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In general, synthesis models provide the mean value of the distribution of possible integrated luminosities, this distribution (and not only its mean value) being the actual description of the integrated luminosity. Therefore, to obtain the closest model to an observation only provides confi- dence about the precision of such a fit, but not information about the accuracy of the result. In this contribution we show how to overcome this drawback and we propose the use of the theoretical mean-averaged dispersion that can be produced by synthesis models as a metric of fitting to infer accurate physical parameters of observed systems., Comment: Invited talk in "New Quests in stellar astrophysics II. Ultraviolet properties of evolved stellar populations" in press. 8 pages, 5 figures. (Part II on metric of fitting for population synthesis)

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., Comment: Stellar Populations as Building Blocks of Galaxies, Proceedings of IAU Symposium #241. Edited by A. Vazdekis and R. F. Peletier. Cambridge: Cambridge University Press, 2007, pp. 171-172

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., Comment: Stellar Populations as Building Blocks of Galaxies, Proceedings of IAU Symposium #241. Edited by A. Vazdekis and R. F. Peletier. Cambridge: Cambridge University Press, 2007, pp. 173-174

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., Comment: 2 pages, 2 figure, going to be published in the proceedings of the IAU Symp. 241, "Stellar Populations as Building Blocks of Galaxies"

The combination of stellar population synthesis models and photoionization models allows a better understanding of the spectral features of HII regions and HII galaxies. In this work we show that sampling effects in the initial mass function (IMF) are very important in the low cluster mass case. To this aim, we compute photoionization models ionized by realistic clusters made up of various combinations of individual stars and clusters made up with a synthesis model. We discuss the differences in the position on diagnostic diagrams and their implications., Comment: 2 pages, 1 figure, going to be published in the proceedings of the IAU Symp. 241, "Stellar Populations as Building Blocks of Galaxies"

The surface brightness fluctuations (SBF) method is a statistical method applied on image pixels in different bands. This contribution aims to distinguish between the observational (statistical) method and the theoretical (probabilistic) method based on stellar population synthesis and needed for the calibration of observational SBF. We find that the commonly used SBF theoretical definition as the "mean luminosity-weighted luminosity of the stellar population" is only compatible with the observational method under quite strong hypotheses, and that it is not compatible with stellar population theory results., Comment: 2 pages, ASP LaTeX. Proceedings of From Stars to Galaxies: Building the pieces to build up the Universe (Venice, October 16-20, 2006)

For a number of reasons, the properties of integrated stellar populations are distributed. Traditional synthesis models usually return the mean value of such distribution, and a perfect fitting to observational data is sought for to infer the age and metallicity of observed stellar populations. We show here that, while this is correct on average, it is not in individual cases because the mean may not be representative of actual values. We present a simple mathematical formalism to derive the shape of the population's luminosity distribution function (pLDF), and an abridged way to estimate it without computing it explicitly. This abridged treatment can be used to establish whether, for a specific case, the pLDF is Gaussian and the application of Gaussian tools, such as the chi^2 test, is correct. More in general, our formalism permits to compute weights to be attributed to different properties (spectral features or band luminosities) in the fitting process. We emphasize that our formalism does not supersede the results of traditionaly synthesis models, but permits to reinterpret and extend them into more powerful tools. The reader is referred to the original paper for further details., Comment: 8 pages, 4 figures. ASP LaTeX. Proceedings of From Stars to Galaxies: Building the pieces to build up the Universe (Venice, October 16-20, 2006)

In this review we address the uncertainties implicit in evolutionary synthesis model computations. After describing the general structure of synthesis codes, we discuss several source of uncertainties that may affect their results. In particular, we discuss the uncertainties arising in the computation of isochrones from evolutionary tracks; those related to atmosphere models; those that are a consequence of the incompleteness of the input ingredients; and those associated with the computational aspect used in synthesis codes. We also discuss the issue of the inclusion of distributed properties in synthesis models; as a paradigm of this case, we illustrate the difficulties implied by the inclusion of tracks with rotation in synthesis models. Finally, we describe several examples of the statistical approach to population synthesis. We report on the failure of the fuel consumption theorem (FCT) and the isochrone synthesis code to produce mutually consistent results. However, we argue that FCT and isochrone synthesis results are reliable for application to real systems in the wavelength range where they coincide. On the constructive side, we derive several useful survival strategies to bypass uncertainties. We show that single stellar populations at the turn-off ages of the tabulated tracks can be safely compared, as they are scarcely affected by the interpolation scheme used to compute isochrones. Finally, we suggest to use derivative quantities, such as the SN-rate, as bug detectors. On the recommendation side, we advocate for greater transparency and more documentation in synthesis modeling. We also ask stellar model makers to think of us and include more mass values in the tracks., Comment: 35 pages, 16 figures. Invited review to "Resolved Stellar Populations", Cancun (Mexico) Apr 2005. To appear on the proceedings of the conference edited by D. Valls-Gabaud & M. Chavez, ASP Conf. Ser. (ASP: San Francisco)

