arXiv subjects

M. Molla

Publications and source records attributed to M. Molla.

33 records · Page 2Linked to original sources

arXiv2005

A grid of chemical evolution models as a tool to interpret spiral and irregular galaxies data

We present a generalization of the multiphase chemical evolution model applied to a wide set of theoretical galaxies with different masses and evolutionary rates. This generalized set of models has been computed using the so-called Universal Rotation Curve from Persic et al (1996) to calculate the radial mass distribution of 44 theoretical protogalaxies. This distribution is a fundamental input which, besides its own effect on the galaxy evolution, defines the characteristic collapse time-scale or gas infall rate onto the disc.We have adopted 10 sets of values, between 0 and 1, for the molecular cloud and star formation efficiencies, as corresponding to their probability nature, for each one of the radial distributions of total mass. Thus, we have constructed a bi-parametric grid of models, depending on those efficiency sets and on the rotation velocity, whose results are valid in principle for any spiral or irregular galaxy. The model results provide the time evolution of different regions of the disc and the halo along galactocentric distance, measured by the gas (atomic and molecular) and stellar masses, the star formation rate and chemical abundances of 14 elements, for a total of 440 models. This grid may be used to estimate the evolution of a given galaxy for which only present time information -- such as radial distributions of elemental abundances, gas densities and/or star formation, which are the usual observational constraints of chemical evolution models -- is available.

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arXiv2004

Low and intermediate mass star yields: The evolution of carbon abundances

We present a set of low and intermediate mass star yields based on a modeling of the TP--AGB phase which affects the production of nitrogen and carbon. These yields are evaluated by using them in a Galaxy Chemical Evolution model, with which we analyze the evolution of carbon abundances. By comparing the results with those obtained with other yield sets, and with a large amount of observational data, we conclude that the model using these yields combined with those from Woosley & Weaver (1995) for massive stars properly reproduce all the data. The model reproduces well the increase of C/O with increasing O/H abundances. Since these massive star yields do not include winds, it implies that these stellar winds might have a smoother dependence on metallicity than usually assumed and that a significant quantity of carbon proceeds from LIM stars.

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arXiv2004

Predicted chemical evolution for spiral disks from their observed rotation curves

The rotation curves for a sample of 67 spiral galaxies observed by Marquez et al(2002) have been used as input for the multiphase chemicale volution model. By using N[II]/Halpha as estimator of the oxygen abundance, we constraint the possible models for each galaxy. We may, then, predict the time evolution of these galaxies and the present time radial distribution for gas, stars and star formation surface densities and elemental abundances.

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arXiv2003

How to determine the star formation histories in spiral disks

We have computed 500 chemical evolution models for the galaxy NGC 4303. With the obtained evolutionary histories and a synthesis model we have also calculated the spectral indices Mg2 and Fe5270. Fom the 500 models, by using a chi-square technique, we obtain those reproducing the present day observations within a confidence level of 95 %, reducing the number of possible models to 19. Only 6 of them are also able to reproduce the spectral indices radial distributions with the same level of confidence. We conclude that this technique of combining chemical with evolutionary synthesis models is a very powerful tool, and, therefore, we propose to perform more observational campaigns to obtain these and other spectral indices in spiral and irregular galaxies.

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arXiv2003

Chemical Evolution of CNO abundances

New low and intermediate star yields calculated by Buell (1997) are evaluated by using them in a Galactic Chemical Evolution model. We analyze their effects on CNO elemental abundances

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arXiv2003

Dispersion in modeled abundances

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

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arXiv2003

Galactic Evolution along the Hubble Sequence.I. A grid of models parametrized by initial galaxy mass distribution

We present a generalization of the multiphase chemical evolution model applied to a wide set of theoretical galaxies with different masses and morphological types. This generalized set of models has been computed using the so-called Universal Rotation Curve from Persic, Salucci & Steel (1996) to calculate the radial mass distribution of 44 theoretical protogalaxies. This distribution is a fundamental input which, besides its own effect on the galaxy evolution, defines the characteristic collapse time scale or gas infall rate onto the disk. For each mass radial distribution, we have 10 different evolutionary rates. With these two hypotheses we construct a bi-parametric grid of models. The results include the time evolution of different regions of the disk and the halo along the galactocentric distance, measured by the gas and stellar masses, the star formation rate and chemical abundances of 15 elements.

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arXiv2002

On the effect of discrete numbers of stars in chemical evolution models

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.

