The task assigned for this review is to discuss structural features of the Galaxy as they may involve or relate to gas dynamics. Since a topic of such great breadth would permit discussion of essentially any aspect of the Galaxy, we shall narrow the view and direct attention primarily towards the larger-scale features of the system. In particular, effort will be made to place emphasis on those areas of the subject in which problems of interpretation exist and in which new theoretical models are needed.

It is now accepted without any doubts that the central stars of Planetary Nebulae (CPN) are rapidly evolving objects in the transition from the asymptotic giant branch (AGB) to the white-dwarf regime. After the pioneering study of Paczyński (1971) it has been demonstrated by Schönberner (1981) that Paczyński's calculations are too crude for the understanding of post-AGB evolution because the latter depends very sensitively on the detailed internal stellar structure, i.e. on the past AGB evolution. More precisely, the evolution of an AGB remnant is a function of the thermal-pulse cycle phase ϕ during which this remnant has been created by the planetary-nebula (PN) formation process. This has been shown by Schönberner (1979, 1983) and later fully been explored by Iben (1984).

We present the results of a spectroscopic study in which we have measured line-strength indices in a sample of 5 dwarf and 2 compact elliptical galaxies (mostly from Virgo). Some conclusions about the stellar populations in dwarf E's are derived by comparing the sample with classical E galaxies. In the adjacent figure, we plot in the Hβ-[MgFe] plane the central indices of our sample of dwarf and compact E's together with data from González (1993, PhD Thesis). The grid represents the predictions of single-burst stellar population models from Worthey (1994, ApJS, 95, 107). The main conclusion is that, whilst the central regions of giant, intermediate and compact ellipticals span, according to stellar population models, a wide range in mean stellar ages, bright dwarf ellipticals are found to be old, compatible with a ∼ 10 Gyr old stellar population. This means that the dichotomy found in the Fundamental Plane between dwarf ellipticals and the gE-iE-cE sequence (Bender, Burstein & Faber 1992, ApJ, 399, 462) is also observed in the stellar populations.

By using the quantization rule based on the WKB asymptotic method, we present an integral equation to infer the form of the acoustic potential of a fixed ℓ as a function of the acoustic length. Since we analyze the acoustic potential itself by taking account of some factors other than the sound velocity and we can analyze the radial modes by this scheme as well as nonradial modes, this method improves the accuracy and effectiveness of the inverse problem to infer the internal structure of the Sun, in particular, the deep interior of the Sun.

The materials of large asteroids and asteroid families are sampled by meteorites that fall to Earth. The cosmic ray exposure age of the meteorite identifies the collision event from which that meteorite originated. The inclination of the orbit on which the meteoroid impacted Earth measures the inclination of the source region, while the semi-major axis of the orbit points to the delivery resonance, but only in a statistical sense. To isolate the sources of our meteorites requires multiple documented falls for each cosmic ray exposure peak. So far, only 36 meteorites have been recovered from observed falls. Despite these low numbers, some patterns are emerging that suggest CM chondrites originated from near the 3:1 resonance from a low-inclined source (perhaps the Sulamitis family), LL chondrites came to us from the ν6 resonance (perhaps the Flora family), there is an H chondrite source at high inclination (Phocaea?), and one group of low shock-stage L chondrites originates from the inner main belt. Other possible links are discussed.

