The limited observational performances which can be achieved on large collections of faint objects do not allow to derive intrisic stellar parameters such as distance, mass, age, space velocity, chemical composition of individual stars. However some information relevant to the distribution of these quantities is reflected in the n-dimensional distribution of observables: connecting observed distributions to the physical processes they come from is basically a multivariate problem for which some of us developped a synthetic approach of galaxy modelling (Robin, Crézé 1986, Bienaymé et al. 1987).

Recently a subgroup of S0 galaxies has been identified with external gas rotating in retrograde motion with respect to the stars (e.g. NGC 1216, NGC 4546 and NGC 7007). All of these galaxies are seen almost edge-on and present an excellent testing ground for studying the connection between the stellar formation and the counterrotating (non-primordial) gas. Recently, the almost face-on and gas-rich SB0 galaxy NGC 2217 was added to this list (Bettoni et al. 1990, AJ, 99, 1789), which presents a more favorable orientation for spectroscopic studies to determine its stellar population. We present three IUE long-wavelength region (LWP) observations of NGC 2217 taken at different orientations of the aperture centered on the galaxy nucleus. We have taken full advantage of the IUE spatial resolution capability to map the nuclear and bulge region. In addition, our multicolor CCD imagery obtained at CTIO allows us to identify some faint, but spatially resolved, dust extinction patterns which correlate with the weak IRAS fluxes.

We discuss the recent outburst of nova LMC 1991. By comparing its magnitude at maximum and rate of decline with those of 14 historical LMC novae, we show that nova LMC 1991 represents the first detection of a super-bright nova in the Large Magellanic Cloud.

Color gradients, in the sense of becoming bluer inwards, are found in post-core-collapse (PCC) clusters. No gradients are seen in clusters with King-model morphology. The gradients seem to be caused by the demise of red giants and/or subgiants, and possibly an increased number of blue stragglers or some other population of faint, blue objects. Practically all PCC clusters have blue horizontal branches, with faint blue tails. Bright red giants are clearly underabundant in the central regions of PCC clusters, whereas HB stars seem to be unaffected. At a fixed metallicity, PCC clusters also show blue FUV colors, as seen in the archival data from the IUE and ANS (Figure 1). This is consistent with our observations of their HB morphologies, but a presence of an additional hot population cannot be excluded. The gradients may be a consequence of stellar interactions during and after the core collapse, and the mechanism may be important for the formation of millisecond and binary pulsars and LMXB's. These effects represent a strong evidence that dynamical evolution of star clusters can physically modify their stellar populations. It is possible that similar effects may operate in at least some galactic nuclei. More details and further references are given by Djorgovski et al. 1991, ApJ, 372, L41. S.D. acknowledges support from the NASA grant NAG5-1173 and the NSF PYI award AST-9157412.

We are currently implementing the instrumentation and the data analysis tools aimed at performing a deep, complete survey of the Southern Sky (project DENIS) in 3 near-IR photometric bands (1=0.9μm, J=1.25 μm, K=2.2μm). The anticipated 3σ limiting magnitudes for point sources in I, J and K are 18, 16 and 14, respectively. The spatial resolutions will be 3″ in J and K and 1.5″ in I. The survey will be carried out with a dedicated IR camera installed at the ESO 1 m telescope in Chile during 3 years and will start in mid '93. One expects the detection of several 107 sources. The final products that we plan to deliver to the community are, so far: a set of “cleaned” elementary images of 12′ x 12′, a computer accessible data–bank (≍ 3Tbytes) with appropriate software tools and specialized catalogs. One major impact of this survey will be to provide digitized maps, that will facilitate stellar counts and follow-up statistical investigations. Thanks to a better spatial resolution and a much lower sensitivity to cool dust emission, DENIS will be less limited by confusion in crowded regions (galactic plane, bulge, etc…) than was the IRAS [12μm] survey. The moderate interstellar extinction in the near-IR range compared to the optical range (AK ~ AV/10) will allow probing of stellar populations at low galactic latitudes and in optically obscured regions of our Galaxy. The near-IR range corresponds to the emission peak of frequent species of stars of low surface temperature. Knowledge of the faint end of the luminosity function is necessary for the determination of the contribution of low–mass stars to the missing mass in our Galaxy. The search for brown dwarfs is an especially important challenge. Modeling suggests that a few 104 of them could be detected, but the positive identification of even a few of them would be a major discovery. At the other end of the mass distribution, DENIS will detect all red giants out to distances halfway across the Galaxy, it will considerably increase the number of known AGB stars suffering high mass-loss effects and of Young Planetary Nebulae. Many major structural features of our Galaxy will benefit of this survey: stars in the molecular ring at 4 kpc will be detected, the radial and vertical scale lengths of the relevant spectral type stars will be improved. DENIS will provide new knowledge about the luminosity function of the populations of late and young stars and their distribution throughout the Galaxy. The bulge of our Galaxy will be more clearly revealed than in the IRAS survey. Of particular promise are studies of the distribution of late–type stars at large distances from the galactic center. It will be possible to trace aspects of the stellar distribution in the region characterized by the galactic warp and gravitationally dominated by dark matter. Finally the detection of all supergiants, secured by follow–up observations, will lead to new informations on the formation and evolution of massive stars and open clusters.

