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Search Results: 1 - 10 of 223899 matches for " R. Gratton "
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Abundances for Globular Cluster Giants: I. Homogeneous Metallicities for 24 Clusters
E. Carretta,R. G. Gratton
Physics , 1996, DOI: 10.1051/aas:1997116
Abstract: We have obtained high-res, high S/N ratio CCD echelle spectra of 10 bright red giants in 3 GCs (47Tuc, NGC6752 and NGC6397) roughly spanning the range of metallicities of the galactic GC system; they reveal no evidence of star to star variation of [Fe/H] in these 3 GCs. A large set of high quality literature data (EWs from high-res CCD spectra) was re-analyzed in a self-consistent way to integrate our data and derive new [Fe/H] for more than 160 bright red giants in 24 GCs. This set was used to define a new [Fe/H] scale for GCs based on high quality, direct spectroscopic data, on updated model atmospheres from the grid of Kurucz (1992) and on a careful fine abundance analysis (using a common set of atomic and atmospheric parameters for all stars). Given the high internal homogeneity, our new scale supersedes the discrepancies of previous attempts. The internal uncertainty in [Fe/H] is very small: 0.06 dex (24 GCs) on average, that can be interpreted as the mean precision of cluster ranking. Compared to our system, metallicities on the widely used Zinn and West's scale are about 0.10dex higher for [Fe/H]$>-1$, 0.23dex lower for $-1<[Fe/H]<-1.9$ and 0.11dex too high for [Fe/H]$<-1.9$: the non-linearity is significant at 3$\sigma$ level. A quadratic transformation corrects older values to the new scale in the range of our calibrating GCs ($-2.24\le[Fe/H]_{ZW}\le-0.51$). A minor disagreement is found at low [Fe/H] between the metallicity scale based on field and cluster RR Lyrae's (via a new calibration of the $\Delta$S index) and our new GCs metallicities, that could be ascribed to non-linearity in the [Fe/H]$-\Delta$S relationship. The impact of new metallicities on major astrophysical problems is exemplified through the Oosterhoff effect in the classical pair M3 and M15.
Galactic orbits of stars with planets
M. Barbieri,R. G. Gratton
Physics , 2001, DOI: 10.1051/0004-6361:20020011
Abstract: We have reconstructed the galactic orbits of the parent stars of exoplanets. For comparison, we have recalculated the galactic orbits of stars from the Edvardsson et al.(1993) catalog. A comparison between the two samples indicates that stars with planets are not kinematically peculiar. At each perigalactic distance stars with planets have a metallicity systematically larger than the average for the comparison sample. We argue that this result favors scenarios where the presence of planets is the cause of the higher metallicity of stars with planets.
Simple, low-cost flow controllers for time averaged atmospheric sampling and other applications
Gere,J.I.; Gratton,R.;
Latin American applied research , 2010,
Abstract: in air sampling is often necessary to limit the air inflow into an initially void vessel in order to extend the filling time. this requires devices characterized by very low, known and stable conductance. likewise devices may also be used to control the gas flow between two vessels, or the outflow from gas containers. depending on the use, they are called flow controllers, controlled leaks or, as in our case, inflow regulators. in this work we describe simple and cheap inflow regulators designed to collect air samples in 0.5 l vessels with filling times of many days. these inflow regulators may be adjusted within a wide impedance range and proved to be very stable under laboratory conditions and reasonably stable even under much harder operating conditions in field experiments.
Simple, low-cost flow controllers for time averaged atmospheric sampling and other applications
J.I. Gere,R. Gratton
Latin American applied research , 2010,
Abstract: In air sampling is often necessary to limit the air inflow into an initially void vessel in order to extend the filling time. This requires devices characterized by very low, known and stable conductance. Likewise devices may also be used to control the gas flow between two vessels, or the outflow from gas containers. Depending on the use, they are called flow controllers, controlled leaks or, as in our case, inflow regulators. In this work we describe simple and cheap inflow regulators designed to collect air samples in 0.5 L vessels with filling times of many days. These inflow regulators may be adjusted within a wide impedance range and proved to be very stable under laboratory conditions and reasonably stable even under much harder operating conditions in field experiments.
Abundances of light elements in metal-poor stars. I. Atmospheric parameters and a new T_eff scale
R. G. Gratton,E. Carretta,F. Castelli
Physics , 1996,
Abstract: We present atmospheric parameters for about 300 stars of different chemical composition, whose spectra will be used to study the galactic enrichment of Fe and light elements. These parameters were derived using an homogenous iterative procedure, which considers new calibrations of colour-\teff\ relations for F, G and K-type stars based on Infrared Flux Method (IRFM) and interferometric diameters for population~I stars, and the Kurucz (1992) model atmospheres. We found that these calibrations yield a self-consistent set of atmospheric parameters for \teff$>4400$~K, representing a clear improvement over results obtained with older model atmospheres. Using this \teff-scale and Fe equilibrium of ionization, we obtained very low gravities (implying luminosities incompatible with that expected for RGB stars) for metal-poor stars cooler than 4400~K; this might be due either to a moderate Fe overionization (expected from statistical equilibrium calculations) or to inadequacy of Kurucz models to describe the atmospheres of very cool giants. Our \teff\ scale is compared with other scales recently used for metal-poor stars; it agrees well with those obtained using Kurucz (1992) models, but it gives much larger \teff's than those obtained using OSMARCS models (Edvardsson et al. 1993). This difference is attributed to the different treatment of convection in the two sets of models. For the Sun, the Kurucz (1992) model appears to be preferable to the OSMARCS ones because it better predicts the solar limb darkening; furthermore, we find that our photometric \teff's for metal-poor stars agree well with both direct estimates based on the IRFM, and with \teff's derived from H$\alpha$\ wings when using Kurucz models.
