E. Pancino

Na-O anticorrelation and HB - VII. The chemical composition of first and second-generation stars in 15 globular clusters from GIRAFFE spectra

We present abundances of Fe, Na, and O for 1409 red giant stars in 15 galactic globular clusters (GCs), derived from the homogeneous analysis of high-resolution FLAMES/GIRAFFE spectra. Combining the present data with results from our FLAMES/UVES spectra and from previous studies within the project, we obtained a total sample of 1958 stars in 19 clusters, the largest and most homogeneous database of this kind to date. The programme clusters cover a range in metallicity from [Fe/H] = −2. 4d ex to [Fe/H] = −0.4 dex, with a wide variety of global parameters (morphology of the horizontal branch, mass, concentration, etc.). For all clusters we find the Na-O anticorrelation, the classical signature of the operation of proton-capture reactions in H-burning at high temperature in a previous generation of more massive stars that are now extinct. Using quantitative criteria (from the morphology and extension of the Na-O anticorrelation), we can define three different components of the stellar population in GCs. We separate a primordial component (P) of first-generation stars, and two components of second-generation stars, that we name intermediate (I) and extreme (E) populations from their different chemical composition. The P component is present in all clusters, and its fraction is almost constant at about one third. The I component represents the bulk of the cluster population. On the other hand, E component is not present in all clusters, and it is more conspicuous in some (but not in all) of the most massive clusters. We discuss the fractions and spatial distributions of these components in our sample and in two additional clusters (M 3 = NGC 5272 and M 13 = NGC6205) with large sets of stars analysed in the literature. We also find that the slope of the anti-correlation (defined by the minimum O and maximum Na abundances) changes from cluster-to-cluster, a change that is represented well by a bilinear relation on cluster metallicity and luminosity. This second dependence suggests a correlation between average mass of polluters and cluster mass.

DAOSPEC: An Automatic Code for Measuring Equivalent Widths in High-Resolution Stellar Spectra*

DAOSPEC is a Fortran code for measuring equivalent widths of absorption lines in stellar spectra with minimal human involvement. It works with standard FITS-format files and it is designed for use with high-resolution (R > 15,000) and high signal-to-noise-ratio (S/N > 30) spectra that have been binned on a linear wavelength scale. First, we review the analysis procedures that are usually employed in the literature. Next, we discuss the principles underlying DAOSPEC and point out similarities and differences with respect to conventional measurement techniques. Then, experiments with artificial and real spectra are discussed to illustrate the capabilities and limitations of DAOSPEC, with special attention given to the issues of continuum placement, radial velocities, and the effects of strong lines and line crowding. Finally, quantitative comparisons with other codes and with results from the literature are also presented.

Chemical abundance analysis of the open clusters Cr 110, NGC 2099 (M 37), NGC 2420, NGC 7789, and M 67 (NGC 2682)

Context. The present number of Galactic open clusters that have high resolution abundance determinations, not only of [Fe/H], but also of other key elements, is largely insufficient to enable a clear modeling of the Galactic disk chemical evolution. Aims. To increase the number of Galactic open clusters with high quality measurements. Methods. We obtained high resolution (R ∼ 30 000), high quality (S /N ∼ 50–100 per pixel), echelle spectra with the fiber spectrograph FOCES, at Calar Alto, Spain, for three red clump stars in each of five Open Clusters. We used the classical equivalent width analysis method to obtain accurate abundances of sixteen elements: Al, Ba, Ca, Co, Cr, Fe, La, Mg, Na, Nd, Ni, Sc, Si, Ti, V, and Y. We also derived the oxygen abundance using spectral synthesis of the 6300 A forbidden line. Results. Three of the clusters were never studied previously with high resolution spectroscopy: we found [Fe/H] =+ 0.03 ± 0.02 (±0.10) dex for Cr 110; [Fe/H] =+ 0.01 ± 0.05 (±0.10) dex for NGC 2099 (M 37), and [Fe/H] = −0.05 ± 0.03 (±0.10) dex for NGC 2420. This last finding is higher than typical literature estimates by 0.2–0.3 dex approximately and in closer agreement with Galactic trends. For the remaining clusters, we find that [Fe/H] =+ 0.05 ± 0.02 (±0.10) dex for M 67 and [Fe/H] =+ 0.04 ± 0.07 (±0.10) dex for NGC 7789. Accurate (to ∼0.5 km s −1 ) radial velocities were measured for all targets, and we provide the first velocity estimate derived from high resolution data for Cr 110, � Vr� = 41.0 ± 3. 8k m s −1 . Conclusions. With our analysis of the new clusters Cr 110, NGC 2099, and NGC 2420, we increase the sample of clusters with highresolution-based abundances by 5%. All our program stars show abundance patterns which are typical of open clusters, very close to solar with few exceptions. This is true for all the iron-peak and s-process elements considered, and no significant α-enhancement is found. No significant (anti-)correlations for Na, Al, Mg, and O abundances are found. If anticorrelations are present, the involved spreads must be <0.2 dex. We then compile high resolution data of 57 OC from the literature and find a gradient of [Fe/H] with Galactocentric radius of –0.06 ± 0.02 dex kpc −1 , in agreement with past work and with results for Cepheids and B stars in the same range. A change of slope is seen outside RGC = 12 kpc and [α/Fe] shows a tendency to increase with RGC. We also confirm the absence of a significant age-metallicity relation, finding slopes of –2.6 ± 1.1 × 10 −11 dex Gyr −1 and 1.1 ± 5.0 × 10 −11 dex Gyr −1 for [Fe/H] and [α/Fe] respectively.

