A great effort is being made by the international Virtual Observatory community to build tools ready to be used by scientists. Presently, providing access to theoretical spectra in general, and synthetic spectra of galaxies in particular, is a matter of current interest in the Virtual Observatory. Several ways of accessing such spectra are available. We present two of them for accessing PEGASE.HR evolutionary synthesis models: HTTP-access to a limited number of parameters using Simple Spectral Access Protocol (SSAP), and full-featured WEB-service based access using Common Execution Architecture (CEA).

By using synthetic integrated spectral energy distribution (ISED) of open clusters and a new evolutionary scenario involving interactive binaries, the theoretical model of simple stellar population (SSP) models are tested. Blue stragglers stars are commonly present in open clusters, which are usually around 2 magnitudes brighter and much bluer than the MS turn-off of the cluster, and they certainly belong to the same population as the host cluster. Considering the cluster as the nearest counterpart the concept of SSP, inclusion of such stars may have significant consequences on the integrated of properties of the population. We are showing in the present work that if one is to fit the real stellar population using the classical model based only on single star evolution theory, large errors in either age or metallicity (or both) can be made. The new theoretical SSP incorporated with interactive binaries can explain part of the difference between classical SSP model and synthetic ISED built up with open clusters. This suggests that the formation of blue stragglers in real clusters is more complicated as usually assumed in binary stellar population synthesis.

The discovery of an excess of light in the far-ultraviolet (UV) in 1969 in elliptical galaxies was a major surprise. While it is now clear that this UV excess (UV-upturn) is probably caused by an old population of helium-burning stars. Han et al. (2002, 2003) proposed a binary model for the formation of hot subdwarfs (helium burning stars) and the model can reproduce the observations in our Galaxy. By applying the binary model to the study of evolutionary population synthesis, we have obtained an a priori model for the UV-upturn of elliptical galaxies. The model shows that the UV-upturn is most likely resulted from binary interactions and it is universal (not very much metallicity-dependant) in ellipticals. This has major implications for understanding the evolution of the UV-upturn and elliptical galaxies in general; contrary to previous postulates, it implies that the UV-upturn is not a sign of age, but could be a potentially powerful indicator for a recent minor burst of star-forming activity.

We have compared simple stellar populations (SSPs) generated with different population synthesis tools: BC03, Vazdekis and Pégase.HR and different stellar libraries: ELODIE3.1, SteLib and MILES. We find that BC03/SteLib SSPs are biased toward solar metallicity, however Pégase.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 (temperature scale of the giants).

We are using full spectrum fitting to determine the ages and metallicities of Galactic clusters (M67 and globular clusters). We find that the method is very accurate to measure the metallicity. Blue horizontal branches are well identified as a ‘young’ sub-component, and the age of the ‘old’ component is in fair agreement with CMD determinations.

We present the effects due to alpha-element enhancement on surface brightness fluctuation magnitudes and broad-band colors in order to investigate the calibration zero-point. We study these effects at ages covering 0.1 to 13 Gyr and metallicities of Z=0.0003 to 0.04 using the Teramo group's isochrones, BaSTI. Our preliminary results indicate that alpha-enhanced models are needed to match massive red galaxies while solar-scaled models are fine for dwarf blue galaxies to reproduce their independently estimated ages and metallicities.

We try to find some colors that have the potential to study stellar age and metallicity from UBV RIJHK and ugriz colors using the simple population synthesis model of Bruzual & Charlot 2003 (BC03). The principal component analysis (PCA) (See Kong & Cheng 2001) and relative sensitive parameter techniques (see Worthey 1994) are used in this work. As a result, some colors such as (B – K), (R – K), (I – H), (B – V), (U – R), (R – I), are found to have the ability to break the stellar age-metallicity degeneracy partially and can possibly be used for studying stellar populations of galaxies.

A new theoretical calibration of surface-brightness fluctuations (SBF) for single age, single metallicity stellar populations is presented for the optical and near-IR broad-band filters, as well as for the HST WFPC2 filters. The IAC-star code is used together with two Padua and the Teramo stellar evolution libraries. A set of single-burst stellar populations with a wide range of ages (3Gy-15Gy) and metallicities (Z=0.0001-0.03) have been computed. The present theoretical calibration shows that the analysis of near-IR SBF provides a very powerful tool in the study and characterization of unresolved stellar populations.

