We study the chromospheric evolution of low mass Pre Main Sequence Stars using the Call line at 8542 å as a probe of stellar activity. Our sample consists mostly of classical T Tauri Stars (CTTS - WHα > 10 må) and a few weak TTS (WTTS - WHα < 10 må) with simultaneous observations from 4500 å up to 9000 å. After the proper corrections for veiling, we measure the chromospheric flux of the Ca II emission core and compare it with Hα fluxes, veiling and the stellar age. Our main conclusions are the following:

1 - There is no trend between λ8542 and Hα fluxes for the whole set of stars, however a clear correlation is found for WTTS. This last finding is expected if both CaII and Hα are mainly formed in the atmospheric environment which might be the case for WTTS.

2 - Chromospheric flux does not correlate with stellar age for CTTS.

3 - We present a correlation between the accretion rate of the circumstellar disk (veiling) and chromospheric fluxes (see below). Accretion through magnetic loops crossing the circumstellar disk may provide additional emitting area to the CaII fluxes. This last finding demonstrates the strong linking between the disk and the stellar magnetic surface fluxes.

We determine the correlation between spectral and morphological type for distant galaxies, defined by optical - near-infrared colors and image structure. As a point of departure, rest-frame colors of field galaxies observed to redshifts of 0.35 are shown to follow the trend in the UVK plane seen by Aaronson (1978) for local galaxies. Correlations are found between the deviations from this trend and absolute magnitude, compactness, and surface brightness. This works provides the foundation for investigations of galaxy structure at cosmologically interesting distances (Bershady, 1992).

A compilation of integrated UBV photometry for 147 LMC clusters was published by van den Bergh (1981). We have increased by more than a factor 4 the sample of star clusters with integrated UBV photometry in the LMC, totalling now 624. The observations were carried out with the CTIO 0.6m and the 2.14m CASLEO telescopes. The sample is essentially complete up to V≍ 13 but also contains fainter clusters up to V ≍ 14.5. The (U-B)vs(B-V) diagram has provided interesting results such as the detection of the helium flash gap in the color evolution (Bica et al. 1991). We have derived equivalent SWB types and study their spatial distribution. The spatial extent (Fig. 1) varies from diameter ≍ 6° for the disk of HII region clusters (SWB 0) to ≍ 11° at SWB VI, reaching 15° or more for SWB VII, which is probably a halo distribution. The coordinates in degrees are centered at NGC1928, a cluster close to the geometric center of the Bar. Groups SWB V or younger present inhomogeneities in their distributions. Group IVA (Fig. 1b) presents a strong concentration of clusters at the western extremity of the Bar (WEB). Group V is already at the 1-2 Gyr range and presents two clumps, one also coincident with the WEB and another with a structure known as the de Vaucouleurs' arm (Lynga and Westerlund, 1963). Fig. 2 shows a remarkable phenomenum in the Bar: SWB I clusters (dots) are shifted by an angle of 19° clockwise relative to those which trace the Bar, i. e. SWB II ones (triangles). This shift is centered at the WEB. These evidences point to an important mass concentration at the western extremity of the Bar. The age difference between SWB I and II and the spatial scale of the angular shift would indicate shock induced star formation for the SWB I group. The spatial distribution of age groups reveals phenomena which will certainly help to better understand the star formation history of the LMC and its dynamical evolution.

BVR photometry of stellar fields in the NW region of supergiant shell LMC4 has been performed. We present preliminary results on the age of the stellar populations in this part of the LMC.

We report new calculations of M/LB ratios for galaxies, which are considered to be closed homogeneous systems. A Salpeter's law was assumed for the initial mass function. The star formation rate was considered to be proportional to the amount of available gas. Spiral galaxies were considered to be constituted by two components: the bulge and the disk. Each component is characterized by its own timescale for the gas convertion into stars. We used the bulge-to-disk average light ratios by Koppen & Arimoto (1990) in order to derive the integral properties for the different morphological types. In our computatios we used the grid of models by Maeder & Meynet (1988) for stellar masses in the range 100 < m < 1 and the evolutionary tracks compiled by Bruzual (1982) for lower masses.

Using evolutionary tracks computed by Alongi et al.(1991) for the two chemical compositions Z=0.020, Y=0.28 and Z=0.008, Y=0.25 we constructed theoretical isochrones from 20 · 109 yr to 3 · 106 yr.

We analyze the HR diagram of LMC supergiant stars using a new set of evolutionary models for massive stars.

