Ground-Based CCD Astrometry with Wide Field Imagers. I. [Observations just a few years apart allow decontamination of field objects from members in two Globular clusters.]

This paper is the first of a series of papers in which we will apply the methods we have developed for high-precision astrometry (and photometry) with the Hubble Space Telescope to the case of wide-field ground-based images. In particular, we adapt the software originally developed for WFPC2 to ground-based, wide field images from the WFI at the ESO 2.2m telescope. In this paper, we describe in details the new software, we characterize the WFI geometric distortion, discuss the adopted local transformation approach for proper-motion measurements, and apply the new technique to two-epoch archive data of the two closest Galactic globular clusters: NGC 6121 (M4) and NGC 6397. The results of this exercise are more than encouraging. We find that we can achieve a precision of ~7 mas (in each coordinate) in a single exposure for a well-exposed star, which allows a very good cluster-field separation in both M4, and NGC 6397, with a temporal baseline of only 2.8, and 3.1 years, respectively.Comment: 1 pages, 17 figures, 1 table. Accepted for publication in A&A, on April 15 2006. For high resolution version: http://www.eso.org/~lbedin/WFI_method/ms.ps.g

SiC(0001): a surface Mott-Hubbard insulator

We present ab-initio electronic structure calculations for the Si-terminated SiC(0001)$\sqrt{3}\times\sqrt{3}$ surface. While local density approximation (LDA) calculations predict a metallic ground state with a half-filled narrow band, Coulomb effects, included by the spin-polarized LDA+U method, result in a magnetic (Mott-Hubbard) insulator with a gap of 1.5 eV, comparable with the experimental value of 2.0 eV. The calculated value of the inter-site exchange parameter, J=30K, leads to the prediction of a paramagnetic Mott state, except at very low temperatures. The observed Si 2p surface core level doublet can naturally be explained as an on-site exchange splitting.Comment: RevTex, 4 pages, 4 eps-figure

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

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&

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

Absolute motions of globular clusters. II. [HST astrometry and VLT radial velocities in NGC6397]

In this paper we present a new, accurate determination of the three components of the absolute space velocity of the Galactic globular cluster NGC6397 (l 338d, b -12d). We used three HST/WFPC2 fields with multi-epoch observations to obtain astrometric measurements of objects in three different fields in this cluster. The identification of 33 background galaxies with sharp nuclei allowed us to determine an absolute reference point and measure the absolute proper motion of the cluster. The third component has been obtained from radial velocities measured on spectra from the multi-fiber spectrograph FLAMES at UT2-VLT. We find [mu_alpha cos(delta), mu_delta](J2000.0) = [+3.39 +/- 0.15, -17.55 +/- 0.15] mas/yr, and V_rad = +18.36 +/- 0.09 (+/-0.10) km/s. Assuming a Galactic potential, we calculate the cluster orbit for various assumed distances, and briefly discuss the implications.Comment: 7 pages, 5 figures, 4 tables. Accepted for publication in A&A, on April 27 200

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&