This paper reviews current ideas about the possible constituents of galactic nuclei, and the mechanisms for ejecting gas and massive objects.

We describe how observations of pulsars from the Parkes Pulsar Timing Array (PPTA) project have been used to develop a pulsar-based timescale. This is the first such timescale that has a precision comparable to uncertainties in international atomic timescales.

Developments in the field since the review of Roche (1989) are discussed. Section 1 describes the results from detailed models of the dust emission from planetary nebulae and summarises the derived dust-to-gas mass ratios. The properties of the remarkable gas-poor planetary nebula IRAS 18333–2357, located in the globular cluster M22, are discussed in Section 2. The use of thermal infrared imaging to map the distribution of different dust components is reviewed in Section 3, and two recently applied optical probes of nebular dust properties are described: emission line spectropolarimetry, and spectroscopy of the Extended Red Emission (ERE). The final section describes recent results from infrared spectroscopy. The spectra of carbon-rich post-AGB objects have provided valuable new insights into the nature and evolution of the materials responsible for the UIR bands, while unusual oxide grains and crystalline ice emission have been detected around oxygen-rich post-AGB objects. The spectra of a small number of objects show evidence for the simultaneous presence of both oxygen-rich and carbon-rich material.

We present a new observational evidence of the magnetic field reconnection in emerging flux regions (EFR). Our observations show that Hα surges are the first active manifestation of emerging magnetic field in some EFRs. We call them EFR-surges. Examining their morphological and evolutionary characteristics, we conclude that they are produced by the magnetic field reconnection between the EFR and the pre-existing surrounding region.

The study of Wolf-Rayet binaries is important for the information we can gleam about the most fundamental property of any star: its mass. In this regard we are very fortunate, since the masses of WR stars can be determined with far greater confidence than can their luminosities, compositions, colors, effective temperatures, etc. By simply measuring the velocities of both components in a WR+0 system, we can find the minimum masses and the mass ratio of the two stars; if we can get some further handle on the orbital inclination, we then know the mass of the WR star directly.

Starting from a few topical astrophysical questions which require the knowledge of the age of Pop I stars, we discuss the needed precision on the age in order to make progresses in these areas of research. Then we review the effects of various inputs of the stellar models on the age determination and try to identify those affecting the most the lifetimes of stars.

Interplanetary dust probably has a cometary origin. Collectible samples of this material are likely to be more representative of the meteoroid complex than are meteorites. Since 1974, NASA U-2 spacecraft has been able to collect over 200 interplanetary particles in the stratosphere. Roughly 10% have spheroidal shapes indicating previous melting. The rest seems to come from the gentle fragmentation of a single type of parent-body material: a black aggregate of grains mainly 1000 A in size, whose properties are closely similar to C1 and C2 chondritic material.

An outline of the mean-field magnetohydrodynamics suggested and developed by M. Steenbeck and the authors and its application to the dynamo theory of the solar cycle is presented. Four basic requirements are formulated which have to be satisfied by any dynamo model which claims to explain the solar cycle. The models investigated allow conclusions about the differential rotation. In this connection Leighton's work is criticized.

The origin of stony meteorites landing on Earth today is directly linked to the history of the main belt, which evolved both through collisional evolution and dynamical evolution/depletion. In this paper, we focus our attention on the main belt dynamical evolution scenario discussed in Petit et al. (2001). According to Petit et al., during the planet formation epoch, the primordial main belt contained several Earth masses of material, enough to allow the asteroids to accrete on relatively short timescales.

After a few My, the accretion of planetary embryos in the main belt zone dynamically stirred the remaining planetesimals to high enough velocities to initiate fragmentation. After a short interval, perhaps as long as 10 My, the primordial main belt was dynamically depleted of $>99$% of its material via the combined perturbations of the planetary embryos and a newly-formed Jupiter. The small percentage of objects that survived in the main belt zone became the asteroid belt. It has been shown that the wavy-shaped size-frequency distribution of the main belt is a “fossil” left over from this violent period (Bottke et al. 2004).

Using a collisional/dynamical model of this scenario, we tracked the evolution of stony meteoroids produced by catastrophic disruption events over the last several Gy. We show that most stony meteoroids are a byproduct of a collisional cascade derived from large and ancient asteroid families or smaller, more recent breakup events. The meteoroids are then delivered to Earth by drifting in semimajor axis via Yarkovsky thermal drag forces until they reach a resonance powerful enough to place them on an Earth-crossing orbit.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We consider the structure of atoms and atomic chains in the presence of ultra-strong magnetic fields as might be found in pulsars or neutron stars. Some consequences of these models for neutron star surfaces are mentioned.

We show that deuteration in DCN and HDCO probably derives from CH2D+ in warm clouds, and that the levels of deuteration observed in those clouds require that radiative association play an important role in the chemistry of interstellar clouds.

