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Search Results: 1 - 10 of 371970 matches for " R. G. 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.
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
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.
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).
Metal Abundances of one Hundred Hipparcos Dwarfs
R. G. Gratton,E. Carretta,G. Clementini,C. Sneden
Physics , 1997,
Abstract: Abundances for Fe, O, and the alpha-elements (Mg, Si, Ca, and Ti) have been derived from high resolution spectra of a sample of about one hundred dwarfs with high precision parallaxes measured by HIPPARCOS. The stars have metal abundances in the range -2.5<[Fe/H]<0.2. The observational data set consists of high dispersion (20,000200) spectra collected at the Asiago and McDonald Observatories. The abundance analysis followed the same precepts used by Gratton et al. (1997a) for ~300 field stars and for giants in 24 globular clusters Carretta and Gratton (1997), and includes corrections for departures from LTE in the formation of O lines. Our main results are: 1. the equilibrium of ionization of Fe is well satisfied in late F -- early K dwarfs 2. O and alpha-elements are overabundant by ~0.3dex This large homogeneous data set was used in the derivation of accurate ages for globular clusters.
A stellar population synthesis approach to the Oosterhoff dichotomy
A. Sollima,S. Cassisi,G. Fiorentino,R. G. Gratton
Physics , 2014, DOI: 10.1093/mnras/stu1564
Abstract: We use color-magnitude diagram synthesis together with theoretical relations from non-linear pulsation models to approach the long-standing problem of the Oosterhoff dichotomy related to the distribution of the mean periods of fundamental RR Lyrae variables in globular clusters. By adopting the chemical composition determined from spectroscopic observations and a criterion to account for the hysteresis mechanism, we tuned age and mass-loss to simultaneously reproduce the morphology of both the turn-off and the Horizontal Branch of a sample of 17 globular clusters of the Milky Way and of nearby dwarf galaxies in the crucial metallicity range (-1.9<[Fe/H]<-1.4) where the Oostheroff transition is apparent. We find that the Oosterhoff dichotomy among Galactic globular clusters is naturally reproduced by models. The analysis of the relative impact of the various involved parameters indicates that the main responsibles of the dichotomy are the peculiar distribution of clusters in the age-metallicity plane and the hysteresis. In particular, there is a clear connection between the two main branches of the age-metallicity relation for Galactic globular clusters and the Oosterhoff groups. The properties of clusters' RR Lyrae belonging to other Oostheroff groups (OoInt and OoIII) are instead not well reproduced. While for OoIII clusters a larger helium abundance for a fraction of the cluster's stars can reconcile the model prediction with observations, some other parameter affecting both the Horizontal Branch morphology and the RR Lyrae periods is required to reproduce the behavior of OoInt clusters.
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.
The helium content of globular clusters: NGC6121 (M4)
S. Villanova,D. Geisler,G. Piotto,R. Gratton
Physics , 2012, DOI: 10.1088/0004-637X/748/1/62
Abstract: He has been proposed as a key element to interpret the observed multiple MS, SGB, and RGB, as well as the complex horizontal branch (HB) morphology. Stars belonging to the bluer part of the HB, are thought to be more He rich (\Delta Y=0.03 or more) and more Na-rich/O-poor than those located in the redder part. This hypothesis was only partially confirmed in NGC 6752, where stars of the redder zero-age HB showed a He content of Y=0.25+-0.01, fully compatible with the primordial He content of the Universe, and were all Na-poor/O-rich. Here we study hot blue HB (BHB) stars in the GC NGC 6121 (M4) to measure their He plus O/Na content. We observed 6 BHB stars using the UVES@VLT2 spectroscopic facility. In addition to He, O, Na, and Fe abundances were estimated. Stars turned out to be all Na-rich and O-poor and to have a homogeneous enhanced He content with a mean value of Y=0.29+-0.01(random)+-0.01(systematic). The high He content of blue HB stars in M4 is also confirmed by the fact that they are brighter than red HB stars (RHB). Theoretical models suggest the BHB stars are He-enhanced by \Delta Y=0.02-0.03 with respect to the RHB stars. The whole sample of stars has a metallicity of [Fe/H]=-1.06+-0.02 (internal error). This is a rare direct measurement of the (primordial) He abundance for stars belonging to the Na-rich/O-poor population of GC stars in a temperature regime where the He content is not altered by sedimentation or extreme mixing as suggested for the hottest, late helium flash HB stars. Our results support theoretical predictions that the Na-rich/O-poor population is also more He-rich than the Na-poor/O-rich generation and that a leading contender for the 2^{nd} parameter is the He abundance.
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