Aims. We have used FLAMES (the Fibre Large Array Multi Element Spectrograph) at the VLT-UT2 telescope to obtain spectra of a large sample of red giant stars from the inner disk of the LMC, ∼2 kpc from the center of the galaxy. We investigate the chemical abundances of key elements to understand the star formation and evolution of the LMC disk: heavy and light [s-process/Fe] and [α/Fe] give constraints on the time scales of formation of the stellar population. Cu, Na, Sc, and the iron-peak elements are also studied aiming to better understand the build up of the elements of this population and the origin of these elements. We aim to provide a more complete picture of the LMC's evolution by compiling a large sample of field star abundances. Methods. LTE abundances were derived using line spectrum synthesis or equivalent width analysis. We used OSMARCS model atmospheres and an updated line list. The [heavy-s/light-s] ratios are high, showing a slow, increasing trend with metallicity. We were surprised to find an offset for three of the iron-peak elements. We found an offset for the [iron-peak/Fe] ratios of Ni, Cr, and Co, with an underabundant pattern and subsolar values, while Vanadium ratios track the solar value. Copper shows very low abundances in our sample for all metallicities, compatible with those of the Galaxy only for the most metal-poor stars. The overall chemical distributions of this LMC sample indicates a slower star formation history relative to that of the solar neighborhood, with a higher contribution from type Ia supernovae relative to type II supernovae.

The connection between some features of the metallicity gradient in the Galactic disc, best revealed by Open Clusters and Cepheids, and the spiral structure, has been explored. The step-like abrupt decrease in metallicity at 8.5 kpc (with R0= 7.5 kpc, or at 9.5 kpc if R0= 8.5 kpc is adopted) is well explained by the corotation ring-shaped gap in the density of gas, which isolates the internal and external regions of the disc one from the other. This solves the long-standing problem of a lack of understanding of the different chemical characteristics of the inner and outer parts of the disc. The time required to build up the metallicity difference between the two sides of the step is a measure of the minimal lifetime of the present grand-design spiral pattern structure, of the order of 3 Gyr. The plateaux observed on both sides of the step are interpreted in terms of the large-scale radial motion of the stars and of the gas flow induced by the spiral structure. The star formation rate revealed by the density of open clusters is maximum in the Galactic radial range from 6 to 12 kpc (with an exception of a narrow gap at corotation), coinciding with the region where the four-arms mode is allowed to exist. We argue that most of the old open clusters situated at large Galactocentric radii were born in this inner region where conditions more favourable for star formation are found. The ratio of α-elements to Fe of the sample of Cepheids does not vary appreciably with the Galactic radius, which reveals a homogeneous history of star formation. Different arguments are forwarded to show that the usual approximations of chemical evolution models, which assume fast mixing of metallicity in the azimuthal direction and ignore the existence of the spiral arms, are poor ones

The Hyades stream has long been thought to be a dispersed vestige of the Hyades cluster. However, recent analyses of the parallax distribution, of the mass function, and of the actionspace distribution of stream stars have shown it to be rather composed of orbits trapped at a resonance of a density disturbance. This resonant scenario should leave a clearly different signature in the element abundances of stream stars than the dispersed cluster scenario, since the Hyades cluster is chemically homogeneous. Here, we study the metallicity as well as the element abundances of Li, Na, Mg, Fe, Zr, Ba, La, Ce, Nd and Eu for a random sample of stars belonging to the Hyades stream, and compare them with those of stars from the Hyades cluster. From this analysis: (i) we independently confirm that the Hyades stream cannot be solely composed of stars originating in the Hyades cluster; (ii) we show that some stars (namely 2/21) from the Hyades stream nevertheless have abundances compatible with an origin in the cluster; (iii) we emphasize that the use of Li as a chemical tag of the cluster origin of mainsequence stars is very efficient in the range 5500 K ≤ T eff ≤ 6200 K, since the Li sequence in the Hyades cluster is very tight, while at the same time spanning a large abundance range; (iv) we show that, while this evaporated population has a metallicity excess of ∼0.2 dex with respect to the local thin-disc population, identical to that of the Hyades cluster, the remainder of the Hyades stream population has still a metallicity excess of ∼0.06-0.15 dex, consistent with an origin in the inner Galaxy and (v) we show that the Hyades stream can be interpreted as an inner 4:1 resonance of the spiral pattern: this then also reproduces an orbital family compatible with the Sirius stream, and places the origin of the Hyades stream up to 1 kpc inwards from the solar radius, which might explain the observed metallicity excess of the stream population.

A set of 31 oxygen-rich stars has been modelled using corundum and silicate grains. These stars were selected according to their dust-envelope class, as suggested by Little-Marenin and Little in 1990. Then 16 stars classified as Sil were modelled using silicate grains; 10 Broad class stars using corundum (Al 2 O 3 ) grains; and 5`Intermediate' class stars using two kinds of grain simultaneously: corundum and silicate. The temperature of the central stars and some characteristics of their circumstellar envelopes such as their extinction opacities and extensions were determined by fitting the flux curves. The corundum/silicate ratios as well as the energy distributions and temperature laws have been obtained. Based on the authors' results they suggest the existence of chemical and structural evolution of the modelled circumstellar dust shells. The temperature of the central stars and the temperature of the hottest grains decrease from Broad to Intermediate to Sil classes, while the inner radii and optical depths increase in this sequence.