Synthesis models predict the integrated properties of stellar populations. Several problems exist in this field, mostly related to the fact that integrated properties are distributed. To date, this aspect has been either ignored (as in standard synthesis models, which are inherently deterministic) or interpreted phenomenologically (as in Monte Carlo simulations, which describe distributed properties rather than explain them). We approach population synthesis as a problem in probability theory, in which stellar luminosities are random variables extracted from the stellar luminosity distribution function (sLDF). We derive the population LDF (pLDF) for clusters of any size from the sLDF, obtaining the scale relations that link the sLDF to the pLDF. We recover the predictions of standard synthesis models, which are shown to compute the mean of the sLDF. We provide diagnostic diagrams and a simplified recipe for testing the statistical richness of observed clusters, thereby assessing whether standard synthesis models can be safely used or a statistical treatment is mandatory. We also recover the predictions of Monte Carlo simulations, with the additional bonus of being able to interpret them in mathematical and physical terms. We give examples of problems that can be addressed through our probabilistic formalism. Though still under development, ours is a powerful approach to population synthesis. In an era of resolved observations and pipelined analyses of large surveys, this paper is offered as a signpost in the field of stellar populations., Comment: Accepted by A&A. Substantially modified with respect to the 1st draft. 26 pages, 14 figs

We present new yields of 26Al from Wolf--Rayet stellar winds based on rotating stellar models which account well for numerous observed properties of massive stars. We study the impacts on the yields of a change of initial mass, metallicity and initial rotation velocity. We also consider the effects of a change of mass loss rates during the Wolf--Rayet phase. We show that for surface rotation velocities during the core H--burning phase matching to the observed ones, the quantity of 26Al ejected by a star of a given initial mass and metallicity is roughly doubled when the effects of rotation are taken into account. The metallicity dependence of the yield is, on the other hand, very similar to that obtained from non--rotating models. We estimate that at least about 20% to 50% (e.g. 0.6 --1.4 Msun) of the live 26Al detected in the Milky--Way originates from Wolf--Rayet stellar winds. We show the importance of a good knowledge of the present metallicity gradient and star formation rate in our galaxy for modeling both the variation of the 26Al surface density with the galactocentric distance and the global contribution of the Wolf--Rayet stellar winds to the present galactic mass of 26Al., Comment: 12 pages, 8 figures. Replaced version with corrected typos and references, and figure 4 replaced. To be published in Astronomy & Astrophysics

We present new $^{26}$Al stellar yields from rotating Wolf--Rayet stellar models which, at solar metallicity, well reproduce the observed properties of the Wolf-Rayet populations. These new yields are enhanced with respect to non--rotating models, even with respect to non--rotating models computed with enhanced mass loss rates. We briefly discuss some implications of the use of these new yields for estimating the global contribution of Wolf-Rayet stars to the quantity of $^{26}$Al now present in the Milky Way., Comment: 6 pages, 2 figures, to appear in New Astronomy Reviews

This paper describes a new determination of the primordial helium abundance (Y_P), based on the abundance analysis of five metal-poor extragalactic HII regions. For three regions of the sample (SBS 0335-052, I Zw 18, and H29) we present tailored photoionization models based on improved calculations with respect to previous models. In particular, we use the photoionization models to study quantitatively the effect of collisional excitation of Balmer lines on the determination of the helium abundance (Y) in the individual regions. This effect is twofold: first, the intensities of the Balmer lines are enhanced with respect to the pure recombination value, mimicking a higher hydrogen abundance; second, the observed reddening is larger than the true extinction, due to the differential effect of collisions on different Balmer lines. In addition to these effects, our analysis takes into account the following features of HII regions: (i) the temperature structure, (ii) the density structure, (iii) the presence of neutral helium, (iv) the collisional excitation of the HeI lines, (v) the underlying absorption of the HeI lines, and (vi) the optical thickness of the HeI lines. The object that shows the highest increase in Y after the inclusion of collisional effects in the analysis is SBS 0335-052, whose helium abundance has been revised by Delta Y = +0.0107. The revised Y values for the five objects in our sample yield an increase of +0.0035 in Y_P, giving Y_P = 0.2391 +/- 0.0020., Comment: 59 pages, 8 figures. AAS Latex. Accepted for publication in the Astrophysical Journal