The intensity ratio of the continuum to the molecular emissions was estimated in the spectra of eight-five comets. These consisted of 34 new, 1/a (orig) < 100 x 10-6 AU-1, 31 more evolved with 102 < 1/a < 104, and 20 periodic comets, P < 103 years. In each age group the comets were divided into two distance intervals, the first observed at less than 1 AU from the sun and the second category observed at more than 1 AU. No significant difference among the patterns of relative intensity distribution among four age groups was found. No general evidence for a difference between pre- and post-perihelion observations was found although in a few cases a pronounced effect occurs. Four conclusions are drawn. (1) There is no readily apparent difference in continuum to emission intensity ratio between new and more evolved comets. (2) An intrinsic distribution of this characteristic does occur. (3) Periodic comets with weak continua derived from new comets with the same property. (4) No weakening of the continuum in general occurs following perihelion passage. The infrared evidence for Comet Encke suggests that the faintness of its continuum may be caused by a size distribution containing only particles larger than about 10 μm.

As is well known, very rapid and non-periodic changes in brightness have been discovered in several late-type dwarf stars in recent years. In the vicinity of the sun, within a radius not exceeding 10 parsecs, nine or ten such objects have been found and named ‘flare’ stars because of their extraordinarily rapid variations. The prototype of these flare stars is UV Ceti. For the purpose of the present discussion, we shall call these objects ‘classical’ flare stars.

The question of whether non-Cepheids populate the cepheid instability strip has been the subject of much recent interest. Several investigators (see especially Fernie and Hube 1971 and Fernie 1976) have concluded that indeed there are an appreciable number of objects not presently recognized as Cepheids within the strip, though both the size and position of the strip and the placement of supergiants in the H-R diagram are subject to considerable uncertainty. In view of the interest in this matter, I have attempted to shed some light on this situation by simply considering the nature of the stars spectroscopically classified as of high luminosity in spectral classes F and G. For this one needs, of course, a complete sample of the relevant stars, which fortunately is now to some extent available.

Seven compact PNe with high radio surface brightness (and therefore presumably young) were observed with the HST WFPC2 under cycle 8 program 8307 (PI: S. Kwok). Observations were made with three narrow-band filters, F656N (Hα), F658N ([NII]) and F502N ([OIII]). The [NII] images of six of the nebulae are shown in Figure 1.

Ever since Graham’s Strömgren photometry (1972) demonstrated the existence of a single well defined cooling sequence of DA white dwarfs the question of the mass dispersion (or the width of the number-mass distribution) has been in the foreground of my studies (Weidemann, 1970, 1977).

Indeed it turned out that the shape of the white dwarf mass distribution provides strong constraints on the theory of stellar evolution with mass loss, a fact which will be demonstrated again in the following lecture. It therefore seems worthwhile to dwell in some detail on the methods of its determination. For the benefit of the non-specialists I shall first present some of the historical results and then continue to discuss the present situation.

Since the last IAU Colloquium devoted to Ap stars, die continuous development of high signal to noise observations and of computerized analysis tools have greatly increased the effectiveness of abundance anomalies as a probe of hydrodynamical processes in stars. Until the 1980s, they seemed limited to ApBp and AmFm stars where the effects are largest. More recently the abundances of Li and He have been suggested to play important diagnostic roles for hydrodynamics in both the Sun (Proffitt and Michaud 1991b, Pinsonneault et al. 1989) and Halo stars (Proffitt and Michaud 1991a, Pinsonneault et al. 1992, Vauclair 1988, Charbonnel et al. 1992, Deliyannis and Demarque 1991). The observation of a Li abundance gap in F stars of clusters has raised the question of the link between these objects and the AmFm stars (Michaud 1986, Charbonneau and Michaud 1988, 1991) or alternately with solar type stars. The observed Li abundance on die subgiant branch sheds some light on this problem (Charbonneau et al. 1989; Charbonnel and Vauclair 1992). In solar type stars, the observed Li abundance constrains the superficial turbulence that may inhibit surface He gravitational settling (Proffitt and Michaud 1991b) whose effects can be detected using solar seismology (Guzik and Cox 1992, Dziembowski et al. 1992).

A general discussion, based on the ht “Fourier Slice Theorem,” is given for the problem of deprojecting the observed light distribution of galaxies to obtain their intrinsic three dimensional light distribution or “shape.” Several results are obtained: 1) A model-independent deprojection of an axially symmetric galaxy is shown to be possible only if the symmetry axis lies in the plane of the sky. 2) A simple criterion is given to test whether two different galaxies can have the same intrinsic shape, based solely on their observed projections. 3) It is shown that a homogeneous class of galaxies can be deprojected using a sufficiently large number of projections of random perspective.

We discuss follow-up X-ray observations of giant-amplitude, non-recurrent X-ray flares, interpreted as tidal disruptions of stars by supermassive black holes at the centers of the flaring galaxies. The flare observations open up a new window to study supermassive black holes and their environment, and the physics of accretion events, in non-active galaxies.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

A mechanism to create circumstellar envelopes in Be stars is proposed. The mechanism assumes a mass-loaded wind produced by ablated discrete stellar ejecta. The discrete mass ejections are suggested by the long-term spectrophotometric behaviours of Be stars and by their short and long-lived light outbursts discovered recently.