in analogy with the kappa or epsilon processes of stellar pulsation.

We discuss prospects for asteroseismology in stars for which the frequencies of pulsation lie outside the asymptotic regime. The problem of mode identification in this case is the major obstacle. Mode identification techniques from photometric and Spectroscopic observations are discussed.

The interplay between theory and observation has been particularly close in asteroseismology, ranging from white dwarfs through roAp stars. This trend should continue as the field expands to include new classes such as solar-type stars. This synergistic relationship relies on communication between theorists and observers, but also on healthy skepticism in both directions. Here, I outline a (somewhat personal) view of the theoretical side - what kinds of observations are most helpful for optimizing utility of theoretical research efforts ? Where might theory be of closer guidance as observing programs are planned ?

The Delta Scuti Network (DSN) is a collaboration of astronomers all around the globe who study and observe short-period variables. The field of scientific research includes high-precision photometric and Spectroscopic global campaigns, mode identification techniques, and asteroseismological pulsation modeling. We present results for three stars that are receiving the most attention at the moment: 4 CVn, BI CMi, and 44 Tau. Our results demonstrate that a large number of simultaneously excited nonradial oscillations in stars on and above the main sequence can be detected by conventional means.

We assess the potential of asteroseismology for determining the fundamental properties of individual δ Scuti stars. We computed a grid of evolution and adiabatic pulsation models to study systematic changes in l = 0,1,2, and 3 modes as functions of fundamental stellar properties. Mass has the strongest effect on evolution and on pulsation, followed by the metal abundance. Changes to the helium content have very little effect on the frequencies until near the end of the main sequence. Changes to each of the four parameters change the p-mode frequencies more than they do the g- and mixed-mode frequencies, suggesting that these parameters have a greater effect on the outer layers of the star. We also present pulsation models of FG Virginis, outlining a possible method of locating favorable models in the stellar parameter space based upon a definitive identification of only two modes. We plot evolution models on the (period-period ratio) and (temperature-period ratio) planes to select candidate models, and modify the core overshooting parameter to fit the observed star.

For many years, astronomers have promised that the study of pulsating white dwarfs would ultimately lead to useful information about the physics of matter under extreme conditions of temperature and pressure. We can now make good on that promise. Using observational data from the Whole Earth Telescope and a new analysis method employing a genetic algorithm, we empirically determine the central oxygen abundance of the helium-atmosphere variable white dwarf GD 358. We use this value, combined with detailed evolutionary calculations of the internal chemical profiles, to place constraints on the 12C(α, γ)16O nuclear reaction cross-section.

Inversion methods have been used successfully for the Sun. The stars with the next richest set of observed frequencies are the white dwarfs. We consider here the viability of numerical inversions for these stars. We find that, while the number of presently observed modes in the white dwarf GD 358 is too small for structural inversions, such inversions would be possible if the frequencies of all modes with 1 ≤ l ≤ 3 were observed. This is possible for space observations by e.g. Eddington.

The origin of the helium-atmosphere DB white dwarfs is still a matter of debate. In particular, the question is unresolved whether binary evolution produces a significant number of DBs. The pulsating DB white dwarfs (DBV stars) offer a complementary insight into this problem through asteroseismology; DBs descending from binaries will have different interior structures than DBs originating from single stars (Nitta & Winget, 1998).

GD 358 is by far the best-observed pulsating DBV star, and the only one for which asteroseismology has been performed to date. This star’s structure has been shown to be inconsistent with an origin from binary evolution (Nitta & Winget, 1998), but most of the other DBVs are relatively poorly studied.

We therefore analysed archival data on all DBVs and obtained new measurements of stars with very little data available (Table 1), firstly to identify suitable targets for asteroseismological investigations and secondly to examine the pulsation spectra of the DBVs as a group, following the works of Clemens (1994) and Kleinman (1995) on the pulsating DA white dwarfs. Our study also produced new seismological results on individual stars and promising targets for future Whole Earth Telescope (WET, Nather et al., 1990) runs.

We summarize the results of an asteroseismological study of the pulsating planetary nebula central star RXJ 2117+3412.

The equation of state (EOS) is still the big unknown in the physics of neutron stars. An accurate measurement of both the mass and the radius of a neutron star would put severe constraints on the range of possible EOSs. I discuss how the parameters of the oscillation modes of a neutron star, measured from the emitted gravitational waves, can in principle be used to infer its mass and radius, and thus reveal its EOS.