arXiv subjects

E. Carretta

Publications and source records attributed to E. Carretta.

88 records · Page 5Linked to original sources

arXiv2000

Metal Abundances of Red Clump Stars in Open Clusters: I. NGC 6819

We present an analysis of high dispersion spectra (R~40000) of three red clump stars in the old open cluster NGC 6819. The spectra were obtained with SARG, the high dispersion spectrograph of the Telescopio Nazionale Galileo. The spectra were analyzed using both equivalent widths measured with an automatic procedure, and comparisons with synthetic spectra. NGC 6819 is found to be slightly metal-rich ([Fe/H]= +0.09 +/-0.03, internal error); there are no previous high resolution studies to compare with. Most element-to-element abundance ratios are close to solar; we find a slight excess of Si, and a significant Na overabundance. Our spectra can also be used to derive the interstellar reddening towards the cluster, by comparing the observed colours with those expected from line excitation: we derive E(B-V)=0.14 +/-0.04, in agreement with the most recent estimate for this cluster.

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arXiv2000

CU Comae: a new field double-mode RR Lyrae, the most metal poor discovered to date

We report the discovery of a new double-mode RR Lyrae variable (RRd) in the field of our Galaxy: CU Comae. CU Comae is the sixth such RRd identified to date and is the most metal-poor RRd ever detected. Based on BVI CCD photometry spanning eleven years of observations, we find that CU Comae has periods P0=0.5441641 +/-0.0000049d and P1=0.4057605 +/-0.0000018d. The amplitude of the primary (first-overtone) period of CU Comae is about twice the amplitude of the secondary (fundamental) period. The combination of the fundamental period of pulsation P0 and the period ratio of P1/P0=0.7457 places the variable on the metal-poor side of the Petersen diagram, in the region occupied by M68 and M15 RRd's. A mass of 0.83 solar masses is estimated for CU Comae using an updated theoretical calibration of the Petersen diagram. High resolution spectroscopy (R=30,000) covering the full pulsation cycle of CU Comae was obtained with the 2.7 m telescope of the Mc Donald Observatory, and has been used to build up the radial velocity curve of the variable. Abundance analysis done on the four spectra taken near minimum light (phase: 0.54 -- 0.71) confirms the metal poor nature of CU Comae, for which we derive [Fe/H]=-2.38 +/-0.20. This value places this new RRd at the extreme metal-poor edge of the metallicity distribution of the RR Lyrae variables in our Galaxy.

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arXiv1999

Old Open Clusters as Tracers of Galactic Evolution

Old open clusters are useful tools to study the Galactic disk properties, both present and past. Populous samples with properties determined both accurately and homogenously are necessary to draw reliable conclusions. We present here our recent work in the field and introduce and discuss results on metallicities.

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arXiv1999

Light curves and metal abundances of RR Lyrae variables in the bar of the Large Magellanic Cloud

The Large Magellanic Cloud (LMC) is widely considered a corner-stone of the astronomical distance scale, however a difference of 0.2-0.3 mag exists in its distance as predicted by the "short" and "long" distance scales. Distances to the LMC from Population II objects are founded on the RR Lyrae variables. We have undertaken an observational campaign devoted to the definition of the average apparent luminosity and to the study of the mass-metallicity relation for RR Lyraes in the bar of the LMC. These are compared with analogous quantities for cluster RR Lyraes. The purpose is to see whether an intrinsic difference in luminosity, possibly due to a difference in mass, might exist between field and cluster RR Lyraes, which could be responsible for the well-known dichotomy between "short" and "long" distance scales. Preliminary results are presented on the V and B-V light curves, the average apparent visual magnitude, and the pulsational properties of 102 RR Lyrae in the bar of the LMC, observed at ESO in January 1999. The photometric data are accurately tied to the Johnson photometric system. Comparison is presented with the photometry of RR Lyraes in the bar of the LMC obtained by the MACHO collaboration (Alcock et al. 1996). Our sample includes 9 double-mode RR Lyraes selected from Alcock et al. (1997) for which an estimate of the metal abundance from the DeltaS method is presented.

