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

Principais doenças e parasitas que ocorrem no rebanho ovino de Roraima.

Ovino. Consideracoes gerais. Aquisicao de animais. Instalacoes. Aprisco. Bebedouros. Pediluvio. Local de isolamento. Doencas infecto-contagiosas. Linfadenite caseosa. Pododermatite. Necrotica. Mamite. Ectima contagioso. Ceraconjuntivite. Ceratite. Tumores de olhos. Doencas parasitarias. Miiase. Helmintos gastrintestinais.bitstream/item/64864/1/CPAFRR-DOCUMENTOS-01-PRINCIPAIS-DOENCAS-E-PARASITAS-QUE-OCORREN-NO-REBANHO-OVINO-DE-RORAIMA-FL-11.pd

On the peculiar red clump morphology in the open clusters NGC 752 and NGC 7789

The red clump stars in the open cluster NGC 752 present a peculiar distribution in the colour-magnitude diagran (CMD): the clump is observed to present a faint extension, slightly to the blue of the main concentration of clump stars. We point out that a similar structure is present in the CMD of NGC 7789, and discuss their possible origins. This feature may be understood as the result of having, at the same time, stars of low-mass which undergo the helium-flash, and those just massive enough for avoiding it. The ages of both clusters are compatible with this interpretation. Similar features can be produced in theoretical models which assume a non-negligible mass spread for clump stars, of about 0.2 Mo. However, one can probably exclude that the observed effect is due to the natural mass range of core helium burning stars found in single isochrones, although present models do not present the level of detail necessary to completely explore this possibility. Also the possibility of a large age spread among cluster stars can be refuted on observational grounds. We then suggest a few alternatives. This spread may be resulting either from star-to-star variations in the mass-loss rates during the RGB phase. Alternatively, effects sush as stellar rotation or convective core overshooting, could be causing a significant spread in the core mass at He-ignition for star of similar mass. Finally, we point out that similar effects could also help to understand the distribution of clump stars in the CMDs of the clusters NGC 2660 and NGC 2204

Basic physical parameters of a selected sample of evolved stars

We present the detailed spectroscopic analysis of 72 evolved stars, including the [Fe/H] determination for the whole sample. These metallicities, together with the Teff values and the absolute V magnitude derived from Hipparcos parallaxes, are used to estimate basic stellar parameters (ages, masses, radii, (B-V)o and log g using theoretical isochrones and a Bayesian estimation method. The (B-V)o values so estimated turn out to be in excellent agreement with the observed (B-V), confirming the reliability of the (Teff,(B-V)o) relation used in the isochrones. The estimated diameters have been compared with limb darkening-corrected ones measured with independent methods, finding an agreement better than 0.3 mas within the 1-10 mas interval. We derive the age-metallicity relation for the solar neighborhood; for the first time such a relation has been derived from observations of field giants rather than from open clusters and field dwarfs and subdwarfs. The age-metallicity relation is characterized by close-to-solar metallicities for stars younger than ~4 Gyr, and by a large [Fe/H] spread with a trend towards lower metallicities for higher ages. We find that the [Fe/H] dispersion of young stars (less than 1 Gyr) is comparable to the observational errors, indicating that stars in the solar neighbourhood are formed from interstellar matter of quite homogeneous chemical composition. The three giants of our sample which have been proposed to host planets are not metal rich, what is at odds with those for main sequence stars. However, two of these stars have masses much larger than a solar mass so we may be sampling a different stellar population from most radial velocity searches for extrasolar planets. We also confirm that the radial velocity variability tends to increase along the RGB.Comment: 17 pgs, 19 fig

AGB stars in the Magellanic Clouds. III. The rate of star formation across the SMC

