The celestial system maintained by the International Earth Rotation Service is described in terms of physical properties of the fiducial objects, internal consistency of the frame, and agreement with the FK5 and dynamical systems.

We explore a series expansion method to calculate the instabilities and the structure of the perturbations for a variety of uniformly rotating finite stellar disks. This survey focuses on the role of the distribution function in stability analyses. Although the potential does not show differential rotation, it will in many cases be a reasonable approximation for the disk in the central regions of galaxies without massive central mass concentration.

I will discuss the connection between the luminosity function of a population of high-mass X-ray binaries in a galaxy and the star formation rate in the this galaxy. The understanding of this connection provides on the one hand an independent measure of an important galaxy property, and on the other hand new insights into populations of high-mass X-ray binaries. In particular, observations with the Chandra X-ray telescope are uniquely suited to investigate the X-ray part of this connection and I will present examples of this.

Relativistic outflows from AGN can be parameterized by θ, the angle subtended by the direction of the outflow and the line of sight to the observer, and γ, the bulk Lorentz factor of the outflow. The Doppler factor, δ, and the apparent speed in the plane of the sky, βapp, are combinations of θ and γ. The Doppler factor can be estimated using either the equipartition Doppler factor, δeq (Readhead 1994), or the inverse Compton Doppler factor, δIC. These Doppler factor estimates are combined with observed βapp to solve for θ and γ for different categories of AGN.

Ghisellini et al. (1993) compute δIC for 105 compact radio sources, and Güijosa & Daly (1996) compute δeq for the same sample. Daly, Guerra, & Güijosa (1996) estimate θ and γ for the 43 sources that have βapp listed by Vermeulen & Cohen (1994) and δeq computed by Güijosa & Daly (1996).

Solutions and errors for θ and γ are presented in Figures 1 and 2 using δeq and δIC respectively. Guerra & Daly (1996) discuss these estimates and errors in greater detail. These AGN fall into the following categories: BL Lacertae objects (BL Lacs), core-dominated high-polarization quasars (CDHPQ), core-dominated low-polarization quasars (CDLPQ), core-dominated quasars with no polarization information (CDQ(NPI)), lobe-dominated quasars (LDQ), and radio galaxies (RG).

In Spite of Many Years of Research on Interstellar Grains, their composition is still unknown. Evidently it can be found only through the interaction of the grains with radiation, and one is led to the spectroscopic study of the solids composing the grains. If discrete lines, bands, or other spectral features can be discovered, they may be of great importance in the chemical analysis of the grains. So far, data of this type include the diffuse interstellar lines, the optical lines of CH, CH +, and CN (almost certainly once part of the grains), the radio lines of OH, and the distinct “kink” or curvature in the optical extinction curve (see paper by Nandy in present compilation), which may reflect spectral variations in refractive index and hence indicate a specific material.

The advent of ultraviolet and infrared techniques suggests that consideration be given to new features to be expected in those regions.

I give a very brief summary of the state of our knowledge of the symbiotic stars, together with some of my hopes for how the field will develop.

Maser astrometry is now providing parallaxes with accuracies of ±10 micro-arcseconds, which corresponds to 10% accuracy at a distance of 10 kpc! The VLBA BeSSeL Survey and the Japanese VERA project have measured ≈200 parallaxes for masers associated with young, high-mass stars. Since these stars are found in spiral arms, we now are directly mapping the spiral structure of the Milky Way. Combining parallaxes, proper motions, and Doppler velocities, we have complete 6-dimensional phase-space information. Modeling these data yields the distance to the Galactic Center, the rotation speed of the Galaxy at the Sun, and the nature of the rotation curve.

Coulomb interactions for the Free Helmholtz energy and the pressure are studied in a partial new formulation which described more exactly the numerical evaluation of many body theories.

The HIPPARCOS mission will permit a decisive step forward in the comparison between observed and predicted global properties of stars, in producing distances and apparent magitudes with accuracies more than one order of magnitude higher than before. Nearby stars of intermediate and low mass will allow for statistical tests on the validity of the equation of state, like for instance the steepness of the main sequence.

We present new UV, visual and Hα photometry obtained with the WFPC2 of NGC 330, NGC 1818, NGC 2004 and NGC 2100, four young populous clusters in the MCs. We present observational evidence for a degree of convective core overshoot in excess of that currently applied in standard models.