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arXiv1999

The Luminosity Function of M3

We present a high precision, large sample luminosity function (LF) for the Galactic globular cluster M3. With a combination of ground based and Hubble Space Telescope data we cover the entire radial extent of the cluster. The observed LF is well fit by canonical standard stellar models from the red giant branch (RGB) tip to below the main sequence turnoff point. Specifically, neither the RGB LF-bump nor subgiant branch LF indicate any breakdown in the standard models. On the main sequence we find evidence for a flat initial mass function and for mass segregation due to the dynamical evolution of the cluster.

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arXiv1998

Homogeneous photometry and metal abundances for a large sample of Hipparcos metal-poor stars

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 (15,000 200) 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).

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arXiv1998

Distances and ages of globular clusters using Hipparcos parallaxes of local subdwarfs

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.

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arXiv1998

RR Lyrae variables in the globular cluster M3 (NGC5272). I. BVI CCD photometry

New BVI CCD photometry is presented for 60 RR Lyrae variables in the globular cluster M3. Light curves have been constructed and ephemerides have been (re)-derived for all of them. Four stars (i.e. V29, V136, V155 and V209), although recognized as variables, had no previous period determinations. Also, the period derived for V129 is significantly different from the one published by Sawyer-Hogg (1973). Light curve parameters, i.e. mean magnitudes, amplitudes and rise-times, have been derived. The discussion of these results in the framework of the stellar evolution and pulsation theories will be presented in a forthcoming paper.

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arXiv1997

Ages of Globular Clusters from Hipparcos Parallaxes of Local Subdwarfs

We report here initial but still strongly conclusive results on absolute ages of galactic globular clusters (GGCs). This study is based on high precision trigonometric parallaxes from the HIPPARCOS satellite coupled with accurate metal abundances ([Fe/H], [O/Fe], and [α/Fe]) from high resolution spectroscopy for a sample of about thirty subdwarfs. Systematic effects due to star selection (Lutz-Kelker corrections to parallaxes) and to the possible presence of undetected binaries in the sample of bona fide single stars are examined, and appropriate corrections are estimated. They are found to be small for our sample. The new data allowed us to reliably define the absolute location of the main sequence (MS) as a function of metallicity. These results are then used to derive distances and ages for a carefully selected sample of nine globular clusters having metallicities determined from high dispersion spectra of individual giants according to a procedure totally consistent with that used for the field subdwarfs. Very precise and homogeneous reddening values have also been independently determined for these clusters. We find that HIPPARCOS parallaxes are smaller than the corresponding ground-based measurements leading, in turn, to longer distance moduli ($\sim 0.2$ mag) and younger ages ($\sim 2.8$ Gyr). The relation between the zero age HB (ZAHB) absolute magnitude and metallicity for the nine programme clusters turns out to be: $M_V(ZAHB) = (0.22\pm 0.09)([Fe/H]+1.5) + (0.49\pm 0.04)$. The corresponding LMC distance modulus is $(m-M)_0=18.60\pm 0.07$. The age of the bona fide old globular clusters (Oosterhoff II and BHB) based on the absolute magnitude of the turn-off, a theoretically robust indicator, is: Age = {11.8^{+2.1}_{-2.5}} Gyr.

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arXiv1997

Binary stars in globular clusters: detection of a binary sequence in NGC 2808?

A CCD V,V-I colour-magnitude diagram (CMD) of the Galactic globular cluster NGC 2808 has been obtained with the ESO-NTT, reaching down to V ~ 24. The highly populated Main Sequence (MS) presents a significant broadening redward to the MS ridge line, larger than expected from photometric errors alone, which could be interpreted as due to binary candidates.

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arXiv1997

Distances and Absolute Ages of Galactic Globular Clusters from Hipparcos Parallaxes of Local Subdwarfs

High precision trigonometric parallaxes from the HIPPARCOS satellite and accurate metal abundances ([Fe/H], [O/Fe], and [alpha/Fe]) from high resolution spectroscopy for about 30 local subdwarfs have been used to derive distances and ages for a carefully selected sample of nine globular clusters. We find that HIPPARCOS parallaxes are smaller than the corresponding ground-based measurements leading, to a longer distance scale (~ 0.2 mag) and to ages ~ 2.8 Gyr younger. The relation between the zero age horizontal branch (ZAHB) absolute magnitude and metallicity for the nine programme clusters is : Mv(ZAHB) = (0.22 +/- 0.09)([Fe/H]+1.5) + (0.48 +/- 0.04) The corresponding Large Magellanic Cloud distance modulus is (m-M)o=18.61 +/- 0.07. The age of the bona fide old globular clusters (Oosterhoff II and Blue Horizontal Branch) based on the absolute magnitude of the turn-off is: Age = 11.8 (+2.1/-2.5) Gyr The present age of globular clusters does no longer conflict with standard inflationary models for the Universe.

