Yazan Momany

Metallicities on the Double Main Sequence of ω Centauri Imply Large Helium Enhancement

Having shown in a recent paper that the main sequence of ? Centauri is split into two distinct branches, we now present spectroscopic results showing that the bluer sequence is less metal-poor. We have carefully combined VLTs GIRAFFE spectra of 17 stars on each side of the split into a single spectrum for each branch, with adequate signal-to-noise ratio, to show clearly that the stars of the blue main sequence are less metal-poor by 0.3 dex than those of the dominant red one. From an analysis of the individual spectra, we could not detect any abundance spread among the blue main-sequence stars, whereas the red main-sequence stars show a 0.2 dex spread in metallicity. We use stellar structure models to show that only greatly enhanced helium can explain the color difference between the two main sequences, and we discuss ways in which this enhancement could have arisen.

ω Centauri: The Population Puzzle Goes Deeper

We present Hubble Space Telescope observations that show a bifurcation of colors in the middle main sequence of the globular cluster ω Centauri. We see this in three different fields, observed with different cameras and filters. We also present high-precision photometry of a central Advanced Camera for Surveys field, which shows a number of main-sequence turnoffs and subgiant branches. The double main sequence, the multiple turnoffs and subgiant branches, and other population sequences discovered in the past along the red giant branch of this cluster add up to a fascinating but frustrating puzzle. We suggest various explanations, none of them very conclusive.

Transforming observational data and theoretical isochrones into the ACS/WFC Vega-mag system

We propose a zero-point photometric calibration of the data from the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) on board the Hubble Space Telescope, based on a spectrum of Vega and the most up-to-date in-flight transmission curves of the camera. This calibration is accurate at the level of a few hundredths of a magnitude. The main purpose of this effort is to transform the entire set of evolutionary models into a simple observational photometric system for ACS/WFC data, and to make them available to the astronomical community. We provide the zero-points for the most used ACS/WFC bands, and give basic recipes for calibrating both the observed data and the models. We also present the colour-magnitude diagram from ACS data of five Galactic globular clusters, spanning the metallicity range -2.2 <[Fe/H] < -0.04, and we provide fiducial points representing their sequences from several magnitudes below the turn-off to the red giant branch tip. The observed sequences are compared with the models in the newly defined photometric system.

VLT-UVES analysis of 5 giants in 47 Tucanae

High resolution spectra of 5 giants, including a horizontal branch star, of the metal-rich globular cluster 47 Tucanae were obtained with the UVES spectrograph at the 8 m VLT UT2-Kueyen telescope. The atmospheric parameters (Teff ,l ogg, (Fe/H), vt) were derived from VIJK photometry and spectroscopic data based on Fe I and Fe II lines. Fe I and Fe II iron abun- dances and respective total errors (Fe I/H) = -0.66 ± 0.12 and (Fe II/H) = -0.69 ± 0.24 are found. Abundances of α (O, Mg, Ca, Si, Ti), odd-Z (Na, Al), s- (Ba, La, Zr), and r-process (Eu) elements were determined by means of spectrum synthesis. The main results are (O/Fe) =+ 0.35 ± 0.11, (Mg/Fe) ≈ (Si/Fe) ≈ (Ti/Fe) =+ 0.23 ± 0.17, (Ca/Fe) = 0.0 ± 0.15, (Ba/Fe) =+ 0.31 ± 0.22 and (Eu/Fe) =+ 0.33 ± 0.10. Overabundances of the α-elements O, Mg, Si and Ti, and of Eu are similar to those seen in halo metal-poor stars, whereas a solar Ca-to-Fe ratio resembles the values found in bulge stars. An overall metallicity Z = 0.006 or (M/H) = -0.45 is thus obtained, as is a mean heliocentric radial velocity v hel = −22.43 ± 1.99 km s −1 .

