Discovery of a super-Li rich turn-off star in the metal poor globular cluster NGC 6397

We report on the discovery of a super-Li rich turn-off star in the old (12 Gyr), metal poor ([Fe/H]=-2.1 dex) globular cluster (GC) NGC 6397, based on high-resolution MIKE/Magellan spectra. This star shows an unusually high lithium abundance of A(Li)_NLTE = 4.03 +/- 0.06 +/- 0.14 dex (or, 4.21, accounting for possible contamination from a binary companion) that lies above the canonical Li-plateau by a factor of 100. This is the highest Li enhancement found in a Galactic GC dwarf star to date. We discuss several enhancement mechanisms, but none can unambiguously explain such a high overabundance. The spectrum of the star shows a possible indication of binarity, but its line strengths and chemical element abundance ratios are fully compatible with other turn-off stars in this GC, seemingly ruling out mass transfer from an AGB companion as origin of the high A(Li). A possible cause is an interaction with a red giant that has undergone cool bottom processing.Comment: 5 pages, 3 figures, accepted for publication in the ApJ Letter

Observations of the Hot Horizontal-Branch Stars in the Metal-Rich Bulge Globular Cluster NGC 6388 - Indications of Helium Enrichment and a Lesson in Crowded Field Spectroscopy

(abridged) The metal-rich bulge globular cluster NGC6388 shows a distinct blue horizontal-branch tail with a strong upward tilt in its colour-magnitude diagram. Several noncanonical scenarios have been proposed to explain these puzzling observations. In order to test the predictions of these scenarios, we have obtained medium resolution spectra to determine the atmospheric parameters of a sample of the blue HB stars in NGC 6388. Using the medium resolution spectra, we determine effective temperatures, surface gravities and helium abundances by fitting the observed Balmer and helium lines with appropriate theoretical stellar spectra. As we know the distance to the cluster, we can verify our results by determining masses for the stars. During the data reduction we took special care in subtracting the background, which is dominated by the overlapping spectra of cool stars. The physical parameters of the blue HB stars near 10,000 K support the helium pollution scenario. The low gravities and masses found for the hot blue tail stars, however, are probably caused by problems with the data reduction, most likely due to remaining background light in the spectra, which would affect the fainter hot blue tail stars much more strongly than the brighter cool blue tail stars. Our study of the hot blue tail stars in NGC 6388 illustrates the obstacles which are encountered when attempting to determine the atmospheric parameters of hot HB stars in very crowded fields using ground-based observations. We discuss these obstacles and offer possible solutions for future projects.Comment: 11 pages, 9 figures (finding charts as png only), uses aa.cls; accepted for publication in A&

RR Lyrae stars in Galactic globular clusters. VI. The Period-Amplitude relation

We compare theory and observations for fundamental RR Lyrae in the solar neighborhood and in both Oosterhoff type I (OoI) and type II (OoII) Galactic globular clusters (GGCs). The distribution of cluster RR_ab in the PA_V plane depends not only on the metal abundance, but also on the cluster Horizontal Branch (HB) morphology. On average the observed k_puls parameter, connecting the period to the visual amplitude, increases when moving from metal-poor to metal-rich GGCs. However, this parameter shows marginal changes among OoI clusters with intermediate to red HB types and iron abundances -1.8<= [Fe/H] <=-1.1, whereas its value decreases in OoII clusters with the bluer HB morphology. Moreover, at [Fe/H]=-1.7+-0.1 the OoI clusters present redder HB types and larger values than the OoII clusters. The RR_ab variables in Omega Cen and in the solar neighborhood further support the evidence that the spread in [Fe/H], at fixed k_puls, is of the order of +-0.5 dex. Synthetic HB simulations show that the PA_V plane can provide accurate cluster distance estimates. The RR_ab variables in OoI and in OoII clusters with very blue HB types obey a well-defined M_V(RR)-k_puls relation, while those in OoII clusters with moderately blue HB types present a zero-point that is ~0.05 mag brighter. Regarding field variables, we show that with [Fe/H]=> -1.0 a unique M_V(RR)-k_puls relation can be adopted, independently of the parent HB morphology. Current findings suggest that the PA_V distribution does not seem to be a robust diagnostic for the metal abundance of RR_ab variables. However, the same observables can be used to estimate the absolute magnitude of globular cluster and field RR_ab variables. We show that over the metallicity range -2.4<= [Fe/H] <= 0.0 the M_V(RR)-[Fe/H] relation shows a parabolic behavior.Comment: Paper accepted on A&A, 13 pages, 18 figure

