Here we present new empirical fitting functions for the Lick/IDS line-strength indices as measured in MILES (Medium-resolution INT Library of Empirical Spectra; Sánchez-Blázquez et al., Cenarro et al. 2007). Following previous work in the field (Gorgas et al. 1993; Worthey et al. 1994; Worthey & Ottaviani 1997), these functions describe the empirical behavior of the line-strength indices with the atmospheric stellar parameters. In order to derive the fitting functions we have devised a new procedure which, being fully automatic, provides a better description of the line-strength index variations in the stellar parameter space.

The Indo-US coudé feed stellar spectral library (CFLIB) published recently by Valdes et al. (2004) contains spectra of 1273 stars in the spectral region 3460 to 9464 Å at a resolution of 1 Å. About 500 stars in this database have gaps ranging from a few Å to several tens of Å in this wavelength range. We use a variation of Principal Component Analysis (PCA) technique to fill gaps of up to 5Å in a subset of spectra from the CFLIB. We hope to exploit the full potential of the scheme and attempt to fill larger gaps in stellar spectra in a subsequent study.

In this work we review the details, with emphasis on the near UV (3000-4500 Å), of the molecular database and the continuous opacity sources as implemented in the spectrum synthesis code used to calculate the spectral library presented in Coelho et al. (2005). We discuss our recent and future efforts towards the improvement of the calculation in this spectral range.

It is well established that mass loss from AGB stars due to dust driven winds cannot be arbitrarily low. We model the mass loss from carbon rich AGB stars using detailed frequency-dependent radiation hydrodynamics including dust formation. We present a study of the thresholds for the mass loss rate as a function of stellar parameters based on a subset of a larger grid of such models and compare these results to previous theoretical work. Furthermore, we demonstrate the impact of the pulsation mechanism and dust formation for the creation of a stellar wind and how it affects these thresholds and briefly discuss the consequences for stellar evolution.

The IMF UNIVERSALITY HYPOTHESIS cannot be discarded despite the existence of the CLUSTER IMF THEOREM. This means that the currently existing star-formation theory fails to describe the stellar outcome. The IGIMF THEOREM, however, predicts a variation of galaxy-wide IMFs in dependence of the galaxy's star-formation rate even if the IMF UNIVERSALITY HYPOTHESIS is valid. Taking indirect evidence from chemical evolution studies and the IGIMF THEOREM into account, it follows, however, that bulges and elliptical galaxies may have had a top-heavy IMF. A break-down of the IMF UNIVERSALITY THEOREM would thus be evident in extreme galaxy-wide (≳ 10 M⊙yr) star-formation events.

Previous attempts to explain the observed mass–metallicity relation of galaxies invoked selective outflows that need to be tuned to the particular cases.

Here we demonstrate that the simple concept of a star-formation history (SFH) dependent galaxy-wide integrated galactic initial stellar mass function (the IGIMF) can easily and naturally explain the mass–metallicity relation.

The idea relies on the simple fact that stars form in clusters. So to construct the IGIMF one needs to add all the stellar IMFs in the clusters formed in one epoch. The SFH enters because a galaxy with a high SFR can form more-massive clusters than a galaxy with a low SFR. Dwarf galaxies therefore have, per unit stellar mass formed, far fewer if any supernovae of type II than massive galaxies.

This work rests on the recent paper by Köppen, Weidner & Kroupa (2007).

Recent calculations of evolutionary tracks of TP-AGB stars of different mass and metallicity by Marigo et al. (2007) have been incorporated in the Bruzual & Charlot evolutionary population synthesis models. The mass of the stellar population in HUDF galaxies at z from 1 to 3 determined from fits to the spectro-photometric data of these galaxies is 50 to 80% lower than the mass determined from the BC03 models. The ages inferred for these populations are, with exceptions, 40 to 60% of the BC03 estimates.

We present a new spectral library for old and intermediate-aged single-age, single-metallicity stellar populations for a wide metallicity range. The major ingredient of these models is a new empirical stellar library composed of 985 stars, whose main characteristic is its unprecedented stellar atmospheric parameters coverage. The model SEDs cover the spectral range 3540-7410A at 2.3A (FWHM). We present some advantages and applications of these models.

In the last years, observations made by large international consortia made available hundreds of thousands of high-resolution spectra of galaxies in the local and distant Universe. The interpretation of these spectra in terms of stellar ages and metallicities is the key to reconstructing the star formation and chemical enrichment histories of the Universe. However, current spectral models rely on libraries of observed stars in the solar neighborhood, hampering our hability in interpreting populations that had undergone different star formation histories. Although it is available in the literature Lick/IDS model indices with variable chemical mixtures, we were still lacking high resolution spectral energy distribution models. Those models are required if we want to exploit the total amount of information contained in observed galaxy spectra. We present here our work aiming at the development of such models, constructed with a consistent set of synthetic stellar library and new stellar evolution models.

Using a huge library of 1-Å resolution spectra over a wide range of log Teff, log g and [Fe/H] and for both solar and α-enhanced abundance ratios [α/Fe], we derive the response functions (s) which are commonly used to correct indices with solar α-enhanced ratios to indices with α-enhanced>0. The s vary not only with the type of star but also with metallicity. With the aid of this and the fitting functions (s), we derive the indices for single stellar populations. The new indices for single stellar populations are used to derive the age, metallicity, and degree of enhancement first for sample of Galactic globular clusters for which these key parameters have been independently derived from the colour-magnitude diagram and/or spectroscopic studies, and second to study the classical two index diagram.

