1,721,004 research outputs found
SPECTROPHOTOMETRIC EVOLUTION OF ELLIPTIC GALAXIES .1. ULTRAVIOLET EXCESS AND COLOR-MAGNITUDE-REDSHIFT RELATIONS
No full textWe present new chemical-spectrophotometric models of population synthesis to predict the spectral evolution of galaxies, in which stars span wide ranges of ages and chemical compositions. The library of stellar models assembled here supersedes other existing libraries for the number of tracks and the coverage of stellar phases and chemical parameters. All evolutionary phases, from the main sequence until the white dwarf or C-ignition stage, as appropriate for the star mass, are included. The range of metallicity and helium content goes from Z = 0.0004 and Y = 0.230 to Z = 0.1 and Y = 0.475. Six combinations of Z and Y are considered, however, scaled according to the law DELTAY/DELTAZ = 2.5. The inclusion of stellar models of both low and high metallicity allows us to follow in a great detail the evolution of spectral properties as function of the chemical enrichment and age. The library of stellar spectra stands on the most recent release of the Kurucz library, however, implemented at the low effective temperatures by means of observed spectra for stars of the latest spectral types. The population synthesis technique stems from the calculation of accurate isochrones in the color-magnitude diagram. Detailed results and useful calibrations are presented for single stellar populations aimed at providing, first, the building blocks of galaxy models and, second, the basic tools for studies of star clusters. The chemical-spectrophotometric model presented here is particularly designed for elliptical galaxies. It includes the presence of dark matter and galactic winds triggered by supernova explosion and energy injection by stellar winds from massive stars and allows for different laws of star formation. This model naturally follows the gradual enrichment in metallicity as a function of time both for the gas and the stars, thus providing the distribution in metallicity among the various stellar populations of a galaxy. The chemical model is the entry point for the model of spectral synthesis both for the detailed spectra and the broadband colors of the Johnson system. The results are used to study the color-magnitude relation for nearby galaxies and the origin of the UV excess in elliptical galaxies together with its dependence on metallicity and Mg2 index. We confirm and extend previous predictions that the main sources of the UV excess are the old, hot horizontal-branch (HB) and asymptotic giant branch (AGB) manque stars of high metallicity present in varying percentages in the stellar content of a galaxy. Since in our model the mean and maximum metallicity are ultimately driven by the mass of the galaxy, this provides a natural explanation for the observed correlation between UV excess and metallicity. We seek to disentangle the effects of metallicity and age on the global properties of a galaxy and address the question whether the prototype galaxy M32 suffered from a recent episode of star formation. With the aid of the new calibrations for the magnitudes and colors of single stellar populations in particular stages of evolution and the new chemical-spectrophotometric model, we find that spectrum, turnoff color, integrated colors, brightest red giant branch (RGB) and AGB stars are all compatible with the notion that M32 is an old system that underwent a prominent initial episode of star formation followed by later activity at about 5 X 10(9) yr ago. Finally, we examine the photometric evolution of the models as a function of the redshift, and, in particular, we address the question whether there are prominent features of the evolving spectra of galaxies that might be used as age probes. We suggest that the color (1550-V) measuring the intensity of the UV emission of a galaxy is a good age indicator. Specifically, this color suffers from a sudden variation from red to blue at the onset of the hot HB and AGB manque stages of the old, metal-rich stars, most likely present in massive galaxies. In the framework of the stellar models in use, this transition occurs at about 7.6 X 10(9) yr. EquivalentIy, it is expected to be observed at redshifts perhaps accessible to present-day instrumentation. The detection of this feature at a certain redshift would impose firm constraints on the underlying cosmological model of the universe. Finally, we present the preliminary comparison of the magnitudes and colors as a function of the redshift, with the observational ones limited to a sample of radio galaxies. The implications of the present models for the study of galaxy evolution are shortly summarized in the concluding remarks
Evolutionary sequences of stellar models with new radiative opacities. IV. Z=0.004 and Z=0.008