Constraints on the Formation of the Galactic Bulge from Na, Al, and Heavy Element Abundances in Plaut's Field

We report chemical abundances of Na, Al, Zr, La, Nd, and Eu for 39 red giant branch (RGB) stars and 23 potential inner disk red clump stars located in Plaut-s low extinction window. We also measure lithium for a super Li-rich RGB star. The abundances were determined by spectrum synthesis of high resolution (R~25,000), high signal-to-noise (S/N~50-100 pixel-1) spectra obtained with the Blanco 4m telescope and Hydra multifiber spectrograph. For the bulge RGB stars, we find a general increase in the [Na/Fe] and [Na/Al] ratios with increasing metallicity, and a similar decrease in [La/Fe] and [Nd/Fe]. Additionally, the [Al/Fe] and [Eu/Fe] abundance trends almost identically follow those of the {\alpha}-elements, and the [Zr/Fe] ratios exhibit relatively little change with [Fe/H]. The consistently low [La/Eu] ratios of the RGB stars indicate that at least a majority of bulge stars formed rapidly (<1 Gyr) and before the main s-process could become a significant pollution source. In contrast, we find that the potential inner disk clump stars exhibit abundance patterns more similar to those of the thin and thick disks. Comparisons between the abundance trends at different bulge locations suggest that the inner and outer bulge formed on similar timescales. However, we find evidence of some abundance differences between the most metal-poor and metal-rich stars in various bulge fields. The data also indicate that the halo may have had a more significant impact on the outer bulge initial composition than the inner bulge composition. The [Na/Fe] and to a lesser extent [La/Fe] abundances further indicate that the metal-poor bulge, at least at ~1 kpc from the Galactic center, and thick disk may not share an identical chemistry.Comment: Accepted for publication in ApJ; 66 pages, 17 figures, 3 tables; prior to publication, data tables in electronic form will be made available upon reques

Stellar populations in NGC 5128 with the VLT: evidence for recent star formation

We resolve stars of the nearest giant elliptical galaxy NGC 5128 using VLT with FORS1 and ISAAC. We construct deep U, V and Ks color-magnitude and color-color diagrams in two different halo fields (in the halo and in the north-eastern diffuse shell). In the outer, shell field, at ~14 kpc from the center of the galaxy, there is a significant recent star formation with stars as young as 10 Myr, approximately aligned with the prominent radio and x-ray jet from the nucleus of the host AGN. Ionized gas filaments are evident in ultraviolet images near the area where neutral HI and CO molecular gas was previously observed. The underlying stellar population of the halo of the giant elliptical is predominantly old with a very broad metallicity distribution. The presence of an extended giant branch reaching M_bol=-5 mag suggests the existence of a significant intermediate-age AGB population in the halo of this galaxy.Comment: 18 pages, 18 figures; to be published in A&A; high res. version at http://www.eso.org/~mrejkuba/cenA_starsVLT.p

Abundance patterns of multiple populations in Globular Clusters: a chemical evolution model based on yields from AGB ejecta

A large number of spectroscopic studies have provided evidence of the presence of multiple populations in globular clusters by revealing patterns in the stellar chemical abundances. This paper is aimed at studying the origin of these abundance patterns. We explore a model in which second generation (SG) stars form out of a mix of pristine gas and ejecta of the first generation of asymptotic giant branch stars. We first study the constraints imposed by the spectroscopic data of SG stars in globular clusters on the chemical properties of the asymptotic and super asymptotic giant branch ejecta. With a simple one-zone chemical model, we then explore the formation of the SG population abundance patterns focussing our attention on the Na-O, Al-Mg anticorrelations and on the helium distribution function. We carry out a survey of models and explore the dependence of the final SG chemical properties on the key parameters affecting the gas dynamics and the SG formation process. Finally, we use our chemical evolution framework to build specific models for NGC 2808 and M4, two Galactic globular clusters which show different patterns in the Na-O and Mg-Al anticorrelation and have different helium distributions. We find that the amount of pristine gas involved in the formation of SG stars is a key parameter to fit the observed O-Na and Mg-Al patterns. The helium distribution function for these models is in general good agreement with the observed one. Our models, by shedding light on the role of different parameters and their interplay in determining the final SG chemical properties, illustrate the basic ingredients, constraints and problems encountered in this self-enrichment scenario which must be addressed by more sophisticated chemical and hydrodynamic simulations.Comment: 19 pages, 10 figures, MNRAS accepte

