VLT Observations of Turnoff stars in the Globular Cluster NGC 6397

VLT-UVES high resolution spectra of seven turnoff stars in the metal-poor globular cluster NGC 6397 have been obtained. Atmospheric parameters and abundances of several elements (Li, Na, Mg, Ca, Sc, Ti, Cr, Fe, Ni, Zn and Ba) were derived for program stars. The mean iron abundance is [Fe/H] = -2.02, with no star-to-star variation. The mean abundances of the alpha-elements (Ca, Ti) and of the iron-peak elements (Sc, Cr, Ni) are consistent with abundances derived for field stars of similar metallicity. Magnesium is also almost solar, consistent with the values found by Idiart & Th\'evenin (2000) when non-LTE effects (NLTE hereafter) are taken into account. The sodium abundance derived for five stars is essentially solar, but one object (A447) is clearly Na deficient. These results are compatible with the expected abundance range estimated from the stochastic evolutionary halo model by Argast et al. (2000) when at the epoch of [Fe/H] $\sim$ -2 the interstellar medium is supposed to become well-mixed.Comment: to appear in A&

Abundances in giant stars of the globular cluster NGC 6752

Recent theoretical yields and chemical evolution models demonstrate that intermediate-mass AGB stars cannot reproduce the observed abundance distributions of O, Na, Mg, and Al. As a further observational test of this finding, we present elemental abundance ratios [X/Fe] for 20 elements in 38 bright giants of the globular cluster NGC 6752. Our mean abundance ratios [X/Fe] are in good agreement with previous studies of this cluster and are also consistent with other globular clusters and field stars at the same metallicity. The mean abundance ratios [Ba/Eu] and [La/Eu] exhibit values, in agreement with field stars at the same metallicity, that lie approximately midway between the pure r-process and the solar (s-process + r-process) mix, indicating that AGB stars have played a role in the chemical evolution of the proto-cluster gas. For the first time, we find possible evidence for an abundance variation for elements heavier than Al in this cluster. We find a correlation between [Si/Fe] and [Al/Fe] which is consistent with the abundance anomalies being synthesized via proton captures at high temperatures. Leakage from the Mg-Al chain into 28Si may explain the Si excess in stars with the highest [Al/Fe]. We identify correlations between [Y/Fe] and [Al/Fe], [Zr/Fe] and [Al/Fe], and [Ba/Fe] and [Al/Fe] suggesting that Y, Zr, and Ba abundances may increase by about 0.1 dex as Al increases by about 1.3 dex. While the correlations are statistically significant, the amplitudes of the variations are small. If the small variations in Y, Zr, and Ba are indeed real, then the synthesis of the Al anomalies must have taken place within an unknown class of stars that also ran the s-process. [Abridged]Comment: Accepted for publication in A&

The two metallicity groups of the globular cluster M22: a chemical perspective

We present a detailed chemical composition analysis of 35 red giant stars in the globular cluster M22. High resolution spectra for this study were obtained at five observatories, and analyzed in a uniform manner. We have determined abundances of representative light proton-capture, alpha, Fe-peak and neutron-capture element groups. Our aim is to better understand the peculiar chemical enrichment history of this cluster, in which two stellar groups are characterized by a different content in iron, neutron capture elements Y, Zr and Ba, and alpha element Ca. The principal results of this study are: (i) substantial star-to-star metallicity scatter (-2.0<[Fe/H]<-1.6); (ii) enhancement of s-process/r-process neutron-capture abundance ratios in a fraction of giants, positively correlated with metallicity; (iii) sharp separation between the s-process rich and s-process poor groups by [La/Eu] ratio; (iv) possible increase of [Cu/Fe] ratios with increasing [Fe/H], suggesting that this element also has a significant s-process component; and (v) presence of Na-O and C-N anticorrelations in both the stellar groups.Comment: 26 pages, 19 figures. Accepted for publication in A&

The double sub-giant branch of NGC 6656 (M22): a chemical characterization

We present an abundance analysis of 101 subgiant branch (SGB) stars in the globular cluster M22. Using low resolution FLAMES/GIRAFFE spectra we have determined abundances of the neutron-capture strontium and barium and the light element carbon. With these data we explore relationships between the observed SGB photometric split in this cluster and two stellar groups characterized by different contents of iron, slow neutron-capture process (s-process) elements, and the alpha element calcium, that we previously discovered in M22's red-giant stars. We show that the SGB stars correlate in chemical composition and color-magnitude diagram position: the stars with higher metallicity and relative s-process abundances define a fainter SGB, while stars with lower metallicity and s-process content reside on a relatively brighter SGB. This result has implications for the relative ages of the two stellar groups of M22. In particular, it is inconsistent with a large spread in ages of the two SGBs. By accounting for the chemical content of the two stellar groups, isochrone fitting of the double SGB suggests that their ages are not different by more than 300 Myr.Comment: 21 pages, 12 figures, accepted for publication in A&

