Elena Valenti

High-Resolution Infrared Spectroscopy of the Old Open Cluster NGC 6791

We report abundance analysis for six M giant members of the old open cluster NGC 6791, based on infrared spectroscopy (1.5-1.8 μm) at R = 25,000, using the NIRSPEC spectrograph at the Keck II telescope. We find the iron abundance [Fe/H] = +0.35 ± 0.02, confirming the supersolar metallicity of this cluster as derived from optical medium-high resolution spectroscopy. We also measure C, O, and other α-element abundances, finding a roughly solar value of [α/Fe] and [C/Fe] = -0.35. Our approach constrains [O/Fe] especially well, on the basis of the measurement of a number of OH lines near 1.6 μm; we find [O/Fe] = -0.07 ± 0.03. The solar value of [α/Fe] is in contrast to the composition of similar stars in the Galactic bulge. We also find a low value of 12C/13C ≈ 10, confirming the presence of extramixing processes during the red giant phase of evolution, up to supersolar metallicities.

Red giant branch in near-infrared colour–magnitude diagrams – I. Calibration of photometric indices

We present new high-quality near-infrared photometry of 10 Galactic globular clusters spanning a wide metallicity range (-2.12 ≤ [Fe/H] ≤ -0.49): five clusters belong to the halo (NGC 288, 362, 6752, M15 and M30) and five (NGC 6342, 6380, 6440, 6441 and 6624) to the bulge. By combining J, H and K observations with optical data, we constructed colour-magnitude diagrams in various planes: (K, J-K), (K, V-K), (H, J-H) and (H, V-H). A set of photometric indices (colours, magnitudes and slopes) describing the location and the morphology of the red giant branch (RGB) have been measured. We have combined this new data set with those collected by our group over the last 5 years, and here we present an updated calibration of the various RGB indices in the Two-Micron All-Sky Survey photometric system, in terms of the cluster metallicity.

The RR Lyrae period–K-luminosity relation for globular clusters: an observational approach

The period-metallicity-K-band luminosity (PL K ) relation for RR Lyrae stars in 15 Galactic globular clusters and in the Large Magellanic Cloud (LMC) globular cluster Reticulum has been derived. It is based on accurate near-infrared (K) photometry combined with Two-Micron All-Sky Survey (2MASS) and other literature data. The PL K relation has been calibrated and compared with the previous empirical and theoretical determinations in literature. The zero point of the absolute calibration has been obtained from the K magnitude of RR Lyr whose distance modulus has been measured via trigonometric parallax with Hubble Space Telescope (HST). Using this relation, we obtain a distance modulus to the LMC of (m - M) 0 = 18.54 ± 0.15 mag, in good agreement with recent determinations based on the analysis of Cepheid variable stars.

Red giant branch in near-infrared colour-magnitude diagrams - II. The luminosity of the bump and the tip

We present new empirical calibrations of the red giant branch (RGB) bump and tip based on a homogeneous near-infrared data base of 24 Galactic globular clusters. The luminosities of the RGB bump and tip in the J, H and K bands and their dependence on the cluster metallicity have been studied, yielding empirical relationships. By using recent transformations between the observational and theoretical planes, we also derived similar calibrations in terms of bolometric luminosity. Direct comparisons between updated theoretical models and observations show an excellent agreement. The empirical calibration of the RGB tip luminosity in the near-infrared passbands presented here is a fundamental tool to derive distances to galaxies far beyond the Local Group, in view of using the new ground-based adaptive optics facilities and, in the near future, the James Webb Space Telescope.

Na-O anticorrelation and horizontal branches - V. The Na-O anticorrelation in NGC 6441 from Giraffe spectra

Aims. We present an analysis of FLAMES-Giraffe spectra for several bright giants in NGC 6441, to investigate the presence and extent of the Na-O anticorrelation in this anomalous globular cluster. Methods. The field of NGC 6441 is very crowded, with severe contamination by foreground (mainly bulge) field stars. Appropriate membership criteria were devised to identify a group of 25 likely cluster members among the about 130 stars observed. Combined with the UVES data obtained with the same observations (Gratton et al. 2006), high dispersion abundance analyses are now available for a total of 30 stars in NGC 6441, 29 of them having data for both O and Na. The spectra were analyzed by a standard line analysis procedure; care was taken to minimize the impact of the differential interstellar reddening throughout the cluster, and to extract reliable information from crowded, and moderately high S/N (30–70), moderately high resolution (