(abriged) Stellar clusters with the same general physical properties (e.g., total mass, age, and star-formation mode) may have very different stellar mass spectra due to the incomplete sampling of the underlying mass function; such differences are especially relevant in the high-mass tail due to the smaller absolute number of massive stars. The dispersion in the number of massive stars also produces a dispersion in the properties of the corresponding ionising spectra. In this paper, we lay the bases for the future analysis of this effect by evaluating the dispersion in the ionising fluxes of synthetic spectra. As an important consequence, we found that the intensities of synthetic fluxes at different ionisation edges are strongly correlated, a fact suggesting that no additional dispersion will result from the inclusion of sampling effects in the analysis of diagnostic diagrams; this is true for HII regions on all scales. Additionally, we find convincing suggestions that the He II lines are strongly affected by sampling, and so cannot be used to constrain the evolutionary status of stellar clusters. We also establish the range of applicability of synthesis models set by the Lowest Luminosity Limit for the ionising flux, that is the lowest limit in cluster mass for which synthesis models can be applied to predict ionising spectra. This limit marks the boundary between the situations in which the ionising flux is better modeled with a single star as opposed to a star cluster; this boundary depends on the metallicity and age, ranging from 10^3 to more than 10^6 Mo. As a consequence, synthesis models should not be used to try to account for the properties of clusters with smaller masses., Comment: Replaced with accepted version

(abriged) In this paper we establish a Lowest Luminosity Limit (LLL) for the use of synthesis codes. The limit is defined by the following statement: The total luminosity of the cluster modeled must be larger than the individual contribution of any of the stars included in the assumed isochrones. This limit is independent of the assumptions on the IMF and almost independent on the star formation history. We have obtained the LLL for a wide range of ages (5 Myr to 20 Gyr) and metallicities (Z=0 to Z=0.019) from Girardi et al. (2002) isochrones. Using the LLL and the results of evolutionary synthesis models we have also obtained the minimal cluster mass, Mmin, for which the results of synthesis models may suffer from a severe bias in the computation of colors. We show that the results of synthesis models for clusters with mass equal to Mmin have a relative dispersion about or larger than 32% (i.e. a dispersion of 0.35 mag) in the corresponding photometric bands. This effect is more important for near infrared bands (except in the case where Asymptotic Giant Branch stars dominate the luminosity in optical bands). In particular, Mmin takes values between 10^4 and 10^5 Mo for the K band. From the observational point of view, we show that in surveys that reach luminosities near the LLL, the color distributions will be skewed toward the color with lower effective sources (toward the red in general), and that the skewness is a signature of the distribution of initial cluster masses in the survey. We also apply the LLL to Globular Clusters showing that sampling effects are relevant in these clusters and that they can explain (at least partially) the bias of the observed colors with respect to the predictions of synthesis models., Comment: Replaced by accepted version: 14 pages, 6 Figures, A&A in press

We determine the physical conditions in NGC 6543, obtaining two different estimates of the electron temperature (T_e) and one estimate of the electron density (N_e). The electron temperature is computed by means of the [O III] ratio lambda 5007/lambda 4363, and of the diagnostic diagram combining lambda 5007 to the [O III] infrared lines 52m and 88m. The continuum intensity measured on slit spectra is much higher than theoretically predicted under the simplest assumptions. After considering several possibilities, we suggest enhanced 2-photon emission as the most probable source of the additional continuum. While T_e and N_e derived from the diagnostic diagram agree with the most recent determination, T_e derived from lambda 5007/lambda 4363 is smaller than previously published values, probably due to the bias in lambda 4363 introduced by the uncertainty in the continuum. Our main conclusion is that it is not possible, with present-day data, to derive accurate temperature maps of photoionized nebulae., Comment: 4 pages, 3 figures. RMAA Latex. Proceedings of 'The Eighth Texas-Mexico Conference on Astrophysics: Energetics of Cosmic Plasmas', 30 October-2 November 2002, Mexico D.F., Mexico