There is overwhelming evidence that the Milky Way has formed its stars at a relatively constant rate throughout the Hubble time. This implies that its stock of cold gas was not in place since the beginning but it has been acquired slowly through gas accretion. The gas accretion must have been at low metallicity in order to reconcile the metallicities observed in the disc with the predictions of chemical evolution models. But how does this gas accretion take place? I review the current evidence of gas accretion into the Milky Way and similar galaxies through the infall of cold gas clouds and satellites. The conclusion from these studies is that the infalling gas at high column densities observed in HI emission is a least one order of magnitude below the value required to sustain star formation, thus accretion must come from a different channel. The likely reservoir for gas accretion is the cosmological corona of virial-temperature gas in which every galaxy must be embedded. At the interface between the disc and the corona the cold high-metallicity disc gas and the hot coronal medium must mix efficiently and this mixing causes the cooling and accretion of the lower corona. I show how this mechanism reproduces the kinematics of the neutral extraplanar gas in the Milky Way and other nearby galaxies and the ionised high-velocity clouds observed in HST spectra. I conclude with the speculation that the loss in efficiency of the disc-corona interaction is the ultimate cause for the evolution of disc galaxies towards the red sequence.

Although rare, planetary nebulae have been extensively studied by astronomers. This affection is certainly due in part to the beauty of the gaseous nebulae but it is also due to the belief that many, probably most, stars of intermediate mass (1–4 M) become planetary nebulae at least once during their lifetimes. If the planetary nebula is an (almost) inevitable stage in stellar evolution, it is important to determine its evolutionary precursors and followers. The latter are likely to be the white dwarfs and the former are generally believed to be red giants. With the advent of infrared and radio techniques it now appears possible, for the first time, to specify the immediate progenitors of planetary nebulae.

The magnetic field vectors derived from the Hanle effect measured in prominences during the ascending phase and maximum of Cycle 21 have been plotted on the corresponding synoptic maps of Meudon Observatory (e.g., Figure 2). Most of these measurements have been performed in a single line, He I D3. Our previous analysis of 2-line measurements have already proven that prominence magnetic fields have Inverse polarity (Bommier et al. 1994) and remain close to the horizontal plane (Bommier et al. 1986, 1994). We also found earlier that the field component along the filament axis shows a remarkable large scale pattern, especially at high latitudes (Leroy et al. 1983). Given the geometry of horizontal Inverse field, we reinvestigate this large scale organization through our complete set of observations (296 prominences). These new maps fully confirm and extend to medium latitudes the law of reversal of the axial field from one prominence band to the adjacent ones (see Figure 2). This behavior, together with the Inverse polarity pattern, gives a picture consistent with a North-South global field distorted by the differential rotation (see Figure 3).

Sample return missions offer opportunities to learn things about other objects in our Solar System (and beyond) that cannot be determined by observations using in situ spacecraft. This is largely because the returned samples can be studied in terrestrial laboratories where the analyses are not limited by the constraints - power, mass, time, precision, etc. - imposed by normal spacecraft operations. In addition, the returned samples serve as a scientific resource that is available far into the future; the study of the samples can continue long after the original spacecraft mission is finished. This means the samples can be continually revisited as both our scientific understanding and analytical techniques improve with time.

These advantages come with some additional difficulties, however. In particular, sample return missions must deal with the additional difficulties of proximity operations near the objects they are to sample, and they must be capable of successfully making a round trip between the Earth and the sampled object. Such missions therefore need to take special precautions against unique hazards and be designed to successfully complete relatively extended mission durations.

Despite these difficulties, several recent missions have managed to successfully complete sample returns from a number of Solar System objects. These include the Stardust mission (samples from Comet 81P/Wild 2), the Hayabusa mission (samples from asteroid 25143 Itokawa), and the Genesis mission (samples of solar wind). This paper will review the advantages and difficulties of sample return missions in general and will summarize some key findings of the recent Stardust and Hayabusa missions.

Whereas the identification of existing or potential observatory sites was an important part of the report for the first three years of existence of Commission 50, during the period 1976-78 the emphasis has been mostly on establishing recommendations for the protection of observing conditions.

It is assumed that the acceleration and collimation mechanisms of jets are the same in all the classes of astrophysical objects which are observed to produce jets. These classes now include: active galactic nuclei, young stellar objects, massive x-ray binaries, low mass x-ray binaries, black hole x-ray transients, symbiotic systems, planetary nebulae, and supersoft x-ray sources.

