This thesis investigates the reliability of the D - σ and D - Mg(_2) distance indicators for elliptical galaxies. In particular, we test whether these empirical correlations are affected by differences in the stellar populations of elliptical galaxies associated with their environment. Our final goal is to assess the reality of the large peculiar velocities measured in the local universe as derived using these relations. Our galaxy sample includes ellipticals located in high and low galaxian density regions. The cluster sample is mainly based on new observations of ellipticals in Coma, Abell 2199 and Abell 2634, while the field sample is selected from the literature. For each galaxy we list measurements of the effective radius, effective surface brightness, photometric diameter, velocity dispersion and magnesium index. We first develop a phenomenological framework of galaxy properties that describes all empirical correlations for spheroidal systems, including both giant and dwarf ellipticals. Despite the wide variety of observed correlations, we show that only three provide independent information on the overall structure and metal abundamce of these systems. These three 'fundamental relations' can be expressed in terms of physical variables assuming that both the galaxy mass-to-light ratio (M/L) and the ratio between the effective radii of the dark and luminous matter distributions (R/R(_e)) depend on the mass of the galaxy. The structure of elliptical galaxies is then determined by both the virial theorem and aa intrinsic mass-radius relation. The metallicity of their stellar populations in turn is a function of the galaxy velocity dispersion alone. We compare this framework with a theoretical scenario of galaxy formation that combines the hierarchical clustering and the galactic wind models. This picture provides a consistent explanation of the fundamental relations of all elliptical galaxies assuming R/R(_e) ~ constant for dwarf ellipticals while, for giant ellipticals, we find that R/R(_e) must be a decreasing function of galaxy mass. Our framework strongly suggests that this dependence of R/Re on galaxy mass is the only difference between the two galaxy families. We then study the effect of the environment on the D - σ and D - Mg(_2) distance indicators by comparing ellipticals that reside in the core of the Coma cluster with those in the cluster halo. By studying the variations within one cluster, we avoid the difficulty of decoupling effects induced by distance errors from those due to real environmental differences. We find that ellipticals located in the outer, low-density areas of the Coma cluster have D diameters that are, on average at a given σ or Mg(_2), 10% or 30% larger than their counterparts in the cluster core. Using galaxy evolution models we demonstrate that this effect is consistent with the presence of an intermediate age stellar component in some halo ellipticals. We use the framework of galaxy properties in combination with galaxy evolution models to design a new age-independent distance indicator for ellipticals. This relation allows distances to be estimated with an uncertainty of ~20% and is independent of age/environmental effects. We also offer a simple explanation of the trends observed in the D - σ versus D -Mg(_2) residuals plot. Finally, we use our new distance indicator to readdress previous measurements of peculiar velocities in both the cluster and field environments. For Abell 2199 and Abell 2634 ellipticals, we show that the new distance estimates are in good agreement with those derived using the D - σ and D - Mg(_2) relations. Both clusters are found to have peculiar velocities which are not significantly different from zero. However, when the new distance indicator is applied to field ellipticals we find that the age/environmental effect translates into a spurious positive component of the local peculiar velocity field as derived from D - σ and D - Mg(_2). This effect may provide a simple explanation of the large positive peculiar motions observed towards the Great Attractor.

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Over the past two decades, much has been learned about the atmospheric structure of the pulsating Mira-type variable stars from computer modeling and the analysis of ultraviolet and visible wavelength spectra. This talk reports on the first set of infrared spectra taken of these stars under high dispersion with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope. A sample of 25 galactic Miras was observed in the 10-37 micron spectral regime anywhere from two to several times during their pulsation cycle. Many of the stars observed show marked changes in overall flux levels as a function of phase. We are able to identify many strong emission lines from neutral and singly ionized metals and emission features due to silicate and carbon dusts and molecular constituents. This work was financially supported through a NASA Spitzer grant for Program GO 50717.

This work aims at studying the influence of the stellar mass-loss, resulting from the stellar evolution, on the dynamics of massive star clusters. The emphasis has been put on the mass-loss by low-mass and intermediate-mass stars (m < 8 Mo) that form, at the end of their life, a planetary nebula. The expansion speed of gas released by these stars is lower than the escape speed from sufficiently massive star clusters, and the gas can be retained by the cluster. For modelling of the gas hydrodynamics, a simple sticky-particles method was used. To carry out simulations in which gaseous and stellar particles mutually interact through their gravity, substantial modifications had to be realized in the N-body codes Nbody6 and Hermit. For the sake of comparing the influence of stellar mass-loss and relaxation processes, which are happening in the simplified model, two types of simulations were performed: one with the formation of gaseous particles and the other consisting of purely stellar component. The simulations in which the gas component was present showed out a significantly different evolution in the central part of the cluster than those in which the presence of gas was not considered.