We have recently completed a catalogue of ∼ 5 million galaxies in the Southern sky down to a limit of bj = 20.5. The catalogue is based on the ROE/NRL Southern Sky Object Catalogue (Yentis et al. 1992), which is the result of processing of scans made with the COSMOS machine at the Royal Observatory Edinburgh (ROE) on glass copies of the IIIa-J survey carried out with the UK Schmidt Telescope (UKST) in Australia. The ‘COSMOS/UKST Southern Sky Galaxy Catalogue’ is based on 500 fields with |b| > 25 degrees and outwith the Magellanic Clouds.

High spatial resolution, near-infrared imaging and spectroscopy of the nuclear star cluster obtained in the last few years have given key new insights about the structure, evolution and mass distribution in the Milky Way Center. The central parsec is powered by a cluster of hot, massive stars. Their characteristics imply that there was an active phase of star formation a few million years ago, probably triggered by the infall and collapse of a very dense gas cloud. Other such starburst episodes may have taken place between 100 and 300 million years ago. Measurements of radial and proper motions for more than 200 stars show that stellar velocities increase with a Kepler law down to a scale of a light week from the compact radio source Sgr A∗. The data make a compelling case for the presence of a compact, central dark mass of about 2.6×106 M⊙. Simple physical considerations show that this dark mass cannot consist of a stable cluster of stars, stellar remnants or substellar condensations. Energy equipartition requires that at least five percent of the dark mass (≥105 M⊙) must be associated with Sgr A∗ itself and likely is enclosed within less than 8 light minutes. If one accepts these arguments it is hard to escape the conclusion that Sgr A∗ is indeed a massive black hole at the core of the Milky Way.

Accurate measurements of the 4He/H abundance ratio are important in constraining Big Bang nucleosynthesis, models of stellar and Galactic evolution, and H ii region physics. We discuss observations of radio recombination lines using the Green Bank Telescope toward a small sample of H ii regions and planetary nebulae. We report 4He/H abundance ratio differences as high as 15–20% between optical and ratio data that are difficult to reconcile. Using the H ii regions S206 and M17 we determine 4He production in the Galaxy to be dY/dZ = 1.71 ± 0.86.

A distance of R0 = 7.6 ± 1.6 kpc for the galactic center is given by comparison of expansion parallax of a cluster of H2O masers in W49(N) with the kinematic distance of that source. Data from additional epochs, now being calibrated, should further improve the accuracy of our determination of R0.

The stellar winds of A supergiants can have a significant influence on their emergent spectra. Here we present the hydrogen line profiles of a model based on the stellar parameters of HD 12953. The radiative transfer equation is solved in two dimensions in axial symmetry. We do not include the velocity field by the Sobolev approximation, but in detail using the Lorentz transformation. This allows us to correctly include the stellar wind, since the velocity gradients in A supergiants are too small for the Sobolev approximation to be valid.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We are now at both the maximum of the solar activity cycle and at the most populated part of the saros. I discuss the solar corona over the recent saros and its changes with the solar activity cycle. We consider the scientific value of eclipse studies and how they relate to other ongoing coronal studies on the sun and other stars.

Increasingly better observations of resolved protoplanetary disks show a wide range of conditions in which planets can be formed. Many transitional disks show gaps in their radial density structure, which are usually interpreted as signatures of planets. It has also been suggested that observed inhomogeneities in transitional disks are indicative of dust traps which may help the process of planet formation. However, it is yet to be seen if the configuration of fully evolved exoplanetary systems can yield information about the later stages of their primordial disks. We use synthetic exoplanet population data from Monte Carlo simulations of systems forming under different density perturbation conditions, which are based on current observations of transitional disks. The simulations use a core instability, oligarchic growth, dust trap analytical model that has been benchmarked against exoplanetary populations.

A substantial amount (≲50%) of solar acoustic (p-mode) absorption in sunspot regions has been reported by Braun, Duvall and LaBonte (1988). A number of suggestions have been advanced to account for the observed p-mode absorption in sunspots (see review by Davila in these proceedings). These include processes involving resonance absorption, radiation damping, and non-dissipative mechanisms. The main purpose of the present work is to demonstrate that any dissipative process like radiative, viscous, or resistive dissipation leads to the resonant absorption of acoustic waves incident on the sunspot tube and that the resultant heating rate can be shown to be consistent with the observed absorption of the p-mode power impinging on an isolated inhomogeneously structured sunspot.