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arXiv2002

Constraining the evolutionary histories of spiral disks

We study the old problem of the uniqueness of chemical evolution models. We showed in Molla et al. (1999) that multiphase models for three Virgo cluster galaxies were able to reproduce the observed radial distributions of spectral indices Mg2 and Fe52. But those models may fit the present-epoch radial distributions with different star formation histories. The two spectral indices are in turn affected by the well known age-metallicity degeneracy which prevents the disentagling of age and metallicity for single stellar populations. In this work we face both issues by analyzing a set of multiphase models for the Virgo galaxy NGC 4303

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arXiv2002

Radial Gradients of abundances

We have computed a set of multiphase chemical evolution models in which the radial mass distribution of each theoretical galaxy is calculated using the Universal Rotation Curve from Persic, Salucci & Steel (1996). We obtain the chemical evolution for galaxies of different masses and morphological types by changing the efficiencies to form molecular clouds and stars according these types. We obtain the radial distribution of diffuse and molecular gas, the star formation rate, and abundances for 15 elements for each galaxy.

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arXiv2001

Confidence limits of evolutionary synthesis models III. On time-integrated quantities

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.

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arXiv2000

Models for the interpretation of CaT and the blue Spectral Indices in Elliptical Nuclei

We present a grid of theoretical models where the calculation of absorption line spectral indices in both the blue and red wavelength ranges is done with the same evolutionary synthesis code. We have computed some of these indices: CaT, NaI, MgI in the near infrared and Mgb, Mg2, Fe52, Fe53, NaD and Hbeta, in the blue-visible range, for Single Stellar Population (SSP) of 6 different metallicities, (Z=0.0004,0.001, 0.004, 0.008, 0.02 and 0.05), and ages from 4 Myr to 20 Gyr. From the comparison of these evolutionary synthesis models with a compilation of elliptical galaxy data from the literature, we find that the observed CaT index follows the blue index rather than Mg2 as the models predict. If this implies an over-abundance [Mg/Ca] and we take into account the masses of stars which produce Mg and Ca, these stars could form in a time scale shorter than 5 Myr from the beginning of the star formation process. Alternatively, an IMF biased towards very massive stars (M > 40 Msun) at the early epoch of star formation in elliptical nuclei has to be assumed. We also suggest to revise the calculation of the nucleosynthesis yield of Magnesium. By using the diagnostic diagram CaT-Hbeta to disentangle age and metallicity in such populations, we obtain around solar abundances and a sequence of ages between 4 and 16 Gyr for the galaxy sample.

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arXiv2000

Bulges

We model the evolution of the galactic bulge and of the bulges of a selected sample of external spiral galaxies, via the multiphase multizone evolution model. We address a few questions concerning the role of the bulges within galactic evolution schemes and the properties of bulge stellar populations. We provide solutions to the problems of chemical abundances and spectral indices, the two main observational constraints to bulge structure.

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arXiv1998

The stellar populations of spiral disks.II Measuring and modeling the radial distribution of absorption spectral indices

The radial distributions of the Mg2 and Fe5270 Lick spectral indices have been measured to large radial distances on the disks of NGC 4303 and NGC 4535 using an imaging technique based on interference filters. These data, added to those of NGC 4321 previously published in Paper I of this series are used to constraint chemical (multiphase) evolutionary models for these galaxies. Because the integrated light of a stellar disk is a time average over the history of the galaxy weighted by the star formation rate, these constraints complement the information on chemical gradients provided by the study of HII regions which, by themselves, can only provide the alpha-elements abundance accumulate over the life of the galaxy. The agreement between the observations and the model predictions shown here lends confidence to the models which are then used to describe the time evolution of galaxy parameters such as star formation rates, chemical gradients, and gradients in the mean age of the stellar population.

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arXiv1998

Calcium Triplet Synthesis

We present theoretical equivalent widths for the sum of the two strongest lines of the Calcium Triplet, CaT index, in the near-IR, using evolutionary techniques and the most recent models and observational data for this feature in individual stars. We compute the CaT index for Single Stellar Populations (instantaneous burst, standard Salpeter-type IMF) at four metallicities, Z=0.004, 0.008, 0.02 (solar) and 0.05, and ranging in age from very young bursts of star formation (few Myr) to old stellar populations, up to 17 gyr, representative of globular clusters, elliptical galaxies and bulges of spirals. The interpretation of the observed equivalent widths of CaT in different stellar systems is discussed. Composite-population models are also computed as a tool to interpret the CaT detections in star-forming regions, in order to disantangle between the component due to Red Supergiants stars, RSG, and the underlying, older, population. CaT is found to be an excellent metallicity-indicator for populations older than 1 Gyr, practically independent of the age. We discuss its application to remove the age- metallicity degeneracy, characteristic of all studies of galaxy evolution based on the usual integrated indices (both broad band colors and narrow band indices). The application of the models computed here to the analysis of a sample of elliptical galaxies will be discussed in a forthcoming paper (Gorgas et al. 1998).

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