In this contribution we evaluate the minimal cluster mass for which the effects of an incomplete sampling of the Initial Mass Function (IMF) cannot be neglected. This minimal cluster mass corresponds to the situation in which the integrated luminosity of the cluster modeled equals the luminosity of the most luminous individual star included in the model, and it takes values between 3x10^2 and 6x10^5 Mo depending on the age and the observed band. We show different examples for young (t < 10 Myr) and old (t up to 10 Gyr) stellar populations. We also make here a first release of the spectral energy distribution (SED) from 160 mic. (1.8x10^12 Hz) to 25 keV (6.0x10^18 Hz) for star-forming regions with ages between 0.1 and 10 Myr, and metallicities between Z=0.001 and Z=0.040. The SEDs are available at our WWW server and include the corresponding quantities for the evaluation of sampling effects. These SEDs can be directly used to obtain colors (including the X-rays ones), as input of photoionization codes, or for chi^2 fitting with observed data taking into account the intrinsic uncertainty of the models due to the IMF sampling., Comment: 4 pages, 3 Fig., rmaa.cls included. To appear in "8th Tex-Mex Meeting: Energetics of Cosmic Plasmas"

We revise the physical conditions in the O++ zone of the planetary nebula NGC 6543, obtaining two different estimates of the electron temperature (Te) and one estimate of the electron density (Ne). The electron temperature is computed by means of two independent methods, the nebular-to-auroral ratio [O III] 5007/4363, and the diagnostic diagram that combines 5007 to the [O III] infrared lines 52m and 88m. The optical and infrared fluxes have been obtained from archive HST/WFPC2 images and ISO LWS spectra respectively, and the continuum intensity in the optical has been measured from narrow-slit spectra obtained with the Isaac Newton Telescope at La Palma. The measured continuum intensity is higher than predicted by recombination theory under the hypothesis that all the Ly-alpha photons either escape or are destroyed. This fact can be explained in terms of an enhancement of the 2-photon continuum due to Ly-alpha conversion, a process that depends strongly on the local structure of the nebula. Alternative possibilities, outside the framework of recombination theory, have also been considered: e.g., the optical tail in the X-ray emission of a very hot plasma, and dust scattering of stellar radiation, but these hypotheses are not supported by quantitative estimates. While the electron temperature and density derived from the diagnostic diagram agree with the most recent determination, the temperature derived from 5007/4363,, Te(O++)_opt, is somewhat smaller than previously published values. We discuss several technical issues that contribute to the overall uncertainty in our results, focusing on the instrumental effects that might bias the [O III] 4363 intensity. We also discuss the effects of the collisional de-excitation of the O++ ground terms on the relation between Te(O++)_opt and Te(O++)_IR., Comment: 36 pages, 7 figures. AAS Latex. Accepted for publication in the Astronomical Journal

Chemical abundance data for the Galaxy show a wide dispersion. We try to check if this dispersion may also be found with chemical evolution models as an effect of variations of an Initial Mass Function (IMF) which follows a Poisson's distribution, Comment: Proceeding of the V Scientific Meeting of the Spanish Astronomical Society (Toledo 2002)

In this contribution I show the physical limitations to the application of standard synthesis models from 0 to 10 Gyr. I also present the multiwavelength spectrum (from gamma-rays to radio) of young star forming regions based on evolutionary population synthesis models that takes into account the stochasticity of the star formation proces. I show how the correlation of different observables can allow to establish a better understanding of the star formation processes. Finally I compare the spectral energy distribution of AGNs and star forming regions and I show the possible connection of Star Forming Galaxies with Seyfert 2 AGNs., Comment: 6 pages, 3 figures, to appear in "Star Formation through Time", Perez, Gonzalez Delgado and Tenorio-Tagle (eds.) ASP Conference Series (2003)

Evolutionary synthesis models have been used to study the physical properties of unresolved populations in a wide range of scenarios. Unfortunately, their self-consistency are difficult to test and there are some theoretical open questions without an answer: (1) The change of the homology relations assumed in the computation of isochrones due to the effect of stellar winds (or rotation) and the discontinuities in the stellar evolution are not considered. (2) There is no a consensus about how the isochrones must be integrated. (3) The discreteness of the stellar populations (that produce an intrinsic statistical dispersion) usually are not taken into account, and model results are interpreted in a deterministic way instead a statistical one... The objective of this contribution is to present some inconsistencies in the computation and some cautions in the application of the results of such codes., Comment: 4 pages, to appear in "The Evolution of Galaxies III. From Simple Approaches to Self-consistent Models" G.Hensler et al. (eds.)

