Catálogo de publicaciones - libros

From VLT/UT2 GIRAFFE GTO, we performed a lithium abundance survey along the red giant branch of the metal-rich globular cluster 47 TUC (NGC 104), in order to investigate the efficiency of extra mixing occurring at the RGB bump.

We have derived beryllium abundances for all Li-rich giant stars known in the literature and visible from the southern hemisphere and 10 other Li-normal giants, aimed to investigate the origin of the Lithium in the Li-rich giants. In particular, we test the predictions of the engulfment scenario proposed by Siess & Livio (1999) where the engulfment of a brown dwarf or one or more giant planets would lead to a simultaneous enrichment of Li and Be. We show that regardless their nature, none of the stars observed in this work was found to have beryllium. Using simple dilution arguments we show that the engulfment of an external object as the source of Li enrichment is ruled out by the the Li and Be abundance data. The present results favor the idea that Li has been produced in the interior of the stars and brought up to the surface by a Cameron-Fowler mechanism.

This aims to be an overview of what detailed observations of individual stars in nearby dwarf galaxies may teach us about galaxy evolution. This includes some early results from the DART (Dwarf Abundances and Radial velocity Team) Large Programme at ESO. This project has used 2.2m/WFI and VLT/FLAMES to obtain spectra of large samples of individual stars in nearby dwarf spheroidal galaxies and determine accurate abundances kinematics. These results can be used to trace the formation and evolution of nearby galaxies from the earliest times to the present.

We present abundance studies of dwarf irregular galaxies in the Local Group which appear to have undergone very slow chemical evolution since they have low nebular abundances, but have had ongoing star formation over the past 15 Gyr. They are too distant for red giant abundance analyses to examine the details of their chemical evolution. However the isolated, bright blue supergiants do allow us to determine their present-day iron abundances. We compare the [α/Fe] ratios in the dwarf irregulars to those from recent analyses of red giant branch stars in dwarf spheroidal galaxies, damped Lyman α absorption systems, and the latest model predictions.

This paper reviews recent abundance results of local dSph giants. All dSph systems seem to show evidence of slow star formation rates, compared to the Milky Way, based on the most metal-rich stars exhibiting low [even-Z/Fe] ratios and high [sprocess/ r-process] ratios. The most metal-poor stars in the Draco, Ursa Minor, Sextans and Sculptor dSphs seem to show a split in their light even-Z, Mg and O, and the heavier even-Z, Ca and Ti, abundance ratios, where the light even-Z are halo like and the heavier even-Z elements exhibit sub-halo abundance ratios. This split remains a mystery. A review of the first dSph abundance results from FLAMES+GIRAFFE on the VLT and HRS on the Hobby-Eberly Telescope shows that the study of chemical evolution of dSph galaxies is rapidly moving out of infancy and into an era requiring very large surveys and/or targeted studies.

In this contribution, we present detailed chemical abundances on two stellar systems presently believed to be undergoing tidal merging with the Milky Way (MW). The first one is the Sagittarius Dwarf Spheroidal (Sgr dSph, [5]), a massive (10 M) dSph orbiting along a very short period (< ) almost polar orbit inside the Halo, along which is slowly dissolving in a huge stellar stream [6]. The second object is the recently discovered, and still controversial CMa dwarf galaxy [8], a heavily degraded overdensity embedded in the MW thick disk, believed to be the residual of an in-plane accretion of an object of mass comparable to the Sgr dSph. This interpretation is still controversial, since [9] claimed that the structure almost disappears, when disk warping and/or flaring is properly taken into account in the modeling of the MW contribution.

We present a new calibration of the CaII triplet as metallicity indicator based in 4 globular and 11 open clusters which cover a range of metallicity - 2≤[Fe/H]≤+0.1 and age 13≤(Age/Gyr)≤0.25. We use it to derive the metallicity distribution in two fields situated at 5 and 8 degrees from the center of the LMC. We show that the mean [Fe/H] of the LMC field decreases as we move away from the bar.

We have measured the chemical composition of 14 stars in the Sagittarius dwarf spheroidal galaxy (Sgr dSph) using high S/N Keck HIRES echelle spectra. For the Sgr dSph stars with [Fe/H]≥–1 the abundances are highly unusual, showing a striking enhancement in heavy s-process elements, increasing with [Fe/H], deficiencies of the α-elements (O, Si, Ca, and Ti), deficiencies of Al and Na, and deficiencies of the odd-numbered iron-peak elements Mn and Cu. Our abundances suggest that the composition of the metal-rich Sgr dSph stars is dominated by the ejecta of an old, metal-poor population, including products of AGB stars and type Ia supernovae (SN).

Hierarchical structure formation scenarios postulate that large galaxies form by continuous accretion of smaller subunits. Consequently, the properties of large galaxies (esp. of their old populations) should also reflect the properties of the accreted building blocks. The few surviving dwarf galaxies, in turn, hold the fossil record of the early conditions of galaxy evolution. The Local Group provides a convenient nearby laboratory to study the resulting predictions in detail.

We present metallicities for 487 red giants in the Carina dwarf spheroidal (dSph) galaxy that were obtained from FLAMES low-resolution Ca triplet (CaT) spectroscopy. We find a mean [Fe/H] of –1.91 dex with an intrinsic dispersion of 0.25 dex, whereas the full spread in metallicities is at least one dex. The analysis of the radial distribution of metallicities reveals that an excess of metal poor stars resides in a region of larger axis distances. These results can constrain evolutionary models and are discussed in the context of chemical evolution in the Carina dSph.