Recent work on galactic planetary nebulae (PN) has emphasized the complex nature of this galactic subsystem (W. J. Maciel, 31st Herstmonceux Conference, ed. R. Terlevich, in press). A fairly large amount of data is now available, including distances, galactic distribution, kinematical properties, and chemical composition of the nebular gas, supplemented by data on the central stars for some objects. In this work, a large sample of PN is used to study the different stellar populations associated with the galactic nebulae. It is seen that the available data imply a physical description of the PN phenomenon that is consistent with an evolutionary model based on stellar populations.

In this work we analyze the UV upturn seen in elliptical galaxies using the evolutionary spectral library of Bruzual & Charlot (1991). We present models for 3 representative groups galaxies of the Burstein et al. (1988) sample with different levels of UV flux.

Colliding ring galaxies provide a remarkable testbed for the study of star formation in perturbed galaxies. In the process of passing through a disk system, a small perturbing galaxy generates a density wave of stars and gas which expands into the host disk. This triggers a wave of star formation. As the star forming wave passes through the host galaxy, progressively older burst populations may be found interior to the ring. As part of a multiwavelength study of ring galaxies, we have performed optical and infrared imaging using the Kitt Peak 2.1m telescope. These images are used to explore the relation between stellar density wave amplitude and star formation rate. Color gradients are searched for which would indicate the presence of an aging burst population interior to the ring.

The evolution of the red giant clump as a function of age has been investigated by analyzing the color-magnitude diagrams of a number of intermediate-age star clusters in the LMC. We find that the luminosity of the clump decreases systematically until about 1 Gyr. Thereafter, the mean apparent magnitude of the clump in LMC clusters remains constant at V ~ 19.0 ±0.1, although with significant scatter about this value in clusters with ages between 1 and 2 Gyr. The luminosity width of the clump also varies systematically with cluster age, being largest for clusters with ages near 1-1.5 Gyr. For six clusters older than 2 Gyr, the clump shows an intrinsic luminosity spread (defined as the FWHM of the best-fitting Gaussian) of 0.30 ± 0.07 mag. The systematic behavior of the color of the clump as a function of age was studied in detail by Hatzidimitriou (1991, M. N. R. A. S., 251, 545).

The Sextans dwarf Spheroidal (dSph) galaxy was discovered recently by Irwin, et al. (1990, M. N. R. A. S., 244, 16p). We report results concerning a number of the global properties of the stellar population of this system. Based on deep CCD photometry obtained with the prime-focus CCD camera on the CTIO 4m telescope, we find that the galaxy is dominated by an old stellar population, similar to that observed in the Draco, Sculptor, and Ursa Minor dwarfs. Some blue stragglers are also present; if these are associated with an intermediate age component, they indicate that only a tiny fraction of the stellar population of Sextans is younger than about 10 Gyr. Based on the apparent magnitude of the predominately red horizontal branch, we conclude that the true distance modulus of Sextans is 19.7 ± 0.3, and the total luminosity of the galaxy is 5.2 × 105L⊙. A complete description of these results is given in Mateo, et al. (1991, A. J., 101, 892).

Some results from a new CCD color imaging survey of centrally dominant cluster galaxies selected for their X-ray properties are discussed.

The definitions of the amorphous and blue compact dwarf (BCD) classes of galaxies are very similar. One key difference is that BCDs are often selected for their apparent compactness (i.e. a small angular size), which selects against nearby objects, whereas amorphous galaxies must be extended. We present initial results of a project to determine the population distribution in dwarf (MB < −18) amorphous galaxies (dAgs) and determine which dAgs can be classified as BCDs. We have used the 3.9m Anglo-Australian Telescope to obtain deep B and R CCD images of two dAgs: NGC 216 (MB = −17.3) and NGC 2915 (MB = −14.1). The morphology of NGC 216 is that of a dusty late-type edge-on disk galaxy, with a peculiar one-sided bar. It would not be classified as a BCD if seen face-on. However, NGC 2915 does have all the properties of a BCD, and can be classified so. It has numerous condensations near its center. Many of these are likely to be individual stars. We derive a distance to NGC 2915 of 5 Mpc if the brightest blue non-extended objects are blue supergiants, and if there is little internal extinction. Similar condensations are seen in the dAgs NGC 1705 (Meurer, et al., 1989. Astrophys. Space Sci., 156: 141) and NGC 5253 (Caldwell and Phillips, 1989. Astrophys. J., 338: 789) which are also likely to be BCDs.