I present a report of the work in progress towards building a graphical user interface (GUI) for STECKMAP (STEllar Content and Kinematics via Maximum A Posteriori). It is the first serious attempt at making a GUI for a software for analysis of stellar content from integrated spectra of galaxies. It will allow a wider fraction of the community to use this sort of tools thanks to improved user-friendliness. As a result, better understanding of these methods and their strengths and flaws will be achieved by the community. This software also has a strong pedagogical potential. It will contribute to a higher awareness of the community concerning the challenges and the oppportunities in stellar populations modelling. The homepage of the project is http://www.astro.u-strasbg.fr/Obs/GALAXIES/stecmap_eng.html.

Our knowledge about unresolved stellar systems comes from comparing integrated-light properties to SSP models. Therefore it is crucial to calibrate the latter as well as possible by integrated-light colors of clusters that have reliable ages and metallicities (deep CMDs and/or spectroscopy of individual giants). This is especially true for the NIR and MIR, which contain important population synthesis diagnostics and are often used to derive masses of stellar systems at hight redshifts. Here we present integrated colors of MC clusters using new VRI photometry and 2MASS data. In the imminent future we will include MIR data from Spitzer/IRAC. We compare our new colors with popular SSP models to illustrate their strengths and weaknesses.

We present a cautionary study characterizing the influence of non-solar abundance ratios on Hδ derived ages in the integrated light of early-type galaxies. We measure Hδ using a variety of index measurements designed to assess contamination from CN molecular lines in and around the Hδ feature. We propose that neighboring CN lines have a strong effect on continuum levels and the Hδ feature itself, thereby leading to a difficulty in designing an accurate Hδ index definition in populations with C andor N non-solar abundance ratios with population synthesis models that do not account for such variations.

Surface Brightness Fluctuations (SBF) are an useful distance indicator of gas-free galaxies as far as 100-150 Mpc with an accuracy of 5–10%. Recent applications have proved that the SBF technique is also a powerful tool to analyze the stellar content of elliptical-like galaxies by means of data/models comparison. Here we present a comprehensive study of the method on the base of a careful analysis of SBF models and a set of accurate multibands SBF measures obtained with HST.

Using subspace methods, we study the distribution of physical components of galaxies in wavelength space. We find that it is valid to assume that the stellar and the gaseous components of galaxies span complementary subspaces. To first order, stellar and gaseous spectral features can be extracted from galaxy spectra through a simple matrix multiplication. By comparing the stellar continua obtained respectively using the model-based and the empirical approach through a commonality measure, we conclude that the latter may lose higher-order spectral features.

The presence of blue straggler stars (BSs) as secure members of Galactic open clusters (OCs) poses a major challenge to the conventional picture of evolutionary population synthesis (EPS) based on the stellar evolution theory of single stars, since the major formation ways of BSs are all correlated with stellar interactions. With a working sample including 100 Galactic OCs with age ranging from 0.1 to 10 Gyr, the contributions of BSs to the integrated light of their host clusters are calculated on an individual cluster base. We also show in this work that the intrinsic evolutionary stages of OCs can be enormously misunderstood by the conventional simple stellar population (SSP) models, if the real/observed integrated spectral energy distributions (ISEDs) of OCs are the only thing that we can depend on and are fitted with ISEDs of conventional SSPs, and consequently the huge uncertainties in age and/or metallicity can be conservatively estimated at 50%. Thus, we strongly confirm that when the conventional EPS technique is used to study the properties of stellar populations in complicated systems such as galaxies, the contributions of BSs should be definitely taken into account in order to avoid big fitting mistake.

Using Yunnan evolutionary population synthesis (EPS) models, we present integrated colours, integrated spectral energy distributions (ISEDs) and absorption-line indices defined by the Lick Observatory image dissector scanner (Lick/IDS) system, for an extensive set of instantaneous-burst binary stellar populations (BSPs) with interactions. By comparing the results for populations with and without interactions we show that the inclusion of binary interactions makes the appearance of the population substantially bluer. This effect raises the derived age and metallicity of the population.

To be used in the studies of modern spectroscopic galaxy surveys at intermediate/high spectral resolution, we also present intermediate- (3 Å) and high-resolution (∼ 0.3 Å) ISEDs and Lick/IDS absorption-line indices for BSPs. To directly compare with observations the Lick/IDS absorption indices are also presented by measuring them directly from the ISEDs.