New Analyses of Star-to-Star Abundance Variations Among Bright Giants in the Mildly Metal-Poor Globular Cluster M5

We present a chemical composition analysis of 36 giant stars in the mildly metal-poor globular cluster M5 (NGC 5904). The analysis makes use of high resolution data acquired at the Keck I telescope as well as a re-analysis of high resolution spectra acquired for an earlier study at Lick Observatory. We employed two analysis techniques: one, adopting standard spectroscopic constraints, and two, adopting an analysis consistent with the non-LTE precepts as recently described by Thevenin & Idiart. The abundance ratios we derive for magnesium, silicon, calcium, scandium, titanium, vanadium, nickel, barium and europium in M5 show no significant abundance variations and the ratios are comparable to those of halo field stars. However, large variations are seen in the abundances of oxygen, sodium and aluminum, the elements that are sensitive to proton-capture nucleosynthesis. In comparing the abundances of M5 and M4 (NGC 6121), another mildly metal-poor globular cluster, we find that silicon, aluminum, barium and lanthanum are overabundant in M4 with respect to what is seen in M5, confirming and expanding the results of previous studies. In comparing the abundances between these two clusters and others having comparable metallicities, we find that the anti-correlations observed in M5 are similar to those found in more metal-poor clusters, M3, M10 and M13, whereas the behavior in M4 is more like that of the more metal-rich globular cluster M71. We conclude that among stars in Galactic globular clusters, there is no definitive ``single'' value of [el/Fe] at a given [Fe/H] for at least some alpha-capture, odd-Z and slow neutron-capture process elements, in this case, silicon, aluminum, barium and lanthanum.Comment: 31 pages + 16 figures + 11 tables; accepted for publication in Sept.2001 Astronomical Journa

Low-resolution spectroscopy of main sequence stars belonging to 12 Galactic globular clusters. I. CH and CN band strength variations

Globular clusters show abundance variations for light elements that are not yet well understood. The preferred explanation involves a self-enrichment scenario, with two subsequent generations of stars. Observations of main sequence stars allow us to investigate the signature of this chemically processed material without the complicating effects of internal mixing. Our goal is to investigate the C-N anti-correlation with low-resolution spectroscopy of 20-50 stars fainter than the first dredge-up in seven globular clusters (NGC288, NGC1851, NGC5927, NGC6352, NGC6388, and Pal12) with different properties. We complemented our observations with 47~Tuc archival data, with four additional clusters from the literature (M15, M22, M55, NGC362), and with additional literature data on NGC288. In this first paper, we measured the strength of CN and CH band indices, and we investigated the anti-correlation and bimodality of these indices. We compared r_CN, the ratio of stars belonging to the CN-strong and weak groups, with 15 different cluster parameters. We clearly see bimodal anti-correlation of the CH and CN band stregths in the metal-rich clusters (Pal12, 47Tuc, NGC6352, NGC5927). Only M15 among the metal-poor clusters shows a clearly bimodal anti-correlation. We found weak correlations (sligthly above 1 sigma) of r_CN with the cluster orbital parameters, present-day total mass, cluster concentration, and age. Our findings support the self-enrichment scenario, and suggest that the occurrence of more than two major generations of stars in a GGC should be rare. Small additional generations (<10-20% of the total) would be difficult to detect with our samples. The first generation, which corresponds to the CN-weak stars, usually contains more stars than the second one (=0.82+/-0.29), as opposed to results based on the Na-O anti-correlations.Comment: 18 pages, 10 figures, 1 electronic table, accepted for publication in A&A (language edited version

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.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles', A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 22 pages including 3 tables and 2 postscript figures, uses Kluwer's crckapb.sty LaTeX style file, enclose