On the metallicity of open clusters. III. Homogenised sample

Open clusters are known as excellent tools for various topics in Galactic research. For example, they allow accurately tracing the chemical structure of the Galactic disc. However, the metallicity is known only for a rather low percentage of the open cluster population, and these values are based on a variety of methods and data. Therefore, a large and homogeneous sample is highly desirable. In the third part of our series we compile a large sample of homogenised open cluster metallicities using a wide variety of different sources. These data and a sample of Cepheids are used to investigate the radial metallicity gradient, age effects, and to test current models. We used photometric and spectroscopic data to derive cluster metallicities. The different sources were checked and tested for possible offsets and correlations. In total, metallicities for 172 open cluster were derived. We used the spectroscopic data of 100 objects for a study of the radial metallicity distribution and the age-metallicity relation. We found a possible increase of metallicity with age, which, if confirmed, would provide observational evidence for radial migration. Although a statistical significance is given, more studies are certainly needed to exclude selection effects, for example. The comparison of open clusters and Cepheids with recent Galactic models agrees well in general. However, the models do not reproduce the flat gradient of the open clusters in the outer disc. Thus, the effect of radial migration is either underestimated in the models, or an additional mechanism is at work. [abridged]Comment: Accepted for publication in Astronomy and Astrophysics: 18 pages, 10 figures, 4 table

A spectroscopic survey of faint, high-galactic latitude red clump stars. II. The medium resolution sample

Aims. The goal of our survey is to provide accurate and multi-epoch radial velocities, atmospheric parameters (Teff, log g and [M/H]), distances and space velocities of faint Red Clump stars. Methods. We recorded high signal-to-noise (S/N >= 200) spectra of Red Clump stars, over the 4750-5950 Ang range, at a resolving power 5500. The target stars are distributed over the great circle of the celestial equator. Radial velocities were obtained via cross-correlation against IAU radial velocity standards. Atmospheric parameters were derived via chi^2 fit to a synthetic spectral library. A large number of RC stars from other surveys were re-observed to check the consistency of our results. Results. A total of 245 Red Clump stars were observed (60 of them with a second epoch observation separated in time by about three months), and the results are presented in an output catalog. None of them is already present in other surveys of Red Clump stars. In addition to astrometric and photometric support data from external sources, the catalog provides radial velocities (accuracy sigma(RV)=1.3 km/s), atmospheric parameters (sigma(Teff)=88 K, sigma(log g)=0.38 dex and sigma([M/H])=0.17 dex), spectro-photometric distances, (X,Y,Z) galacto-centric positions and (U,V,W) space velocities.Comment: in press in A&

The ELODIE archive

The ELODIE archive contains the complete collection of high-resolution echelle spectra accumulated over the last decade using the ELODIE spectrograph at the Observatoire de Haute-Provence 1.93-m telescope. This article presents the different data products and the facilities available on the web to re-process these data on-the-fly. Users can retrieve the data in FITS format from http://atlas.obs-hp.fr/elodie and apply to them different functions: wavelength resampling and flux calibration in particular.Comment: 8 pages, 3 figures and 1 tabl

The Great Eruption of Eta Carinae

During the years 1838-1858, the very massive star {\eta} Carinae became the prototype supernova impostor: it released nearly as much light as a supernova explosion and shed an impressive amount of mass, but survived as a star.1 Based on a light-echo spectrum of that event, Rest et al.2 conclude that "a new physical mechanism" is required to explain it, because the gas outflow appears cooler than theoretical expectations. Here we note that (1) theory predicted a substantially lower temperature than they quoted, and (2) their inferred observational value is quite uncertain. Therefore, analyses so far do not reveal any significant contradiction between the observed spectrum and most previous discussions of the Great Eruption and its physics.Comment: To appear in Nature, a brief communication arising in response to Rest et al. 2012. Submitted to Nature February 17, 201

Spectroscopic ages and metallicities of stellar populations: validation of full spectrum fitting

