The age of 47Tuc from self-consistent isochrone fits to colour-magnitude diagrams and the eclipsing member V69

Our aim is to derive a self-consistent age, distance and composition for the globular cluster $47\,$Tucanae ($47\,$Tuc; NGC104). First, we reevaluate the reddening towards the cluster resulting in a nominal $E(B-V)=0.03\pm0.01$ as the best estimate. The $T_{\rm eff}$ of the components of the eclipsing binary member V69 is found to be $5900\pm72$ K from both photometric and spectroscopic evidence. This yields a true distance modulus $(m-M)_0=13.21\pm0.06$(random)$\pm0.03$(systematic) to $47\,$Tuc when combined with existing measurements of V69 radii and luminosity ratio. We then present a new completely self-consistent isochrone fitting method to ground based and $\textit{HST}$ cluster colour-magnitude diagrams and the eclipsing binary member V69. The analysis suggests that the composition of V69, and by extension one of the populations of $47\,$Tuc, is given by [Fe/H]$\sim-0.70$, [O/Fe]$\sim+0.60$, and $Y\sim0.250$ on the solar abundance scale of Asplund, Grevesse & Sauval. However, this depends on the accuracy of the model $T_{\rm eff}$ scale which is 50-75 K cooler than our best estimate but within measurement uncertainties. Our best estimate of the age of $47\,$Tuc is 11.8 Gyr, with firm ($3 \sigma$) lower and upper limits of 10.4 and 13.4 Gyr, respectively, in satisfactory agreement with the age derived from the white dwarf cooling sequence if our determination of the distance modulus is adopted.Comment: 19 pages, 8 figures, accepted for publication in MNRA

A double stellar generation in the Globular Cluster NGC6656 (M 22). Two stellar groups with different iron and s-process element abundance

AIMS. In this paper we present the chemical abundance analysis from high resolution UVES spectra of seventeen bright giant stars of the Globular Cluster M~22. RESULTS. We obtained an average iron abundance of [Fe/H]=-1.76\pm0.02 (internal errors only) and an \alpha enhancement of 0.36\pm0.04 (internal errors only). Na and O, and Al and O follow the well known anti-correlation found in many other GCs. We identified two groups of stars with significantly different abundances of the s-process elements Y, Zr and Ba. The relative numbers of the two group members are very similar to the ratio of the stars in the two SGBs of M22 recently found by Piotto (2009). Y and Ba abundances do not correlate with Na, O and Al. The s-element rich stars are also richer in iron and have higher Ca abundances. The results from high resolution spectra have been further confirmed by lower resolution GIRAFFE spectra of fourteen additional M22 stars. GIRAFFE spectra show also that the Eu -- a pure r-process element -- abundance is not related to the iron content. We discuss the chemical abundance pattern of M22 stars in the context of the multiple stellar populations in GC scenario.Comment: 17 Pages, 21 figures, Accepted for publication in A&

The Puzzling White Dwarf Cooling Sequence in NGC6791: A Simple Solution

In this paper we demonstrate that the puzzling bright peak in the luminosity function of the white dwarf (WD) cooling sequence of NGC6791 can be naturally accounted for if ~34% of the observed WDs are WD+WD binary systems.Comment: 12 pages, 3 figures. Accepted (April 9th 2008) on ApJ Lette

Multiple stellar populations in Magellanic Cloud clusters. II. Evidence also in the young NGC1844?

We use HST observations to study the LMC's young cluster NGC1844. We estimate the fraction and the mass-ratio distribution of photometric binaries and report that the main sequence presents an intrinsic breadth which can not be explained in terms of photometric errors only, and is unlikely due to differential reddening. We attempt some interpretation of this feature, including stellar rotation, binary stars, and the presence of multiple stellar populations with different age, metallicity, helium, or C+N+O abundance. Although we exclude age, helium, and C+N+O variations to be responsible of the main-sequence spread none of the other interpretations is conclusive.Comment: 9 Pages, 11 figures, accepted for publication in A&A

An upper limit to the secular variation of the gravitational constant from white dwarf stars

A variation of the gravitational constant over cosmological ages modifies the main sequence lifetimes and white dwarf cooling ages. Using an state-of-the-art stellar evolutionary code we compute the effects of a secularly varying G on the main sequence ages and, employing white dwarf cooling ages computed taking into account the effects of a running G, we place constraints on the rate of variation of Newton's constant. This is done using the white dwarf luminosity function and the distance of the well studied open Galactic cluster NGC 6791. We derive an upper bound G'/G ~ -1.8 10^{-12} 1/yr. This upper limit for the secular variation of the gravitational constant compares favorably with those obtained using other stellar evolutionary properties, and can be easily improved if deep images of the cluster allow to obtain an improved white dwarf luminosity function.Comment: 15 pages, 4 figures, accepted for publication in JCA

