We analyse the evolution with redshift of the radial gradient of oxygen abundances in spiral disks resulting from our MULCHEM chemical evolution models, computed for galaxies of different sizes or masses, studying the relationships between the gradients and galaxy characteristics as the stellar mass, the size, the gas fraction or the star formation rate for z < 4.

In this work, we report physical parameters and abundances derived for a sample of 15 high extinction planetary nebulae located in the inner 2° of the Galactic bulge, based on low dispersion spectroscopy secured at the SOAR telescope using the Goodman spectrograph. The new data allow us to extend our database including older, weaker objects that are at the faint end of the planetary nebulae luminosity function. The data provide chemical compositions for PNe located in this region of the bulge to explore the chemical enrichment history of the central region of the Galactic bulge. The results show that the abundances of our sample are skewed to higher metallicities than previous data in the outer regions of the bulge. This can indicate a faster chemical enrichment taking place at the Galactic centre.

We have obtained a large sample of PN with accurately determined helium abundances, as well as abundances of several heavy elements. The nebulae are located in the solar neighbourhood, in the galactic bulge, disk and anticentre, and in the Magellanic Clouds. The abundances are analyzed both in terms of the nucleosynthesis of intermediate mass stars and the chemical evolution of the host galaxies. In particular, correlations between the He/H ratio and the abundances of N and O are used as constraints of the nucleosynthetic processes occurring in the progenitor stars.

Open cluster remnants (OCRs) are fundamental objects to investigate open cluster dissolution processes (e.g., Bica et al. 2001; Carraro 2002; Pavani et al. 2003; Carraro et al. 2007; Pavani & Bica 2007). They are defined as poorly populated concentrations of stars, with enough members to show evolutionary sequences in colour–magnitude diagrams (CMDs) as a result of the dynamical evolution of an initially more massive physical system. An OCR is intrinsically poorly populated, which makes its differentiation from field-star fluctuations difficult. Among the possible approaches to establish the nature of OCRs, we adopted CMD analysis combined with a robust statistical tool applied to 2mass data. In addition, photometry is the main information source available for possible OCRs (POCRs). We developed a statistical diagnostic tool to analyse the CMDs of POCRs and verify them as physical systems, explore membership probabilityies taking into account field contamination and derive age, distance and reddening values in a self-consistent way. We present the results of our analysis of 88 POCRs that are part of a larger sample that is widely distributed across the sky, with a significant density contrast of bright stars compared to the Galactic field. The 88 objects are projected onto low-density Galactic fields, at relatively high latitudes (|b| > 15°). Studies of larger POCR samples will provide a better understanding of OCR properties and constraints for theoretical models, including new insights into the evolution of open clusters and their dissolution rates. The results of this ongoing survey will provide a general picture of these fossil stellar systems and their connection to Galactic-disk evolution.

Planetary nebulae (PN) are favourite objects in the study of stellar populations both in the Galaxy and in other objects as well, due to their high brightness and the considerable range in their kinematic properties and chemical composition. In this work, we take into account available data samples of PN in Local Group galaxies and compare the derived information from different objects, particularly regarding their chemical composition, space distribution, kinematics, and the PN luminosity function.

Bipolar Planetary Nebulae (hereafter PNe) are almost all of the Type I and have the most massive progenitors among PNe. They are mostly found at very low galactic latitudes and thus are expected to follow rather closely the differential rotation. Moreover since they are strongly suspected to constitute the youngest population among PNe, then some bipolars may have not achieved a complete galactic revolution yet and we can reasonably suppose that some signs of their recent galactic evolution still exist.

We present lithium abundances for nine chromospherically young, kinematically old late-type stars. The data support the interpretation that these objects can be formed during the coalescence of a short-period binary.

Two aspects of the chemical evolution of 4He in the Galaxy are considered on the basis of a sample of disk planetary nebulae by the application of corrections due to the contamination of 4He from the progenitor stars. First, the He/H radial gradient is analyzed, and then, the helium to heavy element enrichment ratio is determined for metallicities up to the solar value.

We study the local population of AGB stars, and determine their densities and scale heights according to their masses, following the classification scheme proposed by Ortiz and Maciel (1994, A&A 287, 552), which discriminates the precursors of type I, II, and III planetary nebulae. The formation (or death) rates of AGB stars are determined by means of stellar evolution models, and are compared to the formation rates of planetary nebulae from the literature. A good agreement is found between these rates; however, the results are very sensitive to the adopted model, especially regarding the time spent at the thermal-pulse AGB phase.(FAPESP, CNPq)

In the planetary nebulae (PNe) studies, their distance determination remains a confusing issue. The statistical distance scales cannot be regarded as reliable, and there are few PNe with individual distance determinations. In this work, we present a distance scale based on the method of individual distance determination by Méndez et al. (1988, A&A 190, 113).

Recent work on the PN classification scheme initially proposed by Peimbert has shown that objects of types I, II, and III have markedly different properties such as central star mass, height distribution relative to the galactic plane, and kinematic behaviour. This fact can be used in order to determine distances, especially for the younger disk objects.

The synthesis and properties of two polycarbosilanes that have essentially a “SiH2CH2” composition is described. One of these polymers is a highly branched hydridopolycarbosilane (HPCS) derived from Grignard coupling of CI3SiCH2CI followed by LiAIH4 reduction. This synthesis is amenable to large scale production and we are exploring applications of HPCS as a source of SiC coatings and its allyl-derivative, AHPCS, as a matrix source for SiC- and C-fiber-reinforced composites. These polymers thermoset on heating at 200-400 °C (or at 100 °C with a catalyst) and give near stoichiometric SiC with low O content in ca. 80% yield on pyrolysis to 1000 °C. The second method involves ring-opening polymerization of 1,1,3,3-tetrachlorodisilacyclobutane and yields a high molecular weight, linear polymer that can be reduced to [SiH2CH2]n (PSE), the monosilicon analog of polyethylene. In contrast to high density polyethylene which melts at 135 °C, PSE is a liquid at room temperature which crystallizes at ca. 5 °C. On pyrolysis to 1000 °C, PSE gives stoichiometric, nanocrystalline, SiC in virtually quantitative yield. The polymer-to-ceramic conversion was examined for PSE by using TGA, mass spec, solid state NMR, and IR methods yielding information regarding the cross-linking and structural evolution processes. The results of these studies of the polymer-to-ceramic conversion process and our efforts to employ the AHPCS polymer as a source of SiC matrices are described.

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.

A revision is made of the classification scheme of planetary nebulae proposed by Peimbert (1978), taking into account the observed heavy-element abundances and radial abundance gradients (Faúndez-Abans and Maciel, 1986; 1987a). A subdivision is proposed of type II PN into the subtypes a and b, according to their nitrogen abundances relative to hydrogen (Faundez-Abans and Maciel, 1987b). Type IIa PN show a higher enrichment in nitrogen than type IIb, which present a nitrogen abundance close to the value of population I objects and a radial N/H gradient as well.

Most distances of planetary nebulae are known roughly within a factor of 2 or larger, except for some special objects for which the uncertainty can be as low as 50%. In the present work both IUE interstellar Lyman alpha profiles and 21 cm HI line surveys are used to infer the distances of four planetary nebulae.