Sampling fluctuations in stellar populations give rise to dispersions in observables when a small number of sources contribute effectively to the observables. This is the case for a variety of linear functions of the spectral energy distribution (SED) in small stellar systems, such as galactic and extragalactic HII regions, dwarf galaxies or stellar clusters. In this paper we show that sampling fluctuations also introduce systematic biases and multi-modality in non-linear functions of the SED, such as luminosity ratios, magnitudes and colours. In some cases, the distribution functions of rational and logarithmic quantities are bimodal, hence complicating considerably the interpretation of these quantities in terms of age or evolutionary stages. These biases can be only assessed by Monte Carlo simulations. We find that biases are usually negligible when the effective number of stars, Neff, which contribute to a given observable is larger than 10. Bimodal distributions may appear when Neff is between 10 and 0.1. Predictions from any model of stellar population synthesis become extremely unreliable for small Neff values, providing an operational limit to the applicability of such models for the interpretation of integrated properties of stellar systems. In terms of stellar masses, assuming a Salpeter Initial Mass Function in the range 0.08 -- 120 Mo, Neff=10 corresponds to about 10**5 Mo (although the exact value depends on the age and the observable). This bias may account, at least in part, for claimed variations in the properties of the stellar initial mass function in small systems, and arises from the discrete nature of small stellar populations., Comment: 16 pages, 14 figures. Accpeted for publication in MNRAS

We examine the impact of discrete numbers of stars in stellar populations on the results of Chemical Evolution Models. We explore the resulting dispersion in the true yields and their possible relation with the dispersion in observational data based on a Simple Closed-Box model. In this framework we find that the dispersion is larger for the less evolved or low abundance regions. Thus, the age-metallicity relation may be a tracer of the Star Formation History of our Galaxy. This theoretical dispersion is especially high for the relative abundance log(N/O) in regions where the total number of stars created is still low. This may explain part of the scatter in the N/O ratio observed in star forming galaxies. We have also found a first order theoretical estimation for the goodness of a linear fit of the Helium abundance vs. 12 + log (O/H) with values of the regression coefficient between 0.9 and 0.7 (independent of sampling effects). We conclude that it is necessary to include these sampling effects in a more realistic Chemical Evolution Model in order that such a model reproduces, at the same time, the mean value and the dispersion of observed abundances., Comment: 7 pages, 3 figures, A&A in press

In this work we investigate the evolution of the X-ray emission of a cluster of single young massive stars with different metallicities. We have considered the X-ray contribution originated by the diffuse gas heated by the mechanical energy released by stellar winds and SN explosions as well as the X-ray contribution from SN remnants. The resulting ionizing spectrum (i.e. lambda < 912 A) has been used to compute the expected intensity of the nebular He II 4686 A. The observational ratio He II/Hbeta could be reproduced by the models assuming that a fraction of the mechanical energy produced by the star-formation episode is reprocessed by interaction with the ISM as soft X-ray radiation, contributing to the He ionization. However, the discreteness of the stellar populations affects the ionizing flux and may be responsible for the observed dispersion of the ratio. We have finally used the synthesis models to estimate the contribution of circumnuclear star-forming regions to the multiwavelength energy distribution in Active Galactic Nuclei, finding that the UV to soft X-ray continuum in many Seyfert 2 galaxies seems to be dominated by star-formation processes., Comment: 14 pages, 10 fig, A&A accepted

In this contribution, I present the 90% confidence limits on the diagnostic diagrams of EW(WR bump) and L(WR bump)/L(Hb) ratio vs. EW(Hb) resulting from evolutionary synthesis models that include the statistical dispersion due to finite stellar populations in real star forming regions., Comment: 2 pages, 2 figures, To be presented in IAU Symp. 212