To this contribution we present a brief review of our recent abundance surveys (Reddy et al. 2003; Reddy et al. 2006) of the Milky Way galaxy. Survey focussed on controlled samples of stars selected based on their kinematic properties to belong either thin disk, thick disk or halo components of the Galaxy. Abundance and kinematic results were obtained for about 400 F-, G- and K- dwarfs. Abundances for 22 elements representing different production mechanisms (α-process, p-capture, Fe-peak, s- and r-process) and sites (AGB, SNIa, SNII) were obtained using high quality and high resolution echelle spectra.

Results showed thin and thick disk components are distinct stellar populations with different chemical history. The ratios of α-elements and a few other elements (like Al, V, and Co) are clearly enhanced for stars in the thick disk compared to thin disk stars at given metallicity. Abundance ratios for halo and thick disk stars are very similar. Dispersion in [X/Fe] ratios at given metallicity is comparable to measurement errors inferring lack of cosmic' scatter. Thick disk stars are older (10-13 Gyrs) compared to their counter parts (1-10 Gyrs) in the thin disk. Abundance results for thin and thick disk stars favor the models of hierarchical formation of the galaxy.

With the advent of large wide-field cameras and deep, CCD-based “all-sky” surveys, the study of the structure of the Milky Way (MW) through its stellar populations has become feasible. We are developing tools to apply CMD fitting techniques for this purpose. Here we present two applications. First we constrain the properties of the old and young stellar populations of the Canis Major overdensity. Secondly, we demonstrate how CMD fitting can be used to detect faint overdensities in the halo of the MW and determine their properties, using data from the Sloan Digital Sky Survey (SDSS).

We present an overview and the first results of a large ACS survey of Galactic globular clusters. This Hubble Space Telescope (HST) Treasury project is designed to obtain photometry with S/N ≳ 10 for main sequence stars with masses ≳ 0.2M⊙ in a sample of globulars using the Advanced Camera for Surveys (ACS) Wide Field Channel. Our sample consists primarily of globular clusters within about 20 kpc of the Sun with fairly low reddening. We also added a few clusters of special interest toward the Galactic bulge as well as six clusters believed to be members of the Sagittarius dwarf spheroidal galaxy. This sample of 66 globular clusters was observed with HST using 134 orbits. We plan to explore a wide variety of scientific issues with these data including the formation timescale of the Milky Way halo, the age of the Universe, the dynamical evolution of stars in globular clusters (e.g. mass segregation), and the mass of the Milky Way and its gravitational potential field.

We adopted uvby Strömgren photometry to investigate the metallicity distribution of ω Cen Red Giant (RG) stars. We provided a new empirical calibration of the Strömgren m1≡(v − b) − (b − y) metallicity index based on cluster stars. The new calibration has been applied to a sample of ω Cen RGs. The shape of the estimated metallicity distribution is clearly asymmetric, with a sharp cut-off at low metallicities ([Fe/H] < −2.0) and a metal-rich tail up to [Fe/H] ∼ 0.0. Two main metallicity peaks have been identified, around [Fe/H]≈ −1.9 and −1.3 dex, and a metal-rich shoulder at ≈ −0.2 dex.

We present results from a series of projects aimed at better understanding the ages, velocities, metallicities, and distances of populous clusters in the LMC. Using FORS2 on the VLT, we obtained near-infrared spectra for more than 200 stars in 28 LMC clusters, which span a large range of ages and metallicities. The strong absorption lines of the Ca II triplet are used to calculate cluster velocities and abundances. We determine cluster ages through main sequence fitting of theoretical isochrones to deep optical photometry using a combination of published photometry, VLT FORS2 images and archival HST WFPC2 images. As shown by Grocholski & Sarajedini, a cluster's age and [Fe/H] can be used to predict the K-band luminosity of core helium burning red clump (RC) stars in that cluster. Using ISPI on the CTIO 4m telescope, we imaged 17 LMC clusters in the J– and K–bands, with the resulting photometry reaching ∼0.5 mag below the RC. We combine the measured apparent RC magnitudes with the predicted absolute values to determine absolute distances to each of these clusters and thereby probe the structure of the LMC as traced by its intermediate-age clusters.