I discuss how galaxy fundamental parameters that are derived by comparing observational data with stellar population models, depend on the adopted models. The emphasis is on the treatment of the Thermally-Pulsing Asymptotic Giant Branch (TP-AGB) stellar phase, that is a source of major discrepancy between the various model renditions. In particular, I show that ages and stellar masses of galaxies decrease when models including an empirically-calibrated TP-AGB phase are considered. Illustrative examples are provided for a wide redshift range. Constraints to galaxy formation models need revision on the light of these results.

We report on recent progress in the modelling of the near-IR spectra of young stellar populations, i.e. populations in which red supergiants (RSGs) are dominant. First, we discuss the determination of fundamental parameters of RSGs from Phoenix model fits to their near-IR spectra; RSG-specific surface abundances are accounted for and effects of the microturbulence parameter are explored. New population synthesis predictions are then described and, as an example, it is shown that the spectra of young star clusters in M 82 can be reproduced very well from 0.5 to 2.4 μm. We warn of remaining uncertainties in cluster ages.

Recent spectral analysis of the SDSS DR3 has provided a wealth of information determined from the fossil record concerning star formation rates, stellar mass buildup, dust content and stellar mass functions. Essential to this work is an understanding of how model choice influences results gained from the analysis of galaxy spectra. We summarize the results of analyzing two plates of SDSS spectra with five different SSP models which appear in full in Panter et al. (2006) and discuss some residuals seen in a far larger sample of galaxies.

Simulations of the chemical enrichment histories of ten Local Group (LG) dwarf galaxies are presented, employing empirically-derived star formation histories (SFHs), a rich network of isotopic and elemental nucleosynthetic yields, and a range of prescriptions for supernova (SN)-driven outflows. Our main conclusions are that (i) neutron-capture element patterns (particularly that of Ba/Y) suggest a strong contribution from low- and intermediate-mass stars (LIMS), (ii) neutron star mergers may play a relatively larger role in the nucleosynthesis of dwarfs, (iii) SN feedback alone can explain the observed gas fraction in dwarf irregulars (dIrrs), but dwarf spheroidals (dSphs) require almost all their gas to be removed via ram pressure and/or tidal stripping, (iv) the predicted heavy Mg isotope enhancements in the interstellar medium (ISM) of dwarfs may provide an alternate solution to claims of a varying fine structure (v) the gas lost from dwarfs have O,Si/C abundances in broad agreement with intergalactic medium abundances at redshifts 2<z<4, and (vi) the chemical properties of dSphs are well-matched by preventing galactic winds from re-accreting, whilst those of dIrrs are better-matched by incorporating metallicity-dependent cooling and re-accretion of hot winds. Finally, doubts are cast upon a claimed association between LG dSph UMaII and HVC Complex A.

We have derived a new Hβ absorption index definition, which is fully optimized as an age indicator for old and intermediate-aged stellar populations. Rather than using stellar spectra, we employed theoretical SEDs at moderately high resolution for simple stellar populations of different ages and metallicities. The new index, which is virtually insensitive to metallicity, provides us with improved abilities for lifting the age-metallicity degeneracy, that affects the standard Hβ Lick/IDS index definition. Among other advantages this new index does not require spectra of extremely high signal-to-noise and it can be easily applied to observations of faint galaxies or surveys data.

Choosing either an empirical or a theoretical stellar library in stellar population models is a subject of debate. Theoretical models do not perfectly match high-resolution spectra of real stars, but we can use them to overcome the natural limitations of empirical libraries on covering abundance patterns that differ from solar neighbourhood stars. Here, we will mix stellar population synthesis models based on both theoretical and empirical libraries, in order to explore a new method to constrain the enhanced α-elements pattern on galaxies.

Evolutionary synthesis models (ESM) have been extensively used to obtain the star formation history in galaxies by means of SED fitting. Implicit in this use of ESM is that (a) for given evolutionary parameters, the shape of the SED is fixed whatever the size of the observed cluster (b) all regions of the observed SED have the same weight in the fit. However, Nature does not follow these two assumptions, as is implied by the existence of Surface Brightness Fluctuations in galaxies and as can be shown by simple logical arguments.

The theory interest group in the International Virtual Observatory Alliance (IVOA) has the goal of ensuring that theoretical data and services are taken into account in the IVOA standards process. In this poster we present some of the efforts carried out by this group to include evolutionary synthesis models in the VO framework. In particular we present the VO tool PGos3, developed by the INAOE (Mexico) and the Spanish Virtual Observatory which includes most of public SSP models in the VO framework (e.g. VOSpec). We also describe the problems related with the inclusion of synthesis models in the VO framework and we try to encourage people to define the way in which synthesis models should be described. This issue has implications not only for the inclusion of synthesis models in the the VO framework but also for a proper usage of synthesis models.

In this poster we present the analysis of the CMD of M67 (proposed in the Stellar Population Challenge) performed with VO applications. We found that, although the VO environment is still not ready to perform a complete analysis, its use provides highly useful additional information for the analysis. Thanks to the current VO framework, we are able to identify stars in the provided CMD that are not suitable for isochrone fitting. Additionally, we can complete our knowledge of this cluster extending the analysis to IR colors, which were not provided in the original data but that are available thanks to the VO. On the negative side, we find it difficult to access theoretical data from VO applications, so, currently, it is not possible to perform completely the analysis of the cluster inside the VO framework. However it is expected that the situation will improve in a near future.

We present a novel approach for simultaneous extraction of stellar population parameters and internal kinematics from the spectra integrated along a line of sight. We fit a template spectrum into an observed one in a pixel space using a non-linear χ2 minimization in the multidimensional parameter space, including characteristics of the line-of-sight velocity distribution (LOSVD) and parametrized star formation history (SFH). Our technique has been applied to IFU and multi-object spectroscopy of low-luminosity early type galaxies.