Gamma-ray and microwave observations of the Cygnus region reveal an intense signal of 1.809 MeV line emission, attributed to radioactive decay of 26Al, that is closely correlated with 53 GHz free-free emission, originating from the ionised interstellar medium. We modelled both emissions using a multi-wavelength evolutionary synthesis code for massive star associations that we applied to the known massive star populations in Cygnus. For all OB associations and young open clusters in the field, we determined the population age, distance, and richness as well as the uncertainties in all these quantities from published photometric and spectroscopic data. We propagate the population uncertainties in model uncertainties by means of a Bayesian method. The young globular cluster Cyg OB2 turns out to be the dominant 26Al nucleosynthesis and ionisation source in Cygnus. Our model reproduces the ionising luminosity of the Cygnus region very well, yet it underestimates 26Al production by about a factor of 2. We attribute this underestimation to shortcomings of current nucleosynthesis models, and suggest the inclusion of stellar rotation as possible mechanism to enhance 26Al production. We also modelled 60Fe nucleosynthesis in the Cygnus region, yet the small number of recent supernova events suggests only little 60Fe production. Consequently, a detection of the 1.137 MeV and 1.332 MeV decay lines of 60Fe from Cygnus by the upcoming INTEGRAL observatory is not expected., Comment: 24 pages, 7 figures, accepted for publication in A&A

In this contribution I present my current work in a new generation of evolutionary synthesis models that compute the multiwavelength energy distribution (from gamma-rays to radio) as well as the associated dispersion for young stellar systems. I will also show some statistical effects that may appear in the analysis of surveys, like bimodal or multi-modal distributions and bias when color indices computed by the codes are compared with observations. Such new generation of synthesis models may be useful for the analysis of the data expected from GTC., Comment: 2 pages, 2 figures. Contribution to "Science with the GTC", Granada 8-6 Feb (2002)

The probabilistic nature of the IMF in stellar systems implies that clusters of the same mass and age do not present the same unique values of their observed parameters. Instead they follow a distribution. We address the study of such distributions in terms of their confidence limits that can be obtained by evolutionary synthesis models. These confidence limits can be understood as the inherent uncertainties of synthesis models. We will compare such confidence limits arising from the discreteness of the number of stars obtained with Monte Carlo simulations with the dispersion resulting from an analytical formalism. We give some examples of the effects on the kinetic energy, V--K, EW(H$\beta$) and multiwavelength continuum., Comment: 4 pages, 4 figures, To appear in "The evolution of Galaxies II: Basic Building Blocks". The model output can be found in http://www.laeff.esa.es/users/mcs/SED

We compare the rate of kinetic energy injected by stellar winds into the extragalactic H II region NGC 2363 to the luminosity needed to feed the observed temperature fluctuations. The kinetic luminosity associated to the winds is estimated by means of two different evolutionary synthesis codes, one of which takes into account the statistical fluctuations expected in the Initial Mass Function. We find that, even in the most favorable conditions considered by our model, such luminosity is much smaller than the luminosity needed to account for the observed temperature fluctuations. The assumptions underlying our study are emphasized as possible sources of uncertainty affecting our results., Comment: 7 pages, 1 figure; accepted by Astronomy & Astrophysics. 1 erratum found and corrected on p. 4

In terms of statistical fluctuations, stellar population synthesis models are only asymptotically correct in the limit of a large number of stars, where sampling errors become asymptotically small. When dealing with stellar clusters, starbursts, dwarf galaxies or stellar populations within pixels, sampling errors introduce a large dispersion in the predicted integrated properties of these populations. We present here an approximate but generic statistical formalism which allows a very good estimation of the uncertainties and confidence levels in any integrated property, bypassing extensive Monte Carlo simulations, and including the effects of partial correlations between different observables. Tests of the formalism are presented and compared with proper estimates. We derive the minimum mass of stellar populations which is required to reach a given confidence limit for a given integrated property. As an example of this general formalism, which can be included in any synthesis code, we apply it to the case of young (t < 20 Myr) starburst populations. We show that, in general, the UV continuum is more reliable than other continuum bands for the comparison of models with observed data. We also show that clusters where more than 10^5 Mo have been transformed into stars have a relative dispersion of about 10% in Q(He+) for ages smaller than 3 Myr. During the WR phase the dispersion increases to about 25% for such massive clusters. We further find that the most reliable observable for the determination of the WR population is the ratio of the luminosity of the WR bump over the Hbeta luminosity. A fraction of the observed scatter in the integrated properties of clusters and starbursts can be accounted for by sampling fluctuations., Comment: accepted by A&A. 13 figures, a&a style packages see also http://www.laeff.esa.es/users/mcs for tables (by end October) and for high resolution figures

