Context. The rich open cluster M 67 is known to have a chemical composition close to solar and an age of about 3.5-4.8 Gyr. It offers an important opportunity to check and develop our understanding of the physics and the evolution of solar-type stars. Aims. We present a spectroscopic study at high resolution, R ≈ 50 000, of 14 stars located on the main sequence, at the turn-off point, and on the early subgiant branch in the cluster in order to investigate its detailed chemical composition, for comparison with the Sun and solar twins in the solar neighbourhood, and to explore selective atomic diffusion of chemical elements as predicted by stellar-structure theory. Methods. We have obtained VLT/FLAMES-UVES spectra and analysed these strictly differentially in order to explore chemicalabundance similarities and differences between the M 67 stars and the Sun and among the M 67 stars themselves. Results. Individual abundances of 19 different chemical elements are obtained for the stars. They are found to agree very well with solar abundances, with abundance ratios closer to solar than those of most solar twins in the solar neighbourhood. An exception is Li, which shows considerable scatter among the cluster stars. There is a tendency for the cluster-star abundances to be more depleted than the abundances in the field stars in correlation with the condensation temperature of the elements, a tendency also found earlier for the Sun. Moreover, the heavy-element abundances are found to be reduced in the hotter stars and dwarfs by typically ≤0.05 dex, as compared to the abundances of the subgiants. Conclusions. The results support the hypothesis that the gas of the proto-cluster was depleted by formation and cleansing of dust before the stars formed. They also add support to the proposal that the Sun was formed in a dense stellar environment. Moreover, the observed minor reductions of heavy elements, relative to our standard star M 67-1194 and the subgiants, in the atmospheres of dwarfs and turn-off point stars seem to suggest that diffusion processes are at work in these stars, although the evidence is not compelling. Based on theoretical models, the diffusion-corrected initial metallicity of M 67 is estimated to be [Fe/H] = +0.06.

Context. The rich open cluster M67 is known to have a chemical composition close to solar, and an age around 4 Gyr. It thus offers the opportunity to check our understanding of the physics and the evolution of solar-type stars in a cluster environment. Aims. We present the first spectroscopic study at high resolution, R ≈ 50 000, of the potentially best solar twin, M67-1194, identified among solar-like stars in M67. Methods. G dwarfs in M67 (d ≈ 900 pc) are relatively faint (V ≈ 15), which makes detailed spectroscopic studies time-consuming. Based on a pre-selection of solar-twin candidates performed at medium resolution by Pasquini et al. (2008, A&A, 489, 677), we explore the chemical-abundance similarities and differences between M67-1194 and the Sun, using VLT/FLAMES-UVES. Working with a solar twin in the framework of a differential analysis, we minimize systematic model errors in the abundance analysis compared to previous studies which utilized more evolved stars to determine the metallicity of M67. The differential approach yields precise and accurate chemical abundances for M67, which enhances the possibility to use this object in studies of the potential peculiarity, or normality, of the Sun. Results. We find M67-1194 to have stellar parameters indistinguishable from the solar values, with the exception of the overall metallicity which is slightly super-solar ([Fe/H] = 0.023 ± 0.015). An age determination based on evolutionary tracks yields 4.2 ± 1.6 Gyr. Most surprisingly, we find the chemical abundance pattern to closely resemble the solar one, in contrast to most known solar twins in the solar neighbourhood. Conclusions. We confirm the solar-twin nature of M67-1194, the first solar twin known to belong to a stellar association. This fact allows us to put some constraints on the physical reasons for the seemingly systematic departure of M67-1194 and the Sun from most known solar twins regarding chemical composition. We find that radiative dust cleansing by nearby luminous stars may be the explanation for the peculiar composition of both the Sun and M67-1194, but alternative explanations are also possible. The chemical similarity between the Sun and M67-1194 also suggests that the Sun once formed in a cluster like M67.