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arXiv1997

Metal Abundances of one Hundred Hipparcos Dwarfs

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 (20,000 200) 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.

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arXiv1996

The stellar population of the Globular Cluster M 3. II: CCD photometry of additional 10,000 stars

We present BVI CCD photometry for more than 10,000 stars in the innermost region (0.3' < r <~ 4') of the globular cluster M 3. When added to the previous photographic photometry by Buonanno et al. (1994) reaching as far as r ~ 7', this results in an homogeneous data-set including about 19,000 stars measured in this cluster, which can be now regarded as one of the main templates for stellar population studies. Our main results, from the new colour-magnitude diagrams (CMD), are: i) Completeness has been achieved for all objects at V<=18.6 and 0.3'<r<7'. ii) Our new independent photometric calibration is redder than the old calibration (Sandage and Katem 1982) in the blue range, and bluer in the reddest part of the CMD. This colour term has important consequences on some issues (e.g. metallicity, period-shift effect, etc.). iii) The metallicity derived from the photometric indicators (B-V)_{0,g} and ΔV_{1.4} is [Fe/H]~-1.45, i.e. significantly higher than the value -1.66 generally used so far. This is in very good agreement with the most recent high resolution spectroscopic studies of individual stars. iv) There is indication of the presence of a very faint and blue extension of the HB, although scarcely populated. v) The population ratios of HB,RGB and AGB stars confirm the value previously found for the helium content, i.e. Y=0.23+/-0.02. vi) There is a significant population of blue straggler stars, although the exact number must await HST data for a better resolution in the very central regions.

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arXiv1996

Abundances for Globular Cluster Giants: I. Homogeneous Metallicities for 24 Clusters

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$σ$ 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 $Δ$S index) and our new GCs metallicities, that could be ascribed to non-linearity in the [Fe/H]$-Δ$S relationship. The impact of new metallicities on major astrophysical problems is exemplified through the Oosterhoff effect in the classical pair M3 and M15.

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arXiv1996

Abundances of light elements in metal-poor stars. I. Atmospheric parameters and a new T_eff scale

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$α$\ wings when using Kurucz models.

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arXiv1995

The composition of HB stars : RR Lyrae variables

We used moderately high-resolution, high S/N spectra to study the chemical composition of 10 field ab-type RR Lyrae stars. A new temperature scale was determined from literature Infrared Flux Method measures of subdwarfs and the Kurucz (1992) model atmospheres, and used to calibrate colors for both dwarfs and RR Lyraes. The applicability of Kurucz (1992) model atmospheres in the analysis of RR Lyraes at minimum light was analyzed: we found that they are able to reproduce colors, excitation and ionization equilibria as well as the wings of Halpha. We derived abundances for 21 species. The metal abundances of the program stars span the range -2.50<[Fe/H]<+0.17$. Lines of most elements are found to form in LTE conditions. Fe lines satisfy very well the excitation and ionization equilibria. RR Lyraes share the typical abundance pattern of other stars of similar [Fe/H]: alpha-elements are overabundant by about 0.4dex and Mn is underabundant by about 0.6dex in stars with [Fe/H]<-1. Significant departures from LTE are found only for a few species. We used our new [Fe/H] abundances, as well as values from Butler and coworkers (corrected to our system), and from high resolution spectroscopy of globular clusters giants, to obtain a new calibration of the DeltaS index: [Fe/H]= -0.194(\pm 0.011)DeltaS -0.08(\pm 0.18) and to update the metallicity calibration of the Ca II K line index: [Fe/H]= 0.65(\pm 0.17)W'(K) -3.49(\pm 0.39). Finally, our new metallicity scale was used to revise the [Fe/H] dependence of the absolute magnitude of RR Lyrae stars, Mv: Mv = 0.20(\pm 0.03)[Fe/H] + 1.06(\pm 0.04).

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