On the basis of this assumption, an attempt is made, to identify the necessary ingredients for the acceleration and collimation mechanism. It is argued that: (i) jets are produced at the center of accretion disks which are threaded by a vertical magnetic field, (ii) the production of powerful jets requires, in addition, an energy/wind source associated with the central object. Tentative explanations for the presence of jets in some classes of objects and absence in others are given. Some critical observation that can test the ideas presented in this paper are suggested.

We show the utility of the Systematic Ranging technique by analyzing the orbit determination of asteroid 2004 FU$_{162}$, which passed approximately 6400 km from the surface of the Earth on March 31, 2004. The limited observations introduce strong nonlinearities that must be accounted for when estimating the actual encounter distance.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Although it is clear that cosmic rays are of considerable astrophysical significance we know surprisingly little about their origin(s), their mode(s) of propagation and escape from the galaxy, and even of their general properties. Practically nothing is known of the composition and energy spectra of different species at energies below about 200 MeV/nucleon as a consequence of solar modulation, which has a significant effect on the observed spectra at energies up to about 10 GeV/ nucleon. The energy spectra of ions (especially protons) appear to be well approximated by a power law (differential spectral index γ ~ 2.6) in the range 1010–1015 eV/nucleon. At higher energies (up to ~ 1020 eV/nucleon) both the spectrum and the composition of the ion component are less well known, however it is significant that the anisotropy, as evidenced from the sidereal diurnal variation, increases substantially, suggesting that perhaps an extragalactic origin must be considered.

We apply a code that solves grey radial non-adiabatic pulsation equations with consistent treatment of radiative transfer to the roAp frequency regime. We find evidence for the existence of chromospheric modes at frequencies 2.5 mHz, 3.7 mHz, 4.6 mHz and 5.7 mHz in the equilibrium model we used. We also find that, since the oscillations are not adiabatic at the surfaces of roAp stars, better surface boundary conditions need to be considered in future studies.

By observing the transits of exoplanets, one may determine many fundamental system parameters. I review current techniques and results for the parameters that can be measured with the greatest precision, specifically, the transit times, the planetary mass and radius, and the projected spin-orbit angle.

Curves of polarization are available at present for asteroids Vesta, Ceres, Pallas, Iris, Flora, and Icarus. These curves are compared with those of the satellites of Jupiter and Mercury, the Moon, and Mars. Laboratory simulations had already proved that the Moon's surface behaves like a powder of pulverized basalts; the recent confirmation by direct exploration is proving the significance of the method for remote determination of the surface properties of celestial bodies. The simulation of the Martian surface is found on small grained powders oxidized by ferreous limonite or goethite. New laboratory measurements were conducted to prepare the simulation of the asteroidal surfaces. Samples of the lunar surface returned to Earth provide impact-generated regolith and bare rocks superficially pitted and etched by impacts of the types suggested to be found on asteroidal surfaces; they were analyzed polarimetrically.

Preliminary interpretations show that Vesta departs significantly from the other asteroids and cannot be covered by frost deposits or by aggregate cosmic dusts; a regolith-type surface generated by impacts or a coating of cohesive grains is indicated.

Ceres, Pallas, and Iris are darker, and their polarizations do not suggest a pure regolithic surface, but cohesive grains or aggregates of dust are indicated.

Icarus is 108 times smaller in mass; its polarization authorizes a fluffy, loosely aggregated dust deposit; however, a cometary model with stones embedded in ice is perhaps not ruled out on the basis of the present data.

The way in which deep-space missions near the asteroidal belt can improve these results is discussed.