Fitting whole spectra at intermediate spectral resolution (R = 1000 -- 3000), to derive physical properties of stellar populations, appears as an optimized alternative to methods based on spectrophotometric indices: it uses all the redundant information contained in the signal. This paper addresses the validation of the method and it investigates the quality of the population models together with the reliability of the fitting procedures. We are using two algorithms: STECKMAP, a non-parametric regularized program and NBURSTS a parametric non-linear minimization. We compare three spectral synthesis models for single stellar populations: Pegase-HR, Galaxev (BC03) and Vazdekis/Miles, and we analyse spectra of Galactic clusters whose populations are known from studies of color-magnitude diagrams (CMD) and spectroscopy of individual stars. We find that: (1) The quality of the models critically depends on the stellar library they use. Pegase-HR and Vazdekis/Miles are consistent, while the comparison between Pegase-HR and BC03 shows some systematics reflecting the limitations of the stellar library (STELIB) used to generate the latter models; (2) The two fitting programs are consistent; (3) For globular clusters and M67 spectra, the method restitutes metallicities in agreement with spectroscopy of stars within 0.14 dex; (4) The spectroscopic ages are very sensitive to the presence of a blue horizontal branch (BHB) or of blue stragglers. A BHB morphology results in a young SSP-equivalent age. Fitting a free amount of blue stars in addition to the SSP model to mimic the BHB improves and stabilizes the fit and restores ages in agreement with CMDs studies. This method is potentially able to disentangle age or BHB effects in extragalactic clusters.Comment: accepted in MNRAS; Full version available at http://www-obs.univ-lyon1.fr/labo/perso/prugniel/mina/koleva.pd

Evolutionary synthesis of galaxies at high spectral resolution with the code PEGASE-HR

We present PEGASE-HR, a new stellar population synthesis program generating high resolution spectra (R=10 000) over the optical range lambda=400--680 nm. It links the spectro-photometric model of galaxy evolution PEGASE.2 (Fioc & Rocca-Volmerange 1997) to an updated version of the ELODIE library of stellar spectra observed with the 193 cm telescope at the Observatoire de Haute-Provence (Prugniel & Soubiran 2001a). The ELODIE star set gives a fairly complete coverage of the Hertzprung-Russell (HR) diagram and makes it possible to synthesize populations in the range [Fe/H]=-2 to +0.4. This code is an exceptional tool for exploring signatures of metallicity, age, and kinematics. We focus on a detailed study of the sensitivity to age and metallicity of the high-resolution stellar absorption lines and of the classical metallic indices proposed until now to solve the age-metallicity degeneracy. Validity tests on several stellar lines are performed by comparing our predictions for Lick indices to the models of other groups. The comparison with the lower resolution library BaSeL (Lejeune et al. 1997) confirms the quality of the ELODIE library when used for simple stellar populations (SSPs) from 10 Myr to 20 Gyr. Predictions for the evolved populations of globular clusters and elliptical galaxies are given and compared to observational data. Two new high-resolution indices are proposed around the Hgamma line. They should prove useful in the analysis of spectra from the new generation of telescopes and spectrographs.Comment: 18 pages, 18 figures. Astronomy & Astrophysics, in press. The models are available at http://www.iap.fr/pegase

NGC 6994 - clearly not a physical stellar ensemble

The sparse stellar ensemble NGC 6994 = M73 has recently been discussed as a possible remnant of an old open cluster. In order to solve the controversy on the nature of this object we have taken high-resolution spectra of the six brightest stars within 6' angular distance of its nominal position. These stars are the only obvious member candidates for an eventual cluster or cluster remnant since fainter stars do not show any significant concentration in the plane of the sky. The radial velocities, atmospheric parameters, and absolute magnitudes derived from the spectra reveal that the six candidates do not share the same kinematics and lie at different distances from the Sun. The proper motions provided by the Tycho-2 catalogue show that there is also a large spread in the tangential motions of these stars, in agreement with the spectroscopic results. This leads to the conclusion that the few bright stars that constitute NGC 6994 are not a physical system (cluster, cluster remnant, or group). They must instead be understood as a projective chance alignment of physically unrelated field stars.Comment: 9 pages, 4 figures, accepted for publication in A&

Comparison of different spectral population models

We have compared simple stellar populations (SSPs) generated with different population synthesis tools: BC03, Vazdekis and Pegase.HR and different stellar libraries: ELODIE3.1, SteLib and MILES. We find that BC03/SteLib SSPs are biased toward solar metallicity, however Pegase.HR/ELODIE3.1 and Vazdekis/MILES are extremely consistent. The extensive coverage of the space of atmospheric parameters in the large stellar libraries allows precise synthesis for a large range of ages (0.1 .. 10 Gyr) and metallicities (-2 .. +0.4 dex) limited by the quality of the determination of stellar parameters (like temperature scale of the giants)