Ranging from track interpolation techniques through model atmospheres to the stochastic nature of the IMF, there are many uncertainties which need to be taken into account when modelling HR diagrams or performing population synthesis, particularly if comparison with actual data is sought. In this paper, we highlight (1) the problem of discontinuities along evolutionary tracks of massive stars (M> 8 Mo), showing that inconsistencies appear in the computation of the corresponding isochrones, and (2) the sampling fluctuations produced by the stochastic nature of the IMF, presenting a statistical formalism to estimate the dispersion in any given observable of a stellar population due to sampling effects which bypasses the need of performing Monte Carlo simulations., Comment: 6 pages, 4 figures, contribution to "Observed HR diagrams and stellar evolution", T.Lejeune and J. Fernandes (eds) with .sty file. More details of the work in http://www.laeff.esa.es/users/mcs)

The COMPTEL observations of the diffuse galactic 1.809 MeV emission attributed to the radioactive decay of 26Al have confirmed the diffuse nature of this interstellar emission line. One of the most significant features of the reconstructed intensity pattern is a flux enhancement in the direction of the Cygnus region. This region is fairly young and contains a wealth of massive stars, most of them grouped in the Cygnus OB associations. Multi-frequency model fitting strongly supports the hypothesis of massive stars and their descendent supernovae being the dominant sources of interstellar 26Al as observed by COMPTEL. Massive stars and supernovae are known to impart a large amount of kinetic energy into the surrounding ISM which lead to shockregions and large cavities. In addition, a large fraction of the electro-magnetic radiation of massive stars lies in the extreme ultra-violet causing photoionisation of the surrounding interstellar medium. We applied a population synthesis model in combination with an analytic model of the expansion of superbubbles to the Cygnus OB associations. The model predicts the expected 1.809 MeV flux and the gamma-ray line intensity due to interstellar 60Fe. We compute the sizes and expansion parameters of the expected HI-structures and the free-free emission intensities due to the photoionizing radiation from massive stars within this region of the sky. We discuss our present understanding of the Cygnus region with respect to the massive star census and history. Our model assigns about 70% of the 1.809 MeV intensity to six known OB associations, about 20% to known isolated sources and roughly 10% to an unkown diffuse component., Comment: 4 pages, 5 figures, 27. International Cosmic-Ray Conf., Hamburg, Aug. 2001

Evolutionary synthesis models are a fundamental tool to interpret the properties of observed stellar systems. In order to achieve a meaningful comparison between models and real data, it is necessary to calibrate the models themselves, i.e. to evaluate the dispersion due to the discreteness of star formation as well as the possible model errors. In this paper we show that linear interpolations in the log M - log t_k plane, that are customary in the evaluation of isochrones in evolutionary synthesis codes, produce unphysical results. We also show that some of the methods used in the calculation of time-integrated quantities (kinetic energy, and total ejected masses of different elements) may produce unrealistic results. We propose alternative solutions to solve both problems. Moreover, we have quantified the expected dispersion of these quantities due to stochastic effects in stellar populations. As a particular result, we show that the dispersion in the 14N/12C ratio increases with time., Comment: 11 pages, 8 figures, accepted by A&A

The detection of 1.809 MeV gamma-ray line emission from the Cygnus X complex by the COMPTEL telescope is one of the most convincing proves of massive star nucleosynthesis in our Galaxy. The Cygnus X complex is an extremely active nearby region, containing several star forming regions, OB associations and young open star clusters. It houses some of the most massive stars known in our Galaxy and concentrates a large number of Wolf-Rayet stars. Thermal radio continuum emission and intense [C II] line emission reveals widespread ionisation, and at least 60 individual H II regions have been identified. In order to understand the 1.809 MeV gamma-ray line emission from the Cygnus X complex, and to compare the observations to theoretical nucleosynthesis calculations, we modelled the multi-wavelength spectrum of the region by means of an evolutionary synthesis model. Our investigation leads us to the following conclusions: 1. Stellar wind ejection is the dominant mechanism for the observed 26Al enrichment in the Cygnus X region. 2. Cyg OB2 is by far the dominating massive star association in Cygnus X and 1.809 MeV line emission from 26Al produced in this association should be detectable by the spectrometer SPI on INTEGRAL. 3. There is only low supernova activity in the Cygnus X complex and the 60Fe lines should be below INTEGRAL's detection sensitivity. The detectability of an individual massive star cluster (Cyg OB2) by INTEGRAL provides a fantastic opportunity for future nucleosynthesis studies using gamma-ray line spectroscopy. We will explore the scientific potential of such observations, and provide estimates for gamma-ray line intensity distributions based on the massive star census of the Cygnus X region., Comment: 4 pages, 2 figures, to appear in the proceedings of the 4th INTEGRAL workshop (Alicante)