An Empirical Calibration of the Mixing-Length Parameter α*

R\sim 23\,000

Na-O anticorrelation and HB III. The abundances of NGC 6441 from FLAMES-UVES spectra

) spectra. Results. NGC 6441 has the typical abundance pattern seen in several other globular clusters. It is very metal-rich ([Fe/H] = -0.34 ± 0.02 ± 0.04 dex). There is no clear sign of star-to-star scatter in the Fe-peak elements. The α -elements Mg, Si, Ca, and Ti are overabundant by rather large factors, suggesting that the cluster formed from material enriched by massive core collapse SNe. The O-Na anticorrelation is well defined, with about 1/4 of the stars being Na-rich and O-poor. One of the stars is a Ba-rich and moderately C-rich star. Such stars are rare in globular clusters. Conclusions. The distribution of [Na/O] ratios among RGB stars in NGC 6441 appears similar to the distribution of colors of stars along the horizontal branch. The fraction of Na-poor, O-rich stars found in NGC 6441 agrees well with that of stars on the red horizontal branch of this cluster (in both cases about 80%), with a sloping distribution toward lower values of [O/Na] (among RGB stars) and bluer colors (among HB stars).

Near-infrared photometry of four metal-rich bulge globular clusters: NGC 6304, 6569, 6637 and 6638

We present an empirical calibration of the mixing-length free parameter α based on a homogeneous infrared database of 28 Galactic globular clusters spanning a wide metallicity range (-2.15 < [Fe/H] < -0.2). Empirical estimates of the red giant effective temperatures have been obtained from infrared colors. Suitable relations linking these temperatures to the cluster metallicity have been obtained and compared to theoretical predictions. An appropriate set of models for the Sun and Population II giants has been computed by using both the standard solar metallicity (Z/X)☉ = 0.0275 and the most recently proposed value (Z/X)☉ = 0.0177. We find that when the standard solar metallicity is adopted, a unique value of α = 2.17 can be used to reproduce both the solar radius and the Population II red giant temperature. Conversely, when the new solar metallicity is adopted, two different values of α are required: α = 1.86 to fit the solar radius and α ≈ 2.0 to fit the red giant temperatures. However, it must be noted that regardless the adopted solar reference, the α-parameter does not show any significant dependence on metallicity.

Near-IR photometry of five Galactic Globular Clusters

Aims. The aim of the present work is to determine accurate metallicities for a group of red giant branch stars in the field of the bulge Globular Cluster NGC 6441. This is the third paper in a series resulting from a large project aimed at determining the extent of the Na-O anticorrelation among Globular Cluster stars and exploring its relationship with HB morphology. Methods. We present an LTE abundance analysis of these objects, based on data gathered with the FLAMES fiber facility and the UVES spectrograph at VLT2. Results. Five of the thirteen stars observed are members of the cluster. The average Fe abundance for these five stars is [Fe/H] = −0.39 ± 0.04 ± 0.05 dex, where the first error bar includes the uncertainties related to star-to-star random errors, and the second one the systematic effects related to the various assumptions made in the analysis. The overall abundance pattern is quite typical of Globular Clusters, with an excess of the α-elements and of Eu. There is evidence that the stars of NGC 6441 are enriched in Na and Al, while they have been depleted of O and Mg by H-burning at high temperatures, analogous with extensive observations of other Globular Clusters: in particular, one star is clearly Na and Al-rich and O and Mg-poor. We also obtained quite high V abundances, but it is possible that this is an artifact of the analysis, since similar high V abundances are also derived for the field stars. These last are all more metal-rich than NGC 6441 and probably belong to the bulge population.

An empirical tool to derive metallicity, reddening, and distance for old stellar populations from near-infrared color-magnitude diagrams

We present high-quality near-infrared (near-IR) photometry of four bulge metal-rich globular clusters, namely NGC 6304, 6569, 6637 and 6638. By using the observed colour-magnitude diagrams we derived photometric estimates of the cluster reddening and distance. We performed a detailed analysis of the red giant branch (RGB), presenting a complete description of its morphologic parameters and evolutionary features (bump and tip). Photometric estimates of the cluster metallicity were obtained using the updated data set (published by our group) linking metal abundance to a variety of near-IR indices measured along the RGB. The detection of the RGB bump and tip is also presented and briefly discussed.