No full textWe present detailed tabulations of two large grids of stellar models with the initial chemical compositions [Y=0.240, Z=0.004] and [Y=0.250, Z=0.008], computed with the most recent radiative opacities (OPAL) by Iglesias et al. (1992). This paper continues the series initiated by Bressan et al. (1993) for the grid with [Y=0.280, Z=0.020], the reference solar like abundances, followed by Fagotto et al. (1994a) for the grids with compositions [Y=0.230, Z=0.0004] and [Y=0.352, Z=0.05]. The tracks span a wide range of initial masses from 0.6 M. to 120 M. and extend from the zero age main sequence (ZAMS) till very advanced evolutionary phases. Specifically, low- and intermediate-mass stars are followed till the beginning of the thermally pulsing regime of the asymptotic giant branch phase (TP-AGB), while massive stars axe followed till the core C-ignition. The models of low- and intermediate-mass stars are calculated at constant mass, whereas those of massive stars are followed in presence of mass loss by stellar wind incorporating a suitable dependence on the metallicity. The results for all the models are given in extensive tables which summarize also the lifetimes of the various phases and the variations of the surface abundances by dredge-up phenomena and mass loss by stellar wind. The salient features brought by the different metallicity and helium content are briefly outlined. In virtue of their large coverage of masses, evolutionary phases, and chemical compositions, such grids of evolutionary tracks are basic to studies of population synthesis. In particular they are suited to interpret the CMDs of metal-rich globular clusters and of the typical stellar content of the Large Magellanic Cloud
Evolutionary sequences of stellar models with new radiative opacities. III. Z=0.0004 and Z=0.05
No full textWe present detailed tabulations of two large grids of stellar models with the extreme initial chemical composition [Z=0.0004, Y=0.230], and [Z=0.050, Y=0.352]. The models are computed with the most recent radiative opacities (OPAL) by Iglesias et al. (1992) and with the inclusion of overshoot from convective cores and envelopes according to the formalism by Bressan et al. (1981) and Alongi et al. (1991), respectively, and the revision made by Bressan et al. (1993a). These calculations represent the continuation of the series initiated with the paper by Bressan et al. (1993a) for the grid with [Z=0.020, Y=0.280] (the reference solar like abundance). The tracks are calculated for a wide range of initial masses from 0.6 M. to 120 M. and extend from the ZAMS till very advanced evolutionary phases. Specifically, low and intermediate mass stars are followed to the beginning of the TP-AGB, while massive stars are followed till the core C-ignition. The models of low and intermediate mass stars are calculated at constant mass, whereas those of massive stars are followed in presence of mass loss by stellar winds incorporating a suitable dependence on the metallicity. The results of all the models are given in extensive tables which summarize also the lifetimes of the various phases and the variations of surface abundances by dredge-up phenomena and mass loss by stellar wind. The salient features brought by the different metallicity and helium content are briefly outlined, with particular attention to the anomalous behaviour of the low mass, high metallicity stars in core He-burning and later phases. Such grids of evolutionary tracks are well indicated for studies of population synthesis in virtue of their large coverage of masses, evolutionary phases, and chemical composition. The high metallicity set is particularly suited to interpret the stellar content of bulges and elliptical galaxies
Spectro-photometric evolution of elliptical galaxies .II. Models with infall
No full textIn this paper we present new chemo-spectrophotometric models of elliptical galaxies in which infall of primordial gas is allowed to occur. They aim to simulate the collapse of a galaxy made of two components, i.e. luminous material and dark matter. The mass of the dark component is assumed to be constant in time, whereas that of the luminous material is supposed to accrete at a suitable rate. They also include the effect of galactic winds powered by supernova explosions and stellar winds from massive, early-type stars. The models are constrained to match a number of properties of elliptical galaxies, i.e. the slope and mean colours of the colour-magnitude relation (CMR), V versus (V-K), the UV excess as measured by the colour (1550-V) together with the overall shape of the integrated spectral energy distribution (ISED) in the ultraviolet, the relation between the Mg-2 index and (1550-V), the mass to blue luminosity ratio M/L(B) as a function of the B luminosity, and