In this contribution we give a progress report for our work intending to approach in a more complete way the problem of a quantitative calibration of the Mg2 index (Faber et al. 1977, A.J., 82, 941; Buzzoni, Gariboldi & Mantegazza 1991 submitted to A.J.). We have first investigated empirically the relationship between the index and the fundamental parameters for a wide set of Galactic standard stars deriving a detailed calibration for dwarfs and giants. This allowed to build up synthetic models for stellar populations exploring Mg2 in the galaxies with varying overall distinctive parameters of the populations.

A series of simple analytical models for the evolution a galactic disk have been constructed. General solutions can be obtained under the assumption of pure radial gas flows, a star formation rate proportional to the nth power of the surface gas density, a constant IMF, and the instantaneous recycling approximation. Models with small radial flow velocities in the range 0.05 to 0.1 km s–1 and an initial exponential surface mass density can reproduce, for galactocentric radii larger than about 5 kpc, the Galactic 16O abundance gradient and the present surface gas distribution.

The outermost gaseous disk of our Galaxy is warped (Burton and te Lintel Hekkert 1986; Wouterloot et al. 1990), and a stellar component may be present as well (Miyamoto et al. 1988; Djorgovski and Sosin 1989). If so, the outermost edges of the Galaxy's disk may be more readily studied using the warp than relying upon the terribly crowded and nearly to completely opaque mid-plane directions. The study of the outer disk, and of the warp, is crucial for several reasons, including estimating the core radius of the dark halo (Sparke and Casertano 1988), the disk metallicity gradient (which is a signature of the history of star formation efficiency), and of the ages of the oldest stars elsewhere in the Galactic disk (which can be used to determine if the disk grew slowly outward, as suggested by Gunn 1987).

We have begun a program to detect the old stellar population of the outer disk of the Galaxy by imaging fields at selected longitudes and latitudes where the H I maps of Burton and te Lintel Hekkert (1986) suggest the warp has reached maximal elevations above the plane as seen from the solar perspective. We selected control fields to estimate foreground (disk plus thick disk) contamination to have the same longitudes as the warp fields but opposite latitudes. All fields were found to have relatively clear lines of sight, based on the appearance of galaxies on the ESO/SRC or POSS charts. Only the southern hemisphere warp appears to have such clean lines of sight.

Four 8′ x 8′ warp fields and their four control fields were imaged through V and Ic filters with the Tek 1024 × 1024 CCD at the f/2.7 focus of the CTIO 4-meter reflector. Seeing was sub-arcsecond and 30 minutes (V) and 18 minutes (I c) cumulative exposures were obtained, with limiting magnitudes Vlim > 24 in all cases. A total of 15,829 stars were measured in the four warp fields, and 11,722 stars in the four control fields. The excess is due partially to the Sun's location above mid-plane, but is mostly due to our detection of the outer disk's main sequence. The control fields were used to statistically subtract out the foreground components from the warp fields, leaving well-defined main sequences and in one field, a clear main sequence turn-off. We claim the old stellar warp population has been detected, and plan to continue its study using detailed reddening vs. distance maps, and searches for followed by spectroscopy of the outer disk red giants.

We applied multivariate statistical techniques to study the possible systematic differences between the field and cluster ellipticals, in the framework of the fundamental plane (FP) of elliptical galaxies on the samples of elliptical galaxies from the general field and loose groups, and from rich clusters. The data set used here are the ellipticals from the “7 Samurai” (Burstein et al. 1987; Davies et al. 1987), limited to cz < 6000 km/s. There are 55 galaxies in the cluster sample (mostly from the Coma and Virgo clusters), and 57 galaxies in the field sample, which may include galaxies in loose and poor groups. We have also used the data set from Djorgovski & Davis (1987), with very similar results.

CCD échelle spectra in the wavelength range λλ 478 – 580 nm, were obtained at the 3.6m telescope at ESO for the star III-17 of the metal-rich globular cluster NGC 6553. This cluster was chosen because it is a relatively close bulge globular cluster, and it presented the possibility to be among the most metal-rich ones in the Galaxy.

We present two wide sets of SHB models which have been computed for combinations of evolutionary parameters adequate to the GGC system. The effect of the enhancement of α-elements upon theoretical predictions related to the “Sandage effect” and globular cluster ages is also discussed.

A semi-empirical scenario is suggested which tentatively accounts for the (otherwise conflicting) slopes of the several available RR Lyrae absolute magnitude calibrations. This scenario implies a very small dependence of cluster ages upon metallicity.

