V. Luridiana

Revised Primordial Helium Abundance Based on New Atomic Data

We have derived a primordial helium abundance of Yp = 0.2477 ± 0.0029, based on new atomic physics computations of the recombination coefficients of He I and of the collisional excitation of the H I Balmer lines together with observations and photoionization models of metal-poor extragalactic H II regions. The new atomic data increase our previous determination of Yp by 0.0086, a very significant amount. By combining our Yp result with the predictions made by the standard big bang nucleosynthesis model, we find a baryon-to-photon ratio, η, in excellent agreement both with the η-value derived by the primordial deuterium abundance value observed in damped Lyα systems and with the one obtained from the WMAP observations.

Physical limits to the validity of synthesis models. The Lowest Luminosity Limit

In this paper we establish a necessary condition for the application of stellar population synthesis models to observed star clusters. Such a condition is expressed by the requirement that the total luminosity of the cluster modeled be larger than the contribution of the most luminous star included in the assumed isochrones, which is referred to as the Lowest Luminosity Limit (LLL). This limit is independent of the assumptions on the IMF and almost independent of the star formation history. We have obtained the Lowest Luminosity Limit for a wide range of ages (5 Myr to 20 Gyr) and metallicities (Z = 0 to Z = 0.019) from the Girardi et al. (2002) isochrones. Using the results of evolutionary synthesis models, we have also obtained the minimal cluster mass associated with the LLL, M m i n , which is the mass value below which the observed colors are severely biased with respect to the predictions of synthesis models. We explore the relationship between M m i n and the statistical properties of clusters, showing that the magnitudes of clusters with mass equal to M m i n have a relative dispersion of 32% at least (i.e., 0.35 mag) in all the photometric bands considered; analogously, the magnitudes of clusters with mass larger than 10 x M m i n have a relative dispersion of 10% at least. The dispersion is comparatively larger in the near infrared bands: in particular, M m i n takes values between 10 4 and 10 5 M O . for the K band, implying that severe sampling effects may affect the infrared emission of many observed stellar clusters. As an example of an application to observations, we show that in surveys that reach the Lowest Luminosity Limit the color distributions will be skewed toward the color with the smallest number of effective sources, which is usually the red, and that the skewness is a signature of the cluster mass distribution in the survey. We also apply our results to a sample of Globular Clusters, showing that they seem to be affected by sampling effects, a circumstance that could explain, at least partially, the bias of the observed colors with respect to the predictions of synthesis models. Finally, we extensively discuss the advantages and the drawbacks of our method: it is, on the one hand, a very simple criterion for the detection of severe sampling problems that bypasses the need for sophisticated statistical tools; on the other hand, it is not very sensitive, and it does not identify all the objects in which sampling effects are important and a statistical analysis is required. As such, it defines a condition necessary but not sufficient for the application of synthesis models to observed clusters.

The Effect of Collisional Enhancement of Balmer Lines on the Determination of the Primordial Helium Abundance

This paper describes a new determination of the primordial helium abundance (YP), based on the abundance analysis of five metal-poor extragalactic H II 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, as a result of the differential effect of collisions on different Balmer lines. In addition to these effects, our analysis takes into account the following features of H II regions: (1) the temperature structure, (2) the density structure, (3) the presence of neutral helium, (4) the collisional excitation of the He I lines, (5) the underlying absorption of the He I lines, and (6) the optical thickness of the He I 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 ΔY = +0.0107. The revised Y-values for the five objects in our sample yield an increase of +0.0035 in YP, giving YP = 0.2391 ± 0.0020.

Confidence limits of evolutionary synthesis models - IV. Moving forward to a probabilistic formulation

Context. Synthesis models predict the integrated properties of stel lar populations. Several problems exist in this field, mostl y related to the fact that integrated properties are distributed. To date, this a spect 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). Aims. This paper presents a method of population synthesis that accounts for the distributed nature of stellar properties. Methods. 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). Results. With standard distribution theory, 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 luminosity function. 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: calibrating the SBF method, determining the luminosity function of globular clusters, comparing different isochrone sets, tracing the sLDF by means of resolved data, including fuzzy stellar properties in population synthesis, among others. Additionally, the algorithmic nature of our method makes it suitable for developing analysis tools for the Virtual Observatory. Conclusions. 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.

On emission-line spectra obtained from evolutionary synthesis models I. Dispersion in the ionising flux and Lowest Luminosity Limits

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 of the mass function due to the smaller absolute number of massive stars. Since the ionising spectra of star-forming regions are mainly produced by massive stars and their by-products, the dispersion in the number of massive stars across individual clusters also produces a dispersion in the properties of the corresponding ionising spectra. This implies that regions with the same physical properties may produce very different emission line spectra, and occupy different positions in emission-line diagnostic diagrams. In this paper, we lay the basis for a future analysis of this effect by evaluating the dispersion in the ionising fluxes of synthetic spectra computed with evolutionary models. As an important consequence of the explicit consideration of sampling effects, 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

Fluorescent excitation of Balmer lines in gaseous nebulae: case D

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On surface brightness fluctuations: probabilistic and statistical bases - I. Stellar population and theoretical surface brightness fluctuations

regions on all scales, those ionised by single massive stars through those ionised by super stellar clusters. This finding is especially relevant in consideration of the fact that real

The distance to the C component of I Zw 18 and its star formation history - A probabilistic approach

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Synthesis Models in a Probabilistic Framework: Metrics of Fitting

regions are found in a band sensibly narrower than predicted by standard methods. Additionally, we find convincing suggestions that the

Physical Conditions in the O++ Zone from ISO and HST Data: NGC 6543 Revisited*

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