A New Red Giant-based Distance Modulus of 13.3 Mpc to the Antennae Galaxies and Its Consequences*

The Antennae galaxies are the closest example of an ongoing major galaxy merger and, as such, represent a unique laboratory for furthering the understanding of the formation of exotic objects (e.g., tidal dwarf galaxies, ultraluminous X-ray sources, super stellar clusters). In a previous paper HST WFPC2 observations were used to demonstrate that the Antennae system might be at a distance considerably less than that conventionally assumed in the literature. Here we report new, much deeper HST ACS imaging that resolves the composite stellar populations and, most importantly, reveals a well-defined red giant branch. The tip of this red giant branch (TRGB) is unambiguously detected at -->I0TRGB = 26.65 ? 0.09 mag. Adopting the most recent calibration of the luminosity of the TRGB then yields a distance modulus for the Antennae of -->(m ? M)0 = 30.62 ? 0.17 corresponding to a distance of -->13.3 ? 1.0 Mpc. This is consistent with our earlier result, once the different calibrations for the standard candle are considered. We briefly discuss the implications of this now well-determined shorter distance.

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

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.

The lack of close binaries among hot horizontal branch stars in NGC 6752

Aims. We present the results of a spectroscopic search for close binaries among horizontal branch (HB) stars in NGC 6752. Methods. We used the ESO VLT-FORS2 instrument to obtain medium resolution (R = 4100) spectra of 51 hot HB stars with 8000 K ≤ Teff ≤ 32 000 K during four consecutive nights. Eighteen of our targets are extreme horizontal branch (EHB) stars with Teff ≥ 22 000 K. Radial velocity variations were measured with cross-correlation techniques, previously evaluated the statistical and systematic errors associated with them. Results. No close binary system has been detected among our 51 targets. The data corrected for instrumental effects indicate that the radial velocity variations are always below ≈15 km s −1 (3σ level). From a statistical analysis of our results, we conclude that (at 95% confidence

VLT-UVES abundance analysis of four giants in NGC 6553

Context. Metal-rich globular clusters trace the formation of bulges. Abundance ratios in the metal-rich globular clusters such as NGC 6553 can constrain the formation timescale of the Galactic bulge. Aims. The purpose of this study is determine the metallicity and elemental ratios in individual stars of the metal-rich bulge globular cluster NGC 6553. Methods. A detailed abundance analysis of four giants in NGC 6553 is carried out, based on optical high-resolution ´ spectra obtained with UVES at the ESO VLT-UT2 Kueyen telescope. Results. A metallicity [Fe/H]= 0.20 dex is derived, together with α-element enhancement of Mg and Si ([Mg/Fe]=+0.28, [Si/Fe]=+0.21), solar Ca and Ti ([Ca/Fe]=+0.05, [Ti/Fe]= 0.01), and a mild enhancement of the r-process element Eu with [Eu/Fe] = +0.10. A mean heliocentric radial velocity of 1.86 km s −1 is measured. We compare our results with previous investigations of the cluster.

A new search for planet transits in NGC 6791

Context. Searching for planets in open clusters allows us to study the effects of dynamical environment on planet formation and evolution. Aims. Considering the strong dependence of planet frequency on stellar metallicity, we studied the metal rich old open cluster NGC 6791 and searched for close-in planets using the transit technique. Methods. A ten-night observational campaign was performed using the Canada-France-Hawaii Telescope (3.6 m), the San Pedro Martir telescope (2.1 m), and the Loiano telescope (1.5 m). To increase the transit detection probability we also made use of the Bruntt et al. (2003, AA (ii) a smaller planetary radius for planets orbiting super metal rich stars; or (iii) limitations in the basic assumptions. More extensive photometry with 3–4 m class telescopes is required to allow conclusive inferences about the frequency of planets in NGC 6791.

A study of the Galactic plane towards l= 305°

We present optical (UBVI C ) observations of a rich and complex field in the Galactic plane towards l ~ 305° and b~ 0°. Our analysis reveals a significantly high interstellar absorption (A v ~ 10) and an abnormal extinction law in this line of sight. Availing a considerable number of colour combinations, the photometric diagrams allow us to derive new estimates of the fundamental parameters of the two open clusters Danks 1 and Danks 2. Due to the derived abnormal reddening law in this line of sight, both clusters appear much closer (to the Sun) than previously thought. Additionally, we present the optical colours and magnitudes of the WR 48a star, and its main parameters were estimated. The properties of the two embedded clusters, DBS2003 130 and 131, are also addressed. We identify a number of young stellar objects which are probable members of these clusters. This new material is then used to revisit the spiral structure in this sector of the Galaxy showing evidence of populations associated with the inner Galaxy Scutum-Crux arm.