Deep FORS1 Observations of the Double Main Sequence of ω Centauri

We present the results of a deep photometric survey performed with the VLT FORS1 aimed at investigating the complex main-sequence structure of the stellar system ω Centauri. We confirm the presence of a double main sequence and identify its blue component (bMS) over a large field of view up to 26 from the cluster center. We found that bMS stars are significantly more concentrated toward the cluster center than the other normal MS stars. The bMS morphology and its position in the CMD have been used to constrain the helium overabundance required to explain the observed MS morphology.

Mining SDSS in search of multiple populations in globular clusters

Several recent studies have reported the detection of an anomalous color spread along the red giant branch (RGB) of some globular clusters (GC) that appears only when color indices including a near ultraviolet band (such as Johnson U or Stromgren u) are consid- ered. This anomalous spread in color indexes such as U-B or cy has been shown to correlate with variations in the abundances of light elements such as C, N, O, Na, etc., which, in turn, are generally believed to be associated with subsequent star formation episodes that occurred in the earliest few 10 8 yr of the clusters life. Here we use publicly available u, g, r Sloan Digital Sky Survey photometry to search for anomalous u-g spreads in the RGBs of nine Galactic GCs. In seven of them (M 2, M 3, M 5, M 13, M 15, M 92 and M 53), we find evidence of a statistically significant spread in the u-g color, not seen in g-r and not accounted for by observational effects. In the case of M 5, we demonstrate that the observed u-g color spread correlates with the observed abundances of Na, the redder stars being richer in Na than the bluer ones. In all the seven clusters displaying a significant u-g color spread, we find that the stars on the red and blue sides of the RGB, in (g, u-g) color magnitude diagrams, have significantly di fferent radial distributions. In particular, the red stars (generally identified with the second generation of cl uster stars, in the current scenario) are always more centra lly concentrated than blue stars (generally identified with the first generati on) over the range sampled by the data (0.5rh <r <5rh), in qualitative agreement with the predictions of some recent models of the formation and chemical evolution of GCs. Our results suggest that the difference in the radial distribution between first and second ge neration stars may be a general characteristic of GCs.

The calibration of the RGB Tip as a Standard Candle - Extension to Near Infrared colors and higher metallicity

New empirical calibrations of the Red Giant Branch Tip in the I, J, H and K bands based on two fundamental pillars, namely ω Centauri and 47 Tucanae, have been obtained by using a large optical and near infrared photometric database. Our best estimates give M TRGB = −4.05 ± 0.12, M TRGB