The Chemical Evolution of the Galaxy: the two-infall model
C. Chiappini,F. Matteucci,R. Gratton
Physics , 1996, DOI: 10.1086/303726
Abstract: In this paper we present a new chemical evolution model for the Galaxy which assumes two main infall episodes for the formation of halo-thick disk and thin disk, respectively. We do not try to take into account explicitly the evolution of the halo but we implicitly assume that the timescale for the formation of the halo was of the same order as the timescale for the formation of the thick disk. The formation of the thin-disk is much longer than that of the thick disk, implying that the infalling gas forming the thin-disk comes not only from the thick disk but mainly from the intergalactic medium. The timescale for the formation of the thin-disk is assumed to be a function of the galactocentric distance, leading to an inside-out picture for the Galaxy building. The model takes into account the most up to date nucleosynthesis prescriptions and adopts a threshold in the star formation process which naturally produces a hiatus in the star formation rate at the end of the thick disk phase, as suggested by recent observations. The model results are compared with an extended set of observational constraints. Among these constraints, the tightest one is the metallicity distribution of the G-dwarf stars for which new data are now available. Our model fits very well these new data. We show that in order to reproduce most of these constraints a timescale $\le 1$ Gyr for the (halo)-thick-disk and of 8 Gyr for the thin-disk formation in the solar vicinity are required. We predict that the radial abundance gradients in the inner regions of the disk ($R< R_{\odot}$) are steeper than in the outer regions, a result confirmed by recent abundance determinations, and that the inner ones steepen in time during the Galactic lifetime.
Distances and ages of globular clusters using Hipparcos parallaxes of local subdwarfs
R. G. Gratton,E. Carretta,G. Clementini
Physics , 1998,
Abstract: We discuss the impact of Population II and Globular Cluster (GCs) stars on the derivation of the age of the Universe, and on the study of the formation and early evolution of galaxies, our own in particular. The long-standing problem of the actual distance scale to Population II stars and GCs is addressed, and a variety of different methods commonly used to derive distances to Population II stars are briefly reviewed. Emphasis is given to the discussion of distances and ages for GCs derived using Hipparcos parallaxes of local subdwarfs. Results obtained by different authors are slightly different, depending on different assumptions about metallicity scale, reddenings, and corrections for undetected binaries. These and other uncertainties present in the method are discussed. Finally, we outline progress expected in the near future.
The Helium content of Globular Clusters: light element abundance correlations and HB morphology. I. NGC6752
S. Villanova,G. Piotto,R. G. Gratton
Physics , 2009, DOI: 10.1051/0004-6361/200811493
Abstract: Helium has been proposed as the key element to interpret the observed multiple main sequences (MS), subgiant branches (SGB) and red giant branches (RGB), as well as the complex horizontal branch (HB) morphology in Globular Clusters (GC). However, up to now, He was never directly measured in suitable GC stars (8500
Homogeneous photometry and metal abundances for a large sample of Hipparcos metal-poor stars
G. Clementini,R. G. Gratton,E. Carretta,C. Sneden
Physics , 1998, DOI: 10.1046/j.1365-8711.1999.02098.x
Abstract: Homogeneous photometric data (Johnson V, B-V, V-K, Cousins V-I and Stromgren b-y), radial velocities, and abundances of Fe, O, Mg, Si, Ca, Ti, Cr and Ni are presented for 99 stars with high precision parallaxes measured by the Hipparcos satellite. These data have been previously used to assist the derivation of accurate distances and ages of galactic globular clusters. Magnitudes and colours for the programme stars were obtained combining and standardizing carefully selected literature data available in the Simbad data-base and V and B-V values measured by the Hipparcos/Tycho mission. Comparison of colours for our targets suggests: (i) ground-based and Tycho B-V's agree well for colours bluer than 0.75 mag, but have a lot of scatter for redder colours; (ii) the Hipparcos V-I colours have a very large scatter and a zero point offset of +0.02 mag compared to the literature values. The programme stars have metal abundances in the range -2.5<[Fe/H]<0.2. The spectroscopic observational data set consists of high dispersion (15,000200) spectra obtained at the Asiago and McDonald Observatories for 66 stars. The analysis is carried out following the same precepts used in previous papers of this series and includes corrections for departures from LTE in the formation of the O lines. The main results are: (i) the equilibrium of ionization of Fe is well satisfied in late F-early K-dwarfs; (ii) Oxygen and alpha-elements are overabundant by about 0.3 dex. This large homogeneous abundance data set has been used to recalibrate the abundance scales of Schuster and Nissen (1989, AA, 221, 65), Carney et al. (1994, AJ, 107, 2240), and Ryan and Norris (1991, AJ, 101, 1835).
The metallicity of Palomar 1
A. Rosenberg,G. Piotto,I. Saviane,A. Aparicio,R. Gratton
Physics , 1997, DOI: 10.1086/300221
Abstract: Palomar 1 is a peculiar galactic globular cluster, suspected to be younger than the bulk of the Galactic halo objects. However, such a low age can be confirmed only after a reliable determination of the metallicity. In the present paper, we use the equivalent widths (W) of the Ca II triplet on medium resolution spectra in order to determine the metal content of Pal 1. From the comparison of the luminosity corrected W's in four stars of Palomar 1 with those of a sample of stars in each of three calibration clusters (M2, M15, and M71), we derive [Fe/H]=-0.6+/-0.2 on the Zinn & West (1984) scale or [Fe/H]=-0.7+/-0.2 on the Carretta & Gratton (1997) scale. We also obtain a radial velocity Vr=-82.8+/-3.3 Km/s for Pal 1.
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