Spectroscopic versus Photometric Metallicities: Milky Way Dwarf Spheroidal Companions as a Test Case

Aims. The method of deriving photometric metallicities using red giant branch stars is applied to resolved stellar populations under the common assumption that they mainly consist of single-age old stellar populations. We explore the effect of the presence of mixed-age stellar populations on deriving photometric metallicities. Methods. We use photometric data sets for the five Galactic dwarf spheroidals Sculptor, Sextans, Carina, Fornax, and Leo II in order to derive their photometric metallicity distribution functions from their resolved red giant branches using isochrones of the Dartmouth Stellar Evolutionary Database. We compare the photometric metallicities with published spectroscopic metallicities based on the analysis of the near-infrared Ca triplet (Ca T), both on the metallicity scale of Carretta & Gratton and on the scale defined by the Dartmouth isochrones. In addition, we compare the photometric metallicities with published spectroscopic metallicities based on spectral synthesis and medium-resolution spectroscopy, and on high resolution spectra where available. Results. The mean properties of the spectroscopic and photometric metallicity samples are comparable within the intrinsic scatter of each method although the mean metallicities of dSphs with pronounced intermediate-age population fractions may be underestimated by the photometric method by up to a few tenths of dex in [Fe/H]. The star-by-star differences of the spectroscopic minus the photometric metallicities show a wide range of values along the fiducial spectroscopic metallicity range, with the tendency to have systematically lower photometric metallicities for those dwarf spheroidals with a higher fraction of intermediate-age populations. Such discrepancies persist even in the case of the purely old Sculptor dSph, where one would na\"ively expect a very good match when comparing with medium or low resolution metallicity measurements. Overall, the agreement between Ca T metallicities and photometric metallicities is very good in the metallicity range from ~ -2 dex to ~ -1.5 dex. We find that the photometric method is reliable in galaxies that contain small (less than 15%) intermediate-age stellar fractions. Therefore, in the presence of mixed-age stellar populations, one needs to quantify the fraction of the intermediate-age stars in order to assess their effect on determining metallicities from photometry alone. Finally, we note that the comparison of spectroscopic metallicities of the same stars obtained with different methods reveals similarly large discrepancies as the comparison with photometric metallicities.Comment: 17 pages, 12 figures; A&A accepte

Abundances of Red Giants in the Andromeda II Dwarf Spheroidal Galaxy

We have obtained spectra for 50 candidate red giants in Andromeda II (a dwarf spheroidal companion of M31) using LRIS on the Keck II telescope. After eliminating background galaxies and Galactic foreground stars, we are left with a sample of 42 red giants for which membership in Andromeda II can be established unambiguously from radial velocities. Line indices measured on the Lick/IDS system are combined with V and I photometry obtained with the Keck II and Palomar 5m telescopes to investigate the age and metallicity distribution of these stars. Based on a comparison of the measured line indices to those of Lick/IDS standard stars in globular and open clusters, we derive a mean metallicity of [Fe/H] = -1.47+/-0.19 dex. This confirms earlier conclusions based on Thuan-Gunn gr photometry that Andromeda II obeys the familiar relation between mean stellar metallicity and galaxy luminosity. There is also evidence for a dispersion in metallicity of 0.35+/-0.10 dex based on the scatter in the measured Mgb line indices and the observed width of the galaxy's giant branch. Although existing observations of Local Group dwarf galaxies indicate that their mean stellar metallicity depends rather sensitively on host galaxy absolute magnitude, the internal spread in metallicity appears to be relatively independent of luminosity. Finally, a small number of stars are found above the Population II red giant branch, indicating the presence of a modest intermediate-age population in this galaxy. (ABRIDGED)

Discovering and Locating High-Energy Extra-Galactic Sources by Bayesian Mixture Modelling

Discovering and locating gamma-ray sources in the whole sky map is a declared target of the Fermi Gamma-ray Space Telescope collaboration. In this paper, we carry out an unsupervised analysis of the collection of high-energy photons accumulated by the Large Area Telescope, the principal instrument on board the Fermi spacecraft, over a period of around 7.5 years using a Bayesian mixture model. A fixed, though unknown, number of parametric components identify the extra-galactic emitting sources we are searching for, while a further component represents parametrically the dffuse gamma ray background due to both, extragalactic and galactic high energy photon emission. We determine the number of sources, their coordinates on the map and their intensities. The model parameters are estimated using a reversible jump MCMC algorithm which implements four different types of moves. These allow us to explore the dimension of the parameter space. The possible transitions remove from or add a source to the model, while leaving the background component unchanged. We furthermore present an heuristic procedure, based on the posterior distribution of the mixture weights, to qualify the nature of each detected source

