arXiv subjects

V. Luridiana

Publications and source records attributed to V. Luridiana.

At least 37 records · Page 2Linked to original sources

arXiv2007

Sampling effects in the emission line spectra of HII regions

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.

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arXiv2007

On the Primordial Helium Abundance and the DeltaY/DeltaO Ratio

We present a review on the determination of the primordial helium abundance, Yp, based on the study of hydrogen and helium recombination lines in extragalactic H II regions. We also discuss the observational determinations of the increase of helium to the increase of oxygen by mass Delta Y/Delta O, and compare them with predictions based on models of galactic chemical evolution.

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arXiv2007

Gamow Legacy and the Primordial Abundance of Light Elements

The presently accepted "Theory of the Universe" was pioneered 60 years ago by Gamow, Alpher and Herman. As a consequence of the, later dubbed, Hot Big-Bang, matter was neutrons, and after some decay protons, and a history of successive captures built up the elements. It wasn't until some 15 years later (with the discovery of the Cosmic Microwave Background radiation) that Gamow and colleagues theories were validated and present day Standard Big-bang Nucleosynthesis theory was developed. We will discuss the importance of state of the art observations and modelling in the quest to determine precise values of the primordial abundance of D and 4He, using observations of astrophysical objects and modern day atomic parameters. In particular, we will present the search for understanding and coping with systematic errors in such determinations.

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arXiv2006

Progress report: probabilistic and statistical bases of surface brightness fluctuations

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.

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arXiv2006

From deterministic to probabilistic population synthesis (why synthesis models are not what we thought they were, and how they can be much more than that)

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.

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arXiv2006

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

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.

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arXiv2005

Revisiting and assessing uncertainties in stellar populations synthesis models

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.

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arXiv2003

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

(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.

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arXiv2003

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

(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.

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arXiv2003

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 (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.

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arXiv2003

Photoionization Models of Metal-poor Extragalactic HII Regions and the Primordial Helium Abundance

We discuss the effects of collisional enhancement of Balmer lines on the determination of the primordial helium abundance. To this aim, we present a photoionization model of the metal-poor extragalactic HII region SBS 0335--052. We show that the derived helium abundance (Y) of this HII region depends on the amount of collisional excitation affecting the Balmer lines, both directly (through an underestimation of the actual He/H ratio) and indirectly (through an overestimation of the interstellar reddening). We detail how each of these effects affects the derived value of Y.

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arXiv2003

Physical conditions in NGC 6543 from ISO and HST data and the pitfalls of temperature maps

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.

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arXiv2003

Statistical Synthesis Models

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.

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arXiv2003

Physical conditions in the O++ zone from ISO and HST data. NGC 6543 revisited

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.

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arXiv2002

The primordial Helium abundance

The primordial abundance of 4He, Y_P, is one of the hottest themes in present-day astronomy, mostly due to its cosmological relevance. The disagreement between different determinations has been currently reduced to the 1-2 percent level, but these differences are still large enough to have deep implications for Big-Bang nucleosynthesis. It is therefore crucial to estimate precisely the uncertainties involved in the measurement of Y_P. Here, I review the methods used in the determination of Y_P and the related uncertainties. I also discuss some recent results, and emphasize the assumptions underlying the differences among them.

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arXiv2002

Chemical Abundances of NGC 5461 and NGC 5471 derived from Echelle Spectrophotometry

We present high resolution spectroscopic data of the two giant extragalactic H II regions NGC 5461 and NGC 5471 in M101, which have been obtained with the 2.1-m telescope of the Observatorio Astronómico Nacional in San Pedro Mártir, Baja California. We measured the intensities of several H and He recombination lines, and of forbidden lines of a large number of ions. We calculate the physical conditions in the two nebulae with a large number of diagnostics and determine their chemical abundances by applying ionization correction factors icf's to the observed ionic abundances. For NGC 5461, the icf's are based on a tailored photoionization model of the region (Luridiana & Peimbert 2001), while for NGC 5471 they are computed from those predicted by a photoionization model of NGC 2363 (Luridiana, Peimbert, & Leitherer 1999), a region which is similar to NGC 5471 in the ionization structure. For both regions, the icf's are compared to those computed following the prescriptions by Mathis & Rosa (1991). Such comparison shows large discrepancies for several elements, including nitrogen, neon, and chlorine.

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arXiv2002

Confidence Limits of Evolutionary Synthesis Models

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$β$) and multiwavelength continuum.

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arXiv2001

Can stellar winds account for temperature fluctuations in H II regions? The case of NGC 2363

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.

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