1 Visiting Astronomer, European Southern Observatory. 2 Visiting Astronomer, WIYN 0.9 m. The 0.9 m telescope is operated by WIYN, Inc., on behalf of a Consortium of 10 partner Universities and Organizations (see ABSTRACT We report broadband UBV and/or BVR C I C CCD photometry for a total of 1857 stars in the thick-disk and halo populations of the Galaxy. The majority of our targets were selected as candidate field horizontal-branch or other A-type stars (FHB/A, N ¼ 576), or candidate low-metallicity stars (N ¼ 1221), from the HK and Hamburg/ESO objectiveprism surveys. Similar data for a small number of additional stars from other samples are also reported. These data are being used for several purposes. In the case of the FHB/A candidates they are used to accurately separate the lower gravity FHB stars from various higher gravity A-type stars, a subsample that includes the so-called blue metal poor stars, halo and thick-disk blue stragglers, main-sequence A-type dwarfs, and Am and Ap stars. These data are also being used to derive photometric distance estimates to high-velocity hydrogen clouds in the Galaxy and for improved measurements of the mass of the Galaxy. Photometric data for the metal-poor candidates are being used to refine estimates of stellar metallicity for objects with available medium-resolution spectroscopy, to obtain distance estimates for kinematic analyses, and to establish initial estimates of effective temperature for analysis of high-resolution spectroscopy of the stars for which this information now exists.

Context. The relatively wide spread in the derived metallicities ([Fe/H]) of M dwarfs shows that various approacheshave not yet converged to consistency. The presence of strong molecular features and incomplete line lists for the corresponding molecules have made determining the metallicity of M dwarfs difficult. Furthermore, the faint M dwarfs require long exposure times for the signalto-noise ratio needed for a detailed spectroscopic abundance analysis. Aims. We present a high-resolution (R ∼ 50 000) spectroscopic study of a sample of eight single M dwarfs and three wide-binary systems observed in the infrared J band. Methods. The absence of large molecular contributions allows for a precise continuum placement. We derived metallicities based on the best fit of synthetic spectra to the observed spectra. To verify the accuracy of the applied atmospheric models and test our synthetic spectrum approach, three binary systems with a K-dwarf primary and an M-dwarf companion were observed and analysed along with the single M dwarfs. Results. We obtain good agreement between the metallicities derived for the primaries and secondaries of our test binaries, thereby confirming the reliability of our method of analysing M dwarfs. Our metallicities agree well with some earlier determinations, and deviate from others. Conclusions. We conclude that spectroscopic abundance analysis in the J band is a reliable method for establishing the metallicity scale for M dwarfs. We recommend its application to a larger sample covering lower, as well as higher, metallicities. Further prospects for the method include abundance determinations for individual elements.

Context. The use of model atmospheres for deriving stellar fundamental parameters, such as T eff , log g, and [Fe/H], will increase as we find and explore extreme stellar populations where empirical calibrations are not yet available. Moreover, calibrations for upcoming large satellite missions of new spectrophotometric indices, similar to the uvby−Hβ system, will be needed. Aims. We aim to test the power of theoretical calibrations based on a new generation of MARCS models by comparisons with observational photomteric data. Methods. We calculated synthetic uvby-Hβ colour indices from synthetic spectra. A sample of 367 field stars, as well as stars in globular clusters, is used for a direct comparison of the synthetic indices versus empirical data and for scrutinizing the possibilities of theoretical calibrations for temperature, metallicity, and gravity. Results. We show that the temperature sensitivity of the synthetic (b − y) colour is very close to its empirical counterpart, whereas the temperature scale based upon Hβ shows a slight offset. The theoretical metallicity sensitivity of the m 1 index (and for G-type stars its combination with c 1 ) is somewhat higher than the empirical one, based upon spectroscopic determinations. The gravity sensitivity of the synthetic c 1 index shows satisfactory behaviour when compared to obervations of F stars. For stars cooler than the sun, a deviation is significant in the c 1 -(b − y) diagram. The theoretical calibrations of (b − y), (v − y), and c 1 seem to work well for Pop II stars and lead to effective temperatures for globular cluster stars supporting recent claims that atomic diffusion occurs in stars near the turnoff point of NGC 6397. Conclusions. Synthetic colours of stellar atmospheres can indeed be used, in many cases, to derive reliable fundamental stellar parameters. The deviations seen when compared to observational data could be due to incomplete linelists but are possibly also due to the effects of assuming plane-parallell or spherical geometry and LTE.