finally the broad-band colours (U-B), (B-V), (V-I), (V-K), etc. The CMR is interpreted as a mass-metallicity sequence of old, nearly coeval objects, whose mean age is 15 Gyr. Assuming the law of star formation to be proportional to M(g)(k)(t) with k = 1, the rate of star formation as function of time starts small, grows to a maximum, and then declines thus easily avoiding the excess of metal-poor stars found by BCF with the closed-box scheme (the analog of the G-Dwarf Problem in the solar vicinity). Owing to their stellar content, infall models can easily reproduce all the basic data of the galaxies under examination. As far as the UV excess is concerned, the same sources proposed by BCF are found to hold also with the infall scheme. H-HB and AGB manque stars of high metallicity play the dominant role, and provide a robust explanation of the correlation between the (1550-V) colour and the luminosity, mass and metallicity of the galaxies. Furthermore, these models confirm the potential of the (1550-V) colour as an age indicator in cosmology as already suggested by BCF. In the rest frame of a massive and metal-rich elliptical galaxy, this colour suffers from one major variation: at the onset of the so-called H-HB and AGB-manque stars (age about 5.6 Gyr). This transition occurs at reasonably small red-shifts and therefore could be detected with the present-day instrumentation
Evolutionary sequences of stellar models with very high metallicity. V. Z=0.1
No full textWe present detailed tabulations of a large grid of stellar models with initial chemical compositions [Y = 0.475, Z = 0.1] computed with the radiative opacities by Huebner et al. (1977) and convective overshoot. This choice of the opacity is due to the high metallicity in use, because the Livermore Library of radiative opacities (Iglesias et al. 1992) did not contain tabulations for Z = 0.1. The tracks span the range of initial masses from 0.6 M. to 9 M. and extend from the zero age main sequence (ZAMS) till very advanced evolutionary phases. Specifically, low- and intermediate-mass stars axe followed either till the white dwarf (WD) or late stages of the early asymptotic giant branch (AGB) phase, depencling on the mass of the star and on details of the evolutionary behaviour discussed in the text. Massive stars are calculated till core C-ignition. The critical masses separating low- from intermediate-, and intermediate-mass from massive stars axe M(HeF) = 1.3 M. and M(up) = 4 M., respectively. All the models are calculated at constant mass. The results for all the models are given in extensive tables which summarize also the lifetimes of the various phases and the vaxiations of the surface abundances limited to the first dredge-up. The stellar models presented here are particularly suited to interpret the CMDs of very metal-rich dusters and the stellar content of elliptical galaxies. However we warn the reader that, because of the opacity, these models axe not fully homogeneous with the others of the same series by Bressan et al. (1993) and Fagotto et al. (1994a,b) that are calculated with the Livermore opacity
Evolutionary sequences of stellar models with semiconvection and convective overshoot. I. Z=0.008.
No full textWe present two grids of evolutionary stellar models covering the phases of core H- and He-burning for stars having masses in the range 0.6M. to 100M. with composition (Y = 0.25 and Z = 0.008). The computations were performed for two different mixing schemes in the convective regions, i.e. either the classical definition of the convective core during the central H-burning phase and semiconvection during the core He-burning phase or non local overshoot from convective regions all over the major nuclear phases. All the evolutionary models were computed from the main sequence up to the stage of central carbon ignition or to the beginning of the thermally pulsing regime of the asymptotic giant branch phase as appropriate to the value of the initial stellar mass. Massive stars (M greater-than-or-equal-to 12M.) were evolved considering the effect of mass loss by stellar wind, whereas low and intermediate mass stars were calculated at constant mass. For these latter, mass loss during the red giant and asymptotic giant branch phases can be easily included following the standard analytical procedure. Care was paid to upgrade the physical input of the numerical code, i.e. nuclear reaction rates and accompanying nucleosynthesis network (16 elements were actually followed), opacities, neutrino energy losses, boundary conditions in the outer layers, and mass loss rates for massive stars. The results are presented in tabular form giving as much information as possible. Finally, we concisely compare the main characteristics of the evolutionary models obtained with the two mixing schemes
- …