Very strong evidence suggests that Sagittarius A*, a compact radio source at the center of the Milky Way, marks the position of a super massive black hole. The proximity of Sgr A* in combination with its mass makes its apparent event horizon the largest of any black hole candidate in the universe and presents us with a unique opportunity to observe strong-field GR effects. Recent millimeter very long baseline interferometric observations of Sgr A* have demonstrated the existence of structures on scales comparable to the Schwarzschild radius. These observations already provide strong evidence in support of the existence of an event horizon. (Sub)Millimeter VLBI observations in the near future will combine the angular resolution necessary to identify the overall morphology of quiescent emission, such as an accretion disk or outflow, with a fine enough time resolution to detect possible periodicity in the variable component of emission. In the next few years, it may be possible to identify the spin of the black hole in Sgr A*, either by detecting the periodic signature of hot spots at the innermost stable circular orbit or parameter estimation in models of the quiescent emission. Longer term, a (sub)millimeter VLBI “Event Horizon Telescope” will be able to produce images of the Galactic center emission to the see the silhouette predicted by general relativistic lensing. These techniques are also applicable to the black hole in M87, where black hole spin may be key to understanding the jet-launching region.

Mass loss can affect the evolution of binaries in various ways, during different stages of the evolution.

1. For massive stars stellar wind mass loss will change the masses of the components during their main sequence evolution.

2. During the Roche lobe overflow phase (or tidal interaction phase) matter can leave the system.

3. For low mass stars matter can leave the system during the mass exchange phase and can be stored in envelopes, disks or rings.

4. Sufficiently massive stars(>8-15M0) undergo at the end of their life a supernova explosion, where most of the matter is blown away and a compact object, a neutron star or a black hole can be left.

5. For intermediate stars one of the components can evolve into a degenerate He or CO dwarf; a reverse mass transfer can dump matter on this degenerate dwarf. If the conditions are favorable the white dwarf can explode with loss of matter, and a neutron star can be the result.

6. The chemical abundances in the outer layers change.

I will compute representative spectra for all spectral types for which my model calculations (Kurucz 1992; 1993a; 1994a) are valid. High-resolution zero-rotation fluxes and intensities will be distributed on CD-ROMS together with programs (Kurucz 1993B) for broadening the spectra to arbitrary rotation velocities, macroturbulent velocities, and instrumental profiles, and for transmitting the spectrum through an atmosphere.

Since the line data are still not well determined, I actually have to make two calculations, one with only the lines with good wavelengths to preserve wavelength detail, and one with all lines, including predicted, to preserve the energy distribution for photometric calibration. In the first case the spectrum is usable as a radial velocity, abundance, or classification template. Even though the line list is incomplete, its signal-to-noise can be higher than that of observed spectra for cross-correlating.

The models are plane parallel, LTE, in hydrostatic equilibrium, and have no triatomic opacities. Therefore, M stars and supergiants cannot be computed realisticly. The abundances available now are scaled-solar and alpha-enhanced scaled-solar. Individual abundances can be changed for the spectrum calculation. My new model atmosphere program, ATLAS 12 (Kurucz 1993c), which does not yet work, will allow arbitrary abundances.

I have made test calculations for Arcturus, the sun, Vega, Sirius, and a17000K B. star. Figure 1 shows the effect of resolution on the appearance of the spectrum. Figure 2 shows the effect of rotation. The spectrum for Arcturus is displayed in Kurucz (1994b) with the molecular bands labelled.

Wavelength positions of photospheric absorption lines may be affected by surface convection (stellar granulation). Asymmetries and wavelength shifts originate from correlated velocity and brightness patterns: rising (blueshifted) elements are hot (bright), and convective blueshifts result from a larger contribution of such blueshifted photons than of redshifted ones from the sinking and cooler (darker) gas. For the Sun, the effect is around 300 m s−1, expected to increase in F-type stars, and in giants. Magnetic fields affect convection and induce lineshift variations over stellar activity cycles. A sufficient measuring precision reveals also the temporal variability of line wavelengths (due to the evolution of granules on the stellar surface). A major future development to come from adaptive optics and optical interferometry will be the study of wavelength variations across spatially resolved stars, together with their spatially resolved time variability. Thus, precise radial velocities should soon open up new vistas in stellar atmospheric physics.