When evolutionary synthesis models take into account the stochastic nature of the IMF together with the discrete number of stars in real stellar clusters, typical output turns to dispersion band (where real data can be placed) instead of narrow lines. We present here a qualitative analysis of such dispersion in the SN rate, the kinetic energy and the 14N/12C ratio for different amounts of mass transformed into stars., Comment: 2 pages, 2 figures, RMAA tex format. Contribution presented to "Ionized Gaseous Nebulae" Nov 2000, Mexico

We have developed a new diagnostic tool for the study of gamma-ray emission lines from radioactive isotopes (such as 26Al and 60Fe in conjunction with other multi-wavelength observables of Galactic clusters, associations, and alike objects. Our evolutionary synthesis models are based on the code of Cervino & Mas-Hesse (1994), which has been updated to include recent stellar evolution tracks, new stellar atmospheres for OB and WR stars, and nucleosynthetic yields from massive stars during hydrostatic burning phases and explosive SN II and SN Ib events. The temporal evolution of 26Al and 60Fe production, the equivalent yield of 26Al per ionising O7 V star (yal), and other observables are predicted for a coeval population. We also employed our model to estimate the steady state nucleosynthesis of a population of solar metallicity. To allow for a fully quantitative analysis of existing and future multi-wavelength observations, we propose a Bayesian approach that allows the inclusion of IMF richness effects and observational uncertainties in the analysis. In particular, a Monte Carlo technique is adopted to estimate probability distributions for all observables of interest. We outline the procedure of exploiting these distributions by applying our model to a fictive massive star association. Applications to existing observations of the Cygnus and Vela regions will be discussed in companion papers., Comment: 15 pages, 17 Figures, Accepted for A&A Main Journal, A&A format

The stochastic nature of the IMF in young stellar clusters implies that clusters of the same mass and age do not present the same unique values of their observed parameters. Instead they follow a distribution. We address the study of such distributions, parameterised in terms of their confidence limits, in evolutionary synthesis models. These confidence limits can be understood as the inherent uncertainties of the synthesis models. Here we concentrate on some parameters such as EW(Hb) in emission. For instance, we show that for a cluster where 10^5 Mo have been transformed into stars, the dispersion of EW(Hb) is about 18% within the 90% confidence levels at ages between 3.5 and 5 Myrs., Comment: 4 pages, 7 figures, A&A format

We present in this contribution the predictions on the multiwavelength spectral energy distribution of our evolutionary population synthesis models including single and binary stellar systems. The high energy computations include the emission associated to X-ray binaries and supernovae remnants, as well as the mechanical energy released into the interstellar medium, which can be partially reprocessed into thermal X-rays. With these components we compute the spectral energy distribution of starburst galaxies from X-ray to radio ranges, and analyze finally the effects of the high energy emission on the H and He ionizing continuum., Comment: 2 pages, 2 figures, IAU Symp 193 (conference proceeding)

We present in this contribution our set of multiwavelength synthesis models including the evolution of single and binary stars. The main results we have obtained can be summarized as follows: (a) massive close-binary systems will start to experience mass transfer episodes after the first 4 Myr of the starburst evolution; (b) as a result of these mass transfer processes, stars of relatively low initial mass can loose completely their envelope and become a WR. In this way, the formation of WR stars is extended over longer than 15 Myr, and does not stop at 6 Myr as predicted by models including only single stars; (c) WR stars can thus be coeval with red supergiants, which peak at around 10 Myr for solar metallicities; (d) the accretion of mass will originate relatively massive stars at ages for which they should have already disappeared; these stars, together with the WR stars formed in rather evolved clusters, increase the production of ionizing photons, so that the H$\beta$ equivalent width will not drop as rapidly as predicted by models considering only individual stars; and (e) the mass transfer to compact companions will produce an additional source of high-energy radiation in the form of high-mass X-ray binaries, not predicted either by standard synthesis models., Comment: 8 pages, 3 figures. IAU Symp 193 (conference proceeding)