C and N abundances of MS and SGB stars in NGC 1851

We present the first chemical analysis of stars on the double subgiant branch (SGB) of the globular cluster NGC 1851. We obtained 48 Magellan IMACS spectra of subgiants and fainter stars covering the spectral region between 3650-6750\AA, to derive C and N abundances from the spectral features at 4300\AA (G-band) and at ~ 3883\AA (CN). We added to our sample ~ 45 unvevolved stars previously observed with FORS2 at the VLT. These two datasets were homogeneously reduced and analyzed. We derived abundances of C and N for a total of 64 stars and found considerable star-to-star variations in both [C/H] and [N/H] at all luminosities extending to the red giant branch (RGB) base (V~18.9). These abundances appear to be strongly anticorrelated, as would be expected from the CN-cycle enrichment, but we did not detect any bimodality in the C or N content. We used HST and ground-based photometry to select two groups of faint- and bright-SGB stars from the visual and Str\"omgren color-magnitude diagrams. Significant variations in the carbon and nitrogen abundances are present among stars of each group, which indicates that each SGB hosts multiple subgenerations of stars. Bright- and faint-SGB stars differ in the total C+N content, where the fainter SGB have about 2.5 times the C+N content of the brighter ones. Coupling our results with literature photometric data and abundance determinations from high-resolution studies, we identify the fainter SGB with the red-RGB population, which also should be richer on average in Ba and other s-process elements, as well as in Na and N, when compared to brighter SGB and the blue-RGB population.Comment: 17 pages, 3 tables, 16 figures; typos corrected, added checks on temperature scale; A&A accepted (in press

On the origin of the stellar halo and multiple stellar populations in the globular cluster NGC 1851

We propose that the observed stellar halo around the globular cluster (GC) NGC 1851 is evidence for its formation in the central region of its defunct host dwarf galaxy. We numerically investigate the long-term dynamical evolution of a nucleated dwarf galaxy embedded in a massive dark matter halo under the strong tidal field of the Galaxy. The dwarf galaxy is assumed to have a stellar nucleus (or a nuclear star cluster) that could be the progenitor for NGC 1851. We find that although the dark matter halo and the stellar envelope of the host dwarf of NGC 1851 can be almost completely stripped during its orbital evolution around the Galaxy, a minor fraction of stars in the dwarf can remain trapped by the gravitational field of the nucleus. The stripped nucleus can be observed as NGC 1851 with no/little dark matter whereas stars around the nucleus can be observed as a diffuse stellar halo around NGC 1851. The simulated stellar halo has a symmetric distribution with a power-law density slope of ~ -2 and shows no tidal tails within ~200pc from NGC 1851. We show that two GCs can merge with each other to form a new nuclear GC embedded in field stars owing to the low stellar velocity dispersion of the host dwarf. This result makes no assumption on the ages and/or chemical abundances of the two merging GCs. Thus the observed stellar halo and characteristic multiple stellar populations in NGC 1851 suggest that NGC 1851 could have formed initially in the central region of an ancient dwarf galaxy.Comment: 15 pages, 12 figures, accepted in MNRA

Radial Velocity and Metallicity of the Globular Cluster IC4499 Obtained with AAOmega

We present radial velocity and metallicity measurements for the far-southern Galactic globular cluster IC4499. We selected several hundred target red giant stars in and around the cluster from the 2MASS point source catalog, and obtained spectra at the near-infrared calcium triplet using the AAOmega spectrograph. Observations of giants in globular clusters M4, M22, and M68 were taken to provide radial velocity and metallicity comparison objects. Based on velocity data we conclude that 43 of our targets are cluster members, by far the largest sample of IC4499 giants spectroscopically studied. We determine the mean heliocentric radial velocity of the cluster to be 31.5 plus or minus 0.4 km/s, and find the most likely central velocity dispersion to be 2.5 plus or minus 0.5 km/s. This leads to a dynamical mass estimate for the cluster of 93 plus or minus 37 thousand solar masses. We are sensitive to cluster rotation down to an amplitude of about 1 km/s, but no evidence for cluster rotation is seen. The cluster metallicity is found to be [Fe/H] = -1.52 plus or minus 0.12 on the Carretta-Gratton scale. The radial velocity of the cluster, previously highly uncertain, is consistent with membership in the Monoceros tidal stream, but also with a halo origin. The horizontal branch morphology of the cluster is slightly redder than average for its metallicity, but it is likely not unusually young compared to other clusters of the halo. The new constraints on the cluster kinematics and metallicity may give insight into its extremely high specific frequency of RR Lyrae stars.Comment: Accepted to MNRAS, 13 pages, 9 figure