A detailed study of strong spectral features is an important link between stellar spectroscopy and low-resolution spectroscopy or photometry of galaxies. The magnesium triplet at λ 517 nm is among the strongest features in the spectra of normal galaxies. A question arises regarding its use as a metallicity indicator for composite systems (e.g., Faber, 1973), given that it is also used as a gravity indicator for individual stars (Clark & McClure, 1979), indicating that the Mg feature shows a bi-parametric behaviour as a function of metallicity and gravity.

The high rate of gas consumption and evidence for massive stars in NGC 253, implies a prodigious star formation and supernovae rate. However there is no optical confirmation of these supernovae, presumably due to the enormous dust obscuration present. Radio observations (Antonucci and Ulvestad 1988) reveal compact radio sources i.e. young supernova remnants (SNRs).

Recently, Butcher (1987) and Morell et al. (1991) used the abundance of the long lived isotope 232Th (half life 14 Gyrs), compared to that of a stable element neodymium, to determine an upper limit for the age of the Galaxy. However, this method suffers from the fact that neodymium is partly s-process and partly r-process whereas thorium is pure r-process. Europium which is pure r-process may be used to avoid this problem. This poster presents new measurements of the thorium and europium abundances in halo stars.

The He abundance in red giants is not well known. Moreover, some He enhancement in the stellar atmospheres is expected to occur as a consequence of distinct dredge-up episodes. In the present work is reported and discussed the He abundance for 5 symbiotic nebulae (Hen 2-38, RR Tel, RX Pup, HM Sge, KX TrA). Self-absorption effects in both hydrogen and HeI lines were taken into account.

We derived mass loss rates for 5 central stars (CS) having WC spectrum:NGC 5315, BD+30 3639, He2-99, He2-113, SwSt-1. The values are substantially higher than other previous estimates based on UV data. We consider that those stars have winds in which hydrogen is highly deficient (or absent) with helium and carbon being the dominant elements. The new mass loss rates reduce the timescale for the evolution along the horizontal track, but discrepancies with kinematic timescales still persist.

Hydrodynamical time-dependent models allows to follow the evolution of condensations in cooling flows until the phase of optical line emission.

We present a unified model for the detailed chemical evolution of individual elements from 12C to 56Fe and the photometric properties of galaxies via spectral evolutionary synthesis. Observations of narrow, heavy - element QSO absorption lines show an increase in the number of MgII systems per redshift interval for redshifts 0 <Zr ≤ 1.5 and a decrease in the number of CIV systems for 1.3 ≤ Zr ≤4.1. Both can be understood in terms of our galaxy evolution model accounting for SNI contributions which at the same time gives information about the structure of the Universe and about the IMF and star formation history in the intervening absorber galaxies. The spectrophotometric aspect of our unified model predicts spectral and photometric properties of these galaxies testable by optical identifications.

High signal—to-noise ratio, medium resolution spectra have been obtained for ~8 giants in each of 18 LMC clusters with the CTIO 4-m multifiber ARGUS spectrograph. In addition, Washington CCD photometry has been obtained for ~50 SMC and LMC clusters with the CTIO 4-m and 1.5-m from which abundances can be obtained for ~25 giants per cluster. The derivation of metal abundances from these data will be discussed and some preliminary results presented.