LOOKING OUTSIDE THE GALAXY: THE DISCOVERY OF CHEMICAL ANOMALIES IN THREE OLD LARGE MAGELLANIC CLOUD CLUSTERS*

By using the multifiber spectrograph FLAMES mounted at the ESO-VLT, we have obtained high-resolution spectra for 18 giant stars, belonging to 3 old globular clusters of the Large Magellanic Cloud (namely NGC 1786, 2210 and 2257). While stars in each cluster showed quite homogeneous iron content, within a few cents of dex (the mean values being Fe/H]= -1.75+-0.01 dex, -1.65+-0.02 dex and -1.95+-0.02 dex for NGC 1786, 2210 and 2257, respectively), we have detected significant inhomogeneities for the [Na/Fe], [Al/Fe], [O/Fe] and [Mg/Fe] abundance ratios, with evidence of [O/Fe] vs [Na/Fe] and [Mg/Fe] vs [Al/Fe] anticorrelations. The trends detected nicely agree with those observed in Galactic Globular Clusters, suggesting that such abundance anomalies are ubiquitous features of old stellar systems and they do not depend on the parent galaxy environment. In NGC 1786 we also detected two extreme O-poor, Na-rich stars. This is the first time that a firm signature of extreme chemical abundance anomalies has been found in an extragalactic stellar cluster.

Chemical abundance analysis of the open clusters Berkeley 32, NGC 752, Hyades, and Praesepe

Context. Open clusters are ideal test particles for studying the chemical evolution of the Galactic disc. However, the number and accuracy of existing high-resolution abundance determinations, not only of [Fe/H], but also of other key elements, remains largely insufficient. Aims. We attempt to increase the number of Galactic open clusters that have high quality abundance determinations, and to gather all the literature determinations published so far. Methods. Using high-resolution (R ∼ 30 000), high-quality (S /N ≥ 60 per pixel), we obtained spectra for twelve stars in four open clusters with the fibre spectrograph FOCES, at the 2.2 Calar Alto Telescope in Spain. We employ a classical equivalent-width analysis to obtain accurate abundances of sixteen elements: Al, Ba, Ca, Co, Cr, Fe, La, Mg, Na, Nd, Ni, Sc, Si, Ti, V, and Y. We derived oxygen abundances by means of spectral synthesis of the 6300 A forbidden line. Results. We provide the first determination of abundance ratios other than Fe for NGC 752 giants, and ratios in agreement with the literature for the Hyades, Praesepe, and Be 32. We use a compilation of literature data to study Galactic trends of [Fe/H] and [α/Fe] with Galactocentric radius, age, and height above the Galactic plane. We find no significant trends, but some indication for a flattening of [Fe/H] at large Rgc, and for younger ages in the inner disc. We also detect a possible decrease in [Fe/H] with |z| in the outer disc, and a weak increase in [α/Fe] with Rgc.

The Pure Noncollisional Blue Straggler Population in the Giant Stellar System ω Centauri

We have used high spatial resolution data from the Hubble Space Telescope (HST) and wide-field ground-based observations to search for blue straggler stars (BSSs) over the entire radial extent of the large stellar system ? Centauri. We have detected the largest population of BSSs ever observed in any stellar system. Even though the sample is restricted to the brightest portion of the BSS sequence, more than 300 candidates have been identified. BSSs are thought to be produced by the evolution of binary systems (formed either by stellar collisions or mass exchange in binary stars). Since systems like Galactic globular clusters (GGCs) and ? Cen evolve dynamically on timescales significantly shorter than their ages, binaries should have settled toward the center, showing a more concentrated radial distribution than the ordinary, less massive single stars. Indeed, in all GGCs that have been surveyed for BSSs, the BSS distribution is peaked at the center. Conversely, in ? Cen we find that the BSSs share the same radial distribution as the adopted reference populations. This is the cleanest evidence ever found that such a stellar system is not fully relaxed even in the central region. We further argue that the absence of central concentration in the BSS distribution rules out a collisional origin. Thus, the ? Cen BSSs are the purest and largest population of noncollisional BSSs ever observed. Our results allow the first empirical quantitative estimate of the production rate of BSSs via this channel. BSSs in ? Cen may represent the best local template for modeling the BSS populations in distant galaxies where they cannot be individually observed.

The Gaia-ESO Survey: Kinematic structure in the Gamma Velorum cluster

This work was partially supported by the Gaia Research for European Astronomy Training (GREAT-ITN) Marie Curie network, funded through the European Union Seventh Framework Programme [FP7/2007-2013] under grant agreement 264895 and supported by the European Union FP7 programme through ERC grant number 320360 and by the Leverhulme Trust through grant RPG-2012-541. We acknowledge the support from INAF and Ministero dell’ Istruzione, dell’ Universita e della Ricerca (MIUR) in the form of the grant “Premiale VLT 2012”. RJJ acknowledges financial support from the UK Science and Technology Facilities Council.