Multi-wavelength Hubble Space Telescope photometry of stellar populations in NGC288

We present new UV observations for NGC288, taken with the WFC3 detector on board the Hubble Space Telescope, and combine them with existing optical data from the archive to explore the multiple-population phenomenon in this globular cluster (GC). The WFC3's UV filters have demonstrated an uncanny ability to distinguish multiple populations along all photometric sequences in GCs, thanks to their exquisite sensitivity to the atmospheric changes that are tell-tale signs of second-generation enrichment. Optical filters, on the other hand, are more sensitive to stellar-structure changes related to helium enhancement. By combining both UV and optical data we can measure helium variation. We quantify this enhancement for NGC288 and find that its variation is typical of what we have come to expect in other clusters.Comment: 15 pages, 5 figures, accepted for publication in Ap

Search for giant planets in M67 IV: survey results

We present the results of a seven-year-long radial velocity survey of a sample of 88 main-sequence and evolved stars to reveal signatures of Jupiter-mass planets in the solar-age and solar-metallicity open cluster M67. We aim at studying the frequency of giant planets in this cluster with respect to the field stars. In addition, our sample is also ideal to perform a long-term study to compare the chemical composition of stars with and without giant planets in detail. We analyzed precise radial velocity (RV) measurements obtained with five different instruments. We conducted Monte Carlo simulations to estimate the occurrence rate of giant planets in our radial velocity survey. All the planets previously announced in this RV campaign with their properties are summarized here: 3 hot Jupiters around the main-sequence stars YBP1194, YBP1514, and YBP401, and 1 giant planet around the evolved star S364. Two additional planet candidates around the stars YBP778 and S978 are also analyzed in the present work. We discuss stars that exhibit large RV variability or trends individually. For 2 additional stars, long-term trends are compatible with new binary candidates or substellar objects, which increases the total number of binary candidates detected in our campaign to 14. Based on the Doppler-detected planets discovered in this survey, we find an occurrence of giant planets of ~18.0%(+12.0/-8.0%) in the selected period-mass range. This frequency is slightly higher but consistent within the errors with the estimate for the field stars, which leads to the general conclusion that open cluster and field statistics agree. However, we find that the rate of hot Jupiters in the cluster (~5.7%(+5.5/-3.0%)) is substantially higher than in the field.Comment: Accepted by A&

A Double White-Dwarf Cooling Sequence in {\omega} Centauri

We have applied our empirical-PSF-based photometric techniques on a large number of calibration-related WFC3/UVIS UV-B exposures of the core of {\omega} Cen, and found a well-defined split in the right part of the white-dwarf cooling sequence (WDCS). The redder sequence is more populated by a factor of ~2. We can explain the separation of the two sequences and their number ratio in terms of the He-normal and He-rich subpopulations that had been previously identified along the cluster main sequence. The blue WDCS is populated by the evolved stars of the He-normal component (~0.55 Msun CO-core DA objects) while the red WDCS hosts the end-products of the He-rich population (~0.46 Msun objects, ~10% CO-core and ~90% He-core WDs). The He-core WDs correspond to He-rich stars that missed the central He-ignition, and we estimate their fraction by analyzing the population ratios along the cluster horizontal branch.Comment: 6 pages, 4 figures, accepted for publication in ApJ Letters. Fixed a typo in the metadat

The State-of-the-Art HST Astro-Photometric Analysis of the core of \omega Centauri. III. The Main Sequence's Multiple Populations Galore

We take advantage of the exquisite quality of the Hubble Space Telescope 26-filter astro-photometric catalog of the core of Omega Cen presented in the first paper of this series and the empirical differential-reddening correction presented in the second paper in order to distill the main sequence into its constituent populations. To this end, we restrict ourselves to the five most useful filters: the magic "trio" of F275W, F336W, and F438W, along with F606W and F814W. We develop a strategy for identifying color systems where different populations stand out most distinctly, then we isolate those populations and examine them in other filters where their sub-populations also come to light. In this way, we have identified at least 15 sub-populations, each of which has a distinctive fiducial curve through our 5-dimensional photometric space. We confirm the MSa to be split into two subcomponents, and find that both the bMS and the rMS are split into three subcomponents. Moreover, we have discovered two additional MS groups: the MSd (which has three subcomponents) shares similar properties with the bMS, and the MSe (which has four subcomponents), has properties more similar to those of the rMS. We examine the fiducial curves together and use synthetic spectra to infer relative heavy-element, light-element, and Helium abundances for the populations. Our findings show that the stellar populations and star formation history of Omega Cen are even more complex than inferred previously. Finally, we provide as a supplement to the original catalog a list that identifies for each star which population it most likely is associated with.Comment: 22 pages, 17 figures (most in lower res), 2 tables, accepted for publication in Ap