Stellar population models in the UV, I : characterisation of the New Generation Stellar Library

Context. The spectral predictions of stellar population models are not as accurate in the ultra-violet (UV) as in the optical wavelength domain. One of the reasons is the lack of high-quality stellar libraries. The New Generation Stellar Library (NGSL), recently released, represents a significant step towards the improvement of this situation. Aims. To prepare NGSL for population synthesis, we determined the atmospheric parameters of its stars, we assessed the precision of the wavelength calibration and characterised its intrinsic resolution. We also measured the Galactic extinction for each of the NGSL stars. Methods. For our analyses we used ULySS, a full spectrum fitting package, fitting the NGSL spectra against the MILES interpolator. Results. We find that the wavelength calibration is precise up to 0.1 px, after correcting a systematic effect in the optical range. The spectral resolution varies from 3 angstrom in the UV to 10 angstrom in the near-infrared (NIR), corresponding to a roughly constant reciprocal resolution R = lambda/delta lambda approximate to 1000 and an instrumental velocity dispersion sigma(sins) approximate to 130 km s(-1). We derived the atmospheric parameters homogeneously. The precision for the FGK stars is 42 K, 0.24 and 0.09 dex for T-eff, logg and [Fe/H], respectively. The corresponding mean errors are 29K, 0.50 and 0.48 dex for the M stars, and for the OBA stars they are 4.5 percent, 0.44 and 0.18 dex. The comparison with the literature shows that our results are not biased

IAU Symposium 241 - Stellar Populations as Building Blocks of Galaxies

Stellar populations, building blocks of galaxies, are direct tracers of the star formation history, the chemical enrichment and the assembly of galaxies in the Universe. They therfore allow us to understand how galaxies formed and evolved. This last decade has witnessed a revolution in our observations of galaxies; with larger telescopes and new instruments we are not only able to look deeper in the Universe, we can also study nearby galaxies with greater detail. The fact that now is becoming possible to resolve stars up to the distance of Virgo Cluster allows us to rigorously compare and calibrate the analysis of the integrated light with resolved stellar populations. These Proceedings report the considerable progress made in recent years in this topic. Theorists and observers, researchers of resolved and unresolved stellar populations, discussed the ingredients of stellar population models, and rigorously compared them to new data, forcing theorists to develop more refined models and methods to derive the physical parameters of the stellar populations. New results from the Milky Way, the Local Group, and nearby and distant galaxies were presented.Comment: This is the table of contents of the upcoming proceedings of IAU Symposium 241. The book will appear in September, from Cambridge University Press, and will also be available electronically at http://www.journals.cambridge.org/action/displayJournal?jid=IA

Evidence for Blue Straggler Stars Rejuvenating the Integrated Spectra of Globular Clusters

Integrated spectroscopy is the method of choice for deriving the ages of unresolved stellar systems. However, hot stellar evolutionary stages, such as hot horizontal branch stars and blue straggler stars (BSSs), can affect the integrated ages measured using Balmer lines. Such hot, "non-canonical" stars may lead to overestimations of the temperature of the main sequence turn-off, and therefore underestimations of the integrated age of a stellar population. Using an optimized Hbeta index in conjunction with HST/WFPC2 color-magnitude diagrams (CMDs), we show that Galactic globular clusters exhibit a large scatter in their apparent "spectroscopic" ages, which does not correspond to that in their CMD-derived ages. We find for the first time that the specific frequency of BSSs, defined within the same aperture as the integrated spectra, shows a clear correspondence with Hbeta in the sense that, at fixed metallicity, higher BSS ratios lead to younger "apparent" spectroscopic ages. Thus, the specific frequency of BSSs in globular clusters sets a fundamental limit on the accuracy for which spectroscopic ages can be determined for globular clusters, and maybe for other stellar systems like galaxies. The observational implications of this result are discussed.Comment: 5 pages, 2 figures, 1 table. Accepted for publication in ApJ

MIUSCAT: extended MILES spectral coverage. II. Constraints from optical photometry

In the present work we show a comprehensive comparison of our new stellar population synthesis MIUSCAT models with photometric data of globular clusters and early-type galaxies. The models compare remarkably well with the colours of Milky Way globular clusters in the optical range. Likewise, the colours of M31 globular clusters can also be explained by the models by assuming younger ages then their Galactic counterparts. When compared with quiescent galaxies we reproduce the colour evolution at intermediate redshift. On the other hand we find that the colour relations of nearby early-type galaxies are still a challenge for present-day stellar population synthesis models. We investigate a number of possible explanations and establish the importance of alpha-enhanced models to bring down the discrepancy with observations.Comment: 20 pages, 17 figures; MNRAS in press. Model predictions available at our website: http://miles.iac.e