This article compares the Ks magnitude distribution of Small Magellanic Cloud asymptotic giant branch stars obtained from the DENIS and 2MASS data with theoretical distributions. Theoretical Ks magnitude distributions have been constructed using up-to-date stellar evolution calculations for low and intermediate-mass stars, and in particular for thermally pulsing asymptotic giant branch stars. Separate fits of the magnitude distributions of carbon- and oxygen-rich stars allowed us to constrain the metallicity distribution across the galaxy and its star formation rate. The Small Magellanic Cloud stellar population is found to be on average 7-9 Gyr old but older stars are present at its periphery and younger stars are present in the direction of the companion galaxy the Large Magellanic Cloud. The metallicity distribution traces a ring-like structure that is more metal rich than the inner region of the galaxy. The C/M ratio discussed in Paper I is a tracer of the metallicity distribution only if the underlying stellar population is of intermediate-age.Comment: 8 pages, 7 figures, accepted by A&

AGB stars in the Magellanic Clouds II. The rate of star formation across the LMC

Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO)This article compares the distribution of Ks magnitudes of Large Magellanic Cloud (LMC) asymptotic giant branch (AGB) stars obtained from the DENIS and 2MASS data with theoretical distributions. These have been constructed using up-to-date stellar evolution calculations for low and intermediate-mass stars, and in particular for thermally pulsing AGB stars. A fit of the magnitude distribution of both carbon- and oxygen-rich AGB stars allowed us to constrain the metallicity distribution across the LMC and its star formation rate (SFR). The LMC stellar population is found to be on average 5 − 6 Gyr old and is consistent with a mean metallicity corresponding to Z = 0.006. These values may however be affected by systematic errors in the underlying stellar models, and by the limited exploration of the possible SFR histories. Instead our method should be particularly useful for detecting variations in the mean metallicity and SFR across the LMC disk. There are well defined regions where both the metallicity and the mean-age of the underlying stellar population span the whole range of grid parameters. The C/M ratio discussed in paper I is a tracer of the metallicity distribution if the underlying stellar population is older than about a few Gyr. A similar study across the Small Magellanic Cloud is given in paper III of this series.Peer reviewe

Herbaceous Vegetation Dynamic after Cut and Burn Shrub Plants in Southern Brazil

A savanna area at Serra do Sudeste in southern Brazil, was studied during four years to evaluate the influence of cutting and burning shrub plants on dynamic of herbaceous vegetation. The cover of each species in 44 permanent quadrats (0,25 m2) and in each area (cut and burned) was evaluated. The results suggested that grasses and legumes were favored by cutting shrub plants. Burning favored forbs in the first years after disturbance and retarded the development of native forage species desirable for grazing

Young and intermediate-age massive star clusters

An overview of our current understanding of the formation and evolution of star clusters is given, with main emphasis on high-mass clusters. Clusters form deeply embedded within dense clouds of molecular gas. Left-over gas is cleared within a few million years and, depending on the efficiency of star formation, the clusters may disperse almost immediately or remain gravitationally bound. Current evidence suggests that a few percent of star formation occurs in clusters that remain bound, although it is not yet clear if this fraction is truly universal. Internal two-body relaxation and external shocks will lead to further, gradual dissolution on timescales of up to a few hundred million years for low-mass open clusters in the Milky Way, while the most massive clusters (> 10^5 Msun) have lifetimes comparable to or exceeding the age of the Universe. The low-mass end of the initial cluster mass function is well approximated by a power-law distribution, dN/dM ~ M^{-2}, but there is mounting evidence that quiescent spiral discs form relatively few clusters with masses M > 2 x 10^5 Msun. In starburst galaxies and old globular cluster systems, this limit appears to be higher, at least several x 10^6 Msun. The difference is likely related to the higher gas densities and pressures in starburst galaxies, which allow denser, more massive giant molecular clouds to form. Low-mass clusters may thus trace star formation quite universally, while the more long-lived, massive clusters appear to form preferentially in the context of violent star formation.Comment: 21 pages, 3 figures. To appear as invited review article in a special issue of the Phil. Trans. Royal Soc. A: Ch. 9 "Star clusters as tracers of galactic star-formation histories" (ed. R. de Grijs). Fully peer reviewed. PDFLaTeX, requires rspublic.cls style fil