Catálogo de publicaciones - libros

We present preliminary results of lithium abundances in turnoff stars in the open cluster NGC 2506. Some fifty turnoff stars and a few blue stragglers have been observed using the FLAMES facility on the VLT, during half a night from the French Guaranteed Time on this instrument.

We present preliminary results of an extensive low and high-resolution ESO-VLT spectroscopic survey of Subgiant stars in the stellar system ω Centauri. Basing on infrared Ca II triplet lines we derived metallicities and radial velocities for more than 110 stars belonging to different stellar populations of the system. The most metal rich component, the SGB-a, appears to have metallicity [Fe/H] ~ -0.5 . Moreover, SGB-a stars have been found to stray from the dynamical behaviour of the bulk population. Such evidence adds new puzzling questions on the formation and the chemical enrichment history of this stellar system.

Using the NIRSPEC spectrograph at KeckII we have obtained high resolution echelle spectra in the range 1.5.1.8 m of bright giants in Bulge Globular Clusters. We present our abundances of several metals like Fe, C, O and other α-elements.

In this review I briefly discuss the theory of pre-main-sequence (PMS) Li depletion in low-mass (0.075 < M < 1.2) stars and highlight those uncertain parameters which lead to substantial differences in model predictions. I then summarise observations of PMS stars in very young open clusters, clusters that have just reached the ZAMS and briefly highlight recent developments in the observation of Li in very low-mass PMS stars.

We have determined Li abundances (log (Li)) and metallicity ([Fe/H]) in the ~2 Gyr old open cluster NGC 752. The cluster turned out to have a nearly solar Fe content, at variance with previous reports of sub-solar metallicity. The Li distribution vs. effective temperature () of NGC 752 is very similar to those of IC 4651 and NGC 3680, which have similar age but dierent [Fe/H]. Moreover, similarly to the other two clusters, NGC 752 does not show a Li scatter as large as that observed in the solar age cluster M 67. In general, the Li vs. distribution does not appear to depend significantly on metallicity, as shown by the comparison of NGC 752 with IC 4651 and NGC 3680; however, a weak dependence on metallicity might be present when comparing the three clusters in the [log (Li), mass] plane.

We report on preliminary results of VLT/FLAMES observations of the old open clusters NGC 2506, Mel 66 and Cr 261, obtained as part of our Guaranteed Time on this instrument. We focus in particular on the very old cluster Cr 261, one of the oldest open clusters in the Galaxy. We compare the derived Li abundances with those of other old clusters, and we discuss briefly Li depletion on the main-sequence from the age of the Hyades to ~8 Gyr.

We examine outstanding issues in the analysis and interpretation of the halo Li plateau. We show that the majority of very Li-poor halo Li-plateau stars (5 out of 8) have high projected rotation velocities sin between 4.7 and 10.4 km s. Such stars have very different evolutionary histories to Li-normal plateau stars, and hence cannot be included in studies of Li depletion by normal halo dwarfs. Uncertainties in the effective temperature scale for metal-poor stars continue to challenge the analysis of Li.

The observations of light elements (Lithium and Beryllium) in Globular Cluster (GC) stars are reviewed. Light element observations in GC are very powerful tracers of mixing processes in the stellar interior and shed new light on the GC formation history.

Lithium rich K giant stars are found at the luminosity bump of the first ascending red giant branch. The discovery of these stars has given a recent impulse to advances in the theory of stellar evolution. Important connections appeared between thermonuclear processes, rapid mixing, surface activity, rotation and mass loss episodes. How could such lithium surface enrichment be produced? Two concurring scenarios offer completely different explanations: an internal stellar origin and an external one in which planets are engulfed. We will discuss the present state of these matters.

We present initial results from our study of mixing in M Supergiants. C, N and O abundances are measured in five stars. N/C and N/O ratios indicate extensive mixing in excess of the standard models and in support of the rotational models.