A remarkable discovery of recent years has been the detection of various kinds of interstellar matter (ISM) in elliptical galaxies. The presence of dust patches, ionized gas, and hot X-ray gas have proven to be quite common in ellipticals which were once thought to be simple structures, devoid of gas and dust. A review of ISM in ellipticals has recently been published by Forbes (1991). However, the origin and fate of ISM in ellipticals is not fully understood. Its origin may be stellar mass-loss, condensation of hot X-ray gas pervading the galaxy, or merging with gas-rich galaxies. Recently, the merger picture receives much attention, since (i) the ionized gas usually is dynamically decoupled from the stars, and (ii) the great variety in radial line-strength gradients found among ellipticals also favours merging as formation process (cf. Gorgas et al. 1990). To study the origin and fate of ISM in ellipticals we are currently undertaking an optical survey of a complete sample of ellipticals (B0T < 12, cf. Goudfrooij et al. 1990). An important result of our extensive CCD imaging program is that a relevant fraction of the sample objects exhibits dust patches within Hα+[NII] line-emitting filaments. This common occurrence can be easily accounted for if the dust and gas have an external origin. In these cases, the extended line emission often has a peculiar distribution and is more sharply peaked at the nucleus than is the stellar continuum. Furthermore, all of these ellipticals exhibit a compact flat-spectrum radio source in their nucleus, suggesting that this nuclear activity also has an external origin. In this respect it would be interesting to know the excitation mechanism of the gas.

We are acquiring a large-area sample of faint, high-latitude carbon star candidates for the study of halo dynamics by using an intermediate-band color system with CCDs in efficient survey modes. Except for one odd dwarf carbon (C) star, G77-61, it has long been assumed that these faint C stars are distant giants. However, we recently demonstrated that three more faint C stars are high proper motion objects, and therefore dwarfs. Now we are completing a proper motion survey of known faint high-latitude C stars to search for additional C dwarfs. The CCD and proper motion surveys together will place significant limits on the space density of C stars, be they dwarfs or giants.

Spectrophotometry of compact (M32-like) dwarf ellipticals (dE) and nucleated dwarf ellipticals (dE,N) in the Virgo and Fornax clusters has been used to investigate the stellar populations of these two types of dwarf galaxy and their relation to larger ellipticals.

The properties of the QSO absorbing systems have been established by several large scale surveys. Observational results concerning the metal content, the distribution of the systems as a function of the redshift, and clustering, lead to two populations of intervening objects: (a) intergalactic clouds (Ly α systems), and (b) galaxies (heavy-element systems). More recently, observations have shown an analogy between QSO absorbers and gas-rich dwarf galaxies (York et al. 1990).

Current evolutionary models of the disk of the Milky Way incorporate radial inflows of (metal-poor) gas that affect the subsequent evolution of the inner disk. Therefore, knowledge of the metallicity of stars and/or gas as a function of radius and time is necessary for a complete understanding of disk evolution. Present data on stellar abundances (determined mostly from iron or iron-peak elements) for the outer disk are shown in Table 1. Results from different approaches have been converging and now give two important results: at RGC ~ 15 kpc, the stellar disk is metal poor by about 0.6 dex relative to the solar neighborhood, and the data are consistent with a constant gradient of −0.07 kpc−1 from the solar neighborhood out to 15 kpc, where the data are becoming very sparse. Interestingly, the gradient of [O/Fe] measured with HII regions is the same. Because Cepheids have proven to be useful probes of abundances at large distances, efforts are underway to find additional distant Cepheids. The last line in the table indicates that stellar abundances in the Small Magellanic Cloud (with ages <5 Gyr) are remarkably similar to those in the outer disk, as had been tenatively suggested in many earlier studies.

It has recently been found that diffuse dwarf elliptical galaxies (dE), as well as low–luminosity normal ellipticals of relatively low surface–brightness, are supported by an anisotropic velocity distribution. New kinematic observations have been obtained of the dwarf elliptical galaxy NGC 185. The velocity dispersion is constant at 28 ± 8 km s–1 between 3″ and 40″, but may increase to about twice that value in the center. We find an upper limit of 10 km s–1 for the rotation along either axis, so that the velocity distribution is anisotropic. The derived M/LB is ~ 3 in solar units.