Constraining the third dredge-up via carbon stars in the Magellanic Clouds

We use the available data for Magellanic Cloud carbon stars to constrain the efficiency of the third dredge-up process in TP-AGB models. We show that star counts in LMC clusters provide quite stringent limits to the lifetime of the C-star phase, with a duration between 2 and 3 Myr for stars in the mass range from 1.5 to 2.8 Msun. Together with the luminosity functions of field C stars, this information allows us to re-calibrate the third dredge-up parameters log T_b_dred and lambda in TP-AGB models that include variable molecular opacities (Marigo 2002). Preliminary results are presented here.Comment: in proceedings of the St. Luc conference ``CNO in the Universe'', eds. C. Charbonnel, D. Schaerer, & G. Meynet, ASP Conference Series, in pres

Stellar populations in the Galactic bulge

AIMS:The aim of this paper is to study the characteristics of the stellar populations and the metallicity distribution in the Galactic bulge. We study the entire stellar population, but also retrieve information using only the red clump stars. METHODS: To study the characteristics of the stellar populations and the metallicity distribution in the Galactic bulge, we compared the output of the galaxy model TRILEGAL, which implements the Binney et al. (1997) bulge model, with observations from 2MASS and OGLE-II. A minimisation procedure has been set up to retrieve the best fitting model with different stellar populations and metallicity distributions. RESULTS: Using the TRILEGAL code we find that the best model resembling the characteristics of the Galactic bulge is a model with the distance to the Galactic centre $R_0 = 8.7\pm^{0.57}_{0.43}$ kpc, the major axis ratios of the bar $1:\eta:\zeta = 1 : 0.68\pm_{0.19}^{0.05} : 0.31\pm_{0.04}^{0.06}$, and the angle between the Sun-centre line and the bar $\phi = 15\deg\pm_{12.7}^{13.3}$. Using these parameters the best model is found for a burst of 8 Gyr, although it is almost indistinguishable from models with ages of 9 and 10 Gyr. The metallicity distribution found is consistent with metallicity distributions in the literature based on spectroscopic results.Comment: A&A accepte

Clump stars in the Solar Neighbourhood

Hipparcos data has allowed the identification of a large number of clump stars in the Solar Neighbourhood. We discuss our present knowledge about their distributions of masses, ages, colours, magnitudes, and metallicities. We point out that the age distribution of clump stars is ``biased'' towards intermediate-ages. Therefore, the metallicity information they contain is different from that provided by the local G dwarfs. Since accurate abundance determinations are about to become available, these may provide useful constraints to chemical evolution models of the local disc.Comment: 6 pages, proc. of the Sept. 20-24, 1999 Vulcano Workshop "The chemical evolution of the Milky Way: stars vs. clusters", eds. F. Matteucci, F. Giovanell

Evolution of zero-metallicity massive stars

We discuss the evolutionary properties of primordial massive and very massive stars, supposed to have formed from metal-free gas. Stellar models are presented over a large range of initial masses (8 Msun <= Mi <= 1000 Msun), covering the hydrogen- and helium-burning phases up to the onset of carbon burning. In most cases the evolution is followed at constant mass. To estimate the possible effect of mass loss via stellar winds, recent analytic formalisms for the mass-loss rates are applied to the very massive models (Mi >= 120 Msun).Comment: Invited talk at IAU Symp. 212, ``A Massive Star Odyssey, from Main Sequence to Supernova'', K.A. van der Hucht, A. Herrero, C. Esteban (eds.), 7 pages, 5 postscript figure

The Garching-Bonn Deep Survey (GaBoDS) Wide-Field-Imaging Reduction Pipeline

We introduce our publicly available Wide-Field-Imaging reduction pipeline THELI. The procedures applied for the efficient pre-reduction and astrometric calibration are presented. A special emphasis is put on the methods applied to the photometric calibration. As a test case the reduction of optical data from the ESO Deep Public Survey including the WFI-GOODS data is described. The end-products of this project are now available via the ESO archive Advanced Data Products section.Comment: 6 pages, 3 figures, proceedings of ESO Calibration Workshop 200

Extended star formation history of the star cluster NGC 2154 in the Large Magellanic Cloud

The colour-magnitude diagram (CMD) of the intermediate-age Large Magellanic Cloud star cluster NGC 2154 and its adjacent field has been analysed using Padova stellar models to determine the cluster´s fundamental parameters and its star formation history. Deep BR CCD photometry, together with synthetic CMDs and integrated luminosity functions, has allowed us to infer that the cluster experienced an extended star formation period of about 1.2 Gyr, which began approximately 2.3 Gyr ago and ended 1.1 Gyr ago. However, the physical reality of such a prolonged period of star formation is questionable, and could be the result of inadequacies in the stellar evolutionary tracks themselves. A substantial fraction of binaries (70 per cent) seems to exist in NGC 2154.Fil: Baume, Gustavo Luis. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Carraro, Giovanni. Università di Padova; ItaliaFil: Costa, E.. Universidad de Chile; ChileFil: Mendez, R. A.. Universidad de Chile; ChileFil: Girardi, L.. Osservatorio Astronomico di Padova; Itali

The nature of the giant diffuse non-thermal source in the A3411-A3412 complex

VLA deep radio images at 1.4 GHz in total intensity and polarization reveal a diffuse non-thermal source in the interacting clusters A3411 - A3412. Moreover a small-size low power radio halo at the center of the merging cluster A3411 is found. We present here new optical and X-ray data and discuss the nature and properties of the diffuse non-thermal source. We suggest that the giant diffuse radio source is related to the presence of a large scale filamentary structure and to multiple mergers in the A3411-A3412 complex.Comment: 7 pages, 6 figures, accepted for publication in MNRA

Coupling emitted light and chemical yields from stars: a basic constraint to population synthesis models of galaxies

In this paper we emphasize the close connection between the chemical and spectrophotometric evolution of stellar systems: Chemical yields from stars correspond to a precise fraction of their emitted light. We translate this concept quantitatively. Starting from simple stellar populations, we derive useful analytical relations to calculate the stellar fuel consumption (emitted light) as a function of basic quantities predicted by stellar models, i.e. the mass of the core and the chemical composition of the envelope. The final formulas explicate the relation between integrated light contribution (total or limited to particular evolutionary phases), chemical yields and stellar remnants. We test their accuracy in the case of low- and intermediate-mass stars, and indicate the way to extend the analysis to massive stars. This formalism provides an easy tool to check the internal consistency between the different stellar inputs adopted in galaxy models: The fuel computed by means of the analytical formulas (corresponding to a given set of chemical yields) should be compared to the exact values given by the luminosity integration along the stellar evolutionary tracks or isochrones (corresponding to a given set of spectrophotometric models). Only if both estimates of the fuel are similar, the stellar inputs can be considered self-consistent in terms of their energetics. This sets an important requirement to galaxy models, also in consideration of the fact that different sources of input stellar data are frequently used to model their spectro-photometric and chemical evolution.Comment: 17 pages, 7 figures, 3 tables, accepted for publication in A&