C2 and CN bandheads in the wavelength region λλ 550-680 nm are synthesized for 6 stars of ω Centauri, in order to derive their carbon and nitrogen abundances. Oxygen abundances are also derived taking into account molecular associations with C and N compounded molecules, using the appropriate C and N abundances.

These determinations have been made for CO-rich and CO-normal stars according to the CO indices measured by Persson et al. (1980).

We present spectroscopic observations of ~200 giant stars in two fields toward the Galactic bulge. The positions of fields 588 and 589 are at l,b = (8,7) and (12,3), respectively, which corresponds to a Galactocentric distance of ~1.6 kpc for the stars in the bulge. The K giants were selected from color-magnitude and color-color diagrams produced by scans of B, R and I plates from the APM machine. The spectra were obtained with the MX multi-object spectrograph at the Steward 90″ telescope, and with the Red Channel spectrograph at the MMT. Radial velocities good to ≪ 10 km/s were obtained from cross-correlation techniques. A grid of ~100 standard stars was built to calibrate spectrophotometric indices that provide abundances and luminosities.

In this contribution we present observations for a set of three clusters of galaxies at intermediate redshift (z ~ 0.2) selected from the revised Abell catalog (Abell et al. 1989, Ap.J. Suppl., 70, 1) and observed in the Gunn photometric system at the ESO 3.6m telescope in La Silla (Chile) during various runs between 1986 and 1990. This is part of a more extended project addressing the study of the distant clusters, as outlined in more detail in Molinari et al. (1990, M.N.R.A.S., 246, 576).

The galactocentric motion of 14 stars from the solar neighbourhood is studied. The stars are divided into nine groups according to their heliocentric-velocity components. In this way one can follow the motion of the stars belonging to the same group in the past.

Field #3 of the Palomar–Groningen survey (PG3) is thought to be ideal for the study of stellar evolution in the Galactic Bulge (GB). In this field the work of Plaut (1973) and Wesselink (1987) is extended with the aim of constructing a Color Magnitude Diagram (CMD) for a great many stars. This is done by means of automated photographic photometry.

A new formula for the ratio of the planar velocity dispersions containing an asymmetric-drift term is proposed. It yields a good agreement with the flat rotation curve of the Galaxy.

The total kinetic energy of our Galaxy is estimated to be (0.55 – 3.3) x 1016M⊙km2s–2 and the specific one to about 4 × 104km2s–2.

We constructed a model for the Galaxy to reproduce near-infrared (J, H, K, bands) and mid-infrared 12 and 25 micron IRAS bands source counts for a given direction and sensitivity of detection. The Galaxy is conceived as being formed by two populations: a disk, with different scale heights for each spectral type, and a de Vaucouleurs spheroid. The interstellar extinction is considered to be proportional to the amount of gas along the line of sight; both neutral (HI) and molecular (H2) components were taken into account, using the HI density distribution of Burton & Gordon (1978) and the CO density distribution of Sanders et al.(1984) and Gordon & Burton (1976).

The gaseous component in disks of spiral galaxies generally shows strong metallicity gradients of about 0.2 dex per scale length of the disk (e.g. Edmunds and Pagel 1984, MNRAS 211,507). Current models explain these gradients by a radially varying ratio of stellar density to gas density, possibly coupled with infall of gas from outside and radial gas flows in the disk (Lacey and Fall 1985, Ap.J. 290,154). It follows from these models that the final metallicity gradient (i.e. the gradient after infall has ceased and all gas has been locked into stars) should be quite shallow.

I have UBV photometry for roughly 1500 stars and BV photometry for an additional 900 stars in the 30 Doradus region as a part of my thesis work. Spectra and spectral classifications for over 200 stars have been obtained (by me and various other sources) for most of the early stars in the region. The reductions and classifications are nearly complete. It should be noted that the OB associations of 30 Doradus present unique problems in the analysis of the stellar contents due to the extreme crowding and highly variable nebulosity and reddening.

Until recently is was commonly thought that most spiral galaxies were transparent or contained only modest amounts of absorbing dust, making them optically thin in the B-band (Holmberg 1958). However Disney et al. (1989) showed that there was very little evidence for this. Using a very large optical database Valentijn (1990) showed that surface brightness in B is almost independent of inclination, implying that spiral galaxies in general are optically thick in B.