CCD photometry in the Washington system for a sample of dwarf elliptical galaxies in the Fornax cluster is presented. The observations were done in October 1989 with the CTIO 0.9 and 1.5m telescopes; the galaxies being selected from the Fornax Cluster Catalogue.

Observations of QSO absorbers at Z ~ 2 suggest that they represent a population of primeval, dynamically evolving systems which turn into the local galaxy population.

We present a detailed study of the warmers phase in a starburst as a scenario for AGN (Terlevich & Melnick 1985). A numerical code (“ET”) was developed to compute the time evolution of the total spectrum of an ionizing cluster. ET is based in Maeder & Meynet (1988) and Maeder (1990) tracks and in model atmospheres. The ionizing spectra were input into the photoionization code Aangaba (Gruenwald & Viegas 1991) to calculate the nebular emission for a grid of models covering different densities, ionization parameters, metallicities and cluster ages. The emission line spectrum go through an “active phase” beginning at ≍ 3 Myrs, which lasts for about 3 Myrs, when the population of warmers disapears. Metallicities between 1/2 and 2 Z⊙ are necessary to generate AGN line ratios. The lower limmit is due to the fact that fewer warmers occur in metal poor regions (due to the smaller mass loss rates) and even if some do occur the line ratios would resemble an HII galaxy rather than a Seyfert 2 or a LINER. The upper limmit is set by strong line cooling of the emission nebula. The chemical pollution of the gas by the processed stellar winds from the very massive stars produce straightforwardly a N overabundance as that observed in many AGN. In Figure 1a and b, we show the process of heavy element enrichment as a function of time parametrized by γ—a quantity which measures the mass ejected by stars relative to the mass of gas surrounding the stellar association (see Cid Fernandes et al. 1991). The chemical yields were taken from Maeder (1990) models.

We have applied two slightly different formulations of the Baade-Wesselink (B-W) method: (i) the Infrared Flux (IF) method (Fernley et al 1990a), and (ii) the Surface Brightness (SB) method (Cacciari et al 1989a), to derive the absolute magnitude of the field RR Lyrae stars UU Cet, RV Phe and W Tuc. The photometric (BVRIJHK) and radial velocity data used in the analysis have been published in Cacciari et al (1987, 1989b, 1991) and Clementini et al (1990). The Walraven photometry used in the IF method is by Lub (1977). Typical accuracies for individual data points are ±0.01-0.02 mag for the magnitudes and colours, and ±1-2km/sec for the radial velocity data. Excluding RV Phe, which may be affected by Blazhko effects, the results obtained from the IF and SB methods are rather consistent especially when the V–K colour is used in the SB method. For details see Cacciari et al. (1991).

Recent studies (Gilmore and Reid 1983, Gilmore and Wyse 1985, 1986) concerning the halo stars have showed that a new structure, the thick disc, must be retained to describe the Galaxy. We suppose here that this structure exists and we deduce some results concerning the difference between the halo stars and the thick disc stars. The first assumption is that the inclination of the orbits of the halo stars must be randomly distributed. This must be true if the halo has been formed from a merger or from a non-rotating protogalactic cloud. Using the catalog of Carney et al.1990 (804 stars selected for their high velocities) and Norris' one 1986 (386 stars selected for their poor metallicity [Fe/H] < 0.6), we calculate the orbital inclinations of all the stars and we display them on an histogram (fig.1). The two samples (built on different criterions) show a flat distribution from 180° to 65° (fig. 1-a). From 65° to 0° there is clearly a mixing of halo stars and thick disc stars. If, as we assume, the halo stars have a flat distribution from 180° to 0°, we can separate the two population using another kinematical parameter: the eccentricity. We calculate it by using a logarithmic potential that gives a flat rotation curve. Now if we keep all the stars with an inclination between 180° and 65° and the stars with an inclination between 65° and 0° but with an eccentricity e > 0.6, we obtain a flat distribution of the inclination between 180° to 0° (fig. 1-b). In that case all these stars may be considered as halo stars. Plotting [Fe/H] against orbital inclination we obtain a gaussian symetrical distribution centered on −1.7. It seems that the apocenters of the selected halo stars reach a limit for 45 kpc.

We have obtained slit spectra of 299 candidate early G-dwarf stars from the objective prism and photometric surveys of Corbally and Garrison (1988a, 1988b) in the North and South Galactic Pole regions between 90 and 1700 pc from the sun. The slit spectra from reticon detectors on the S.A.A.O. 1.9 m and the Steward Observatory 2.3 m telescopes have a resolution of 2 å and a signal- to- noise of around 35.