1,721,083 research outputs found
Stellar evolution with turbulent diffusion .2. The HR diagram of supergiant stars
No full textWe present new evolutionary models for high and intermediate mass stars in which the fully convective regions (inner cores, external envelopes and intermediate shells) are let extend into their surrounding regions of overshoot. However,in these layers instead of assuming mixing to be instantaneous and fully efficient, we treat it according to the diffusive scheme elaborated by Deng et al. (1996a). The major difference with respect to standard stellar models is that, while fully unstable regions turn out to be completely homogenized by the diffusive algorithm in use, this is not the case for the overshoot regions which undergo partial mixing and build up smooth chemical profiles. The diffusion algorithm makes use of the so-called scale length most effective for mixing expressed as l(d) = P-dif X 10(-5) l(o), where l(o) is the largest scale in the unstable region in units of the local cal pressure scale height Hp, and finally P-dif is a fine tuning parameter of the order of unity (see the text for more details). The analysis by Deng et al. (1996a) has clarified that P-dif = 0.4 leads to stellar models that are able to match a number of properties of massive and intermediate mass stars. The characterizing feature of these models is that they possess at the same time evolutionary characteristics that were separately typical of model calculated with different schemes of mixing. In other words, they share the same properties of models with standard overshoot, namely a wider main sequence band, higher luminosity, and longer lifetimes, but also perform extended loops that are the main signature of the semiconvective description of convection at the border of the core. The stellar models presented in this paper span the mass range 5 to 100 M(.) and go from the zero age main sequence to the stage of central He-exhaustion. They have been calculated assuming P-dif = 0.4 over the whole range of masses. Two sets of initial chemical compositions are considered, namely [Z=0.008 and Y=0.25] suited to the Large Magellanic Cloud (LMC) supergiant stars, and [Z=0.020 and Y=0.28] suited to the same stars in the solar vicinity. With the aid of these stellar models we analyze the HR diagram (HRD) of supergiant stars in the LMC by Fitzpatrick & Garmany (1990) and that of supergiant stars in the Milky Way by Blaha & Humphreys (1989). Particular attention is paid to the star counts across the HRD, the stars in the Hertzsprung-Russell gap and related possible widening of the main sequence band, the so-called ledge, the ratio of blue to red supergiants N-B/N-R, the location of Wolf-Rayet stars (WR), and the blue progenitor of the SN 1987A. We find that taking into account a plausible scatter in the chemical composition of the supergiant stars in the samples, the many stars in the gap and the ledge can be easily accounted for. However, problems remains as far as the number frequencies of stars among the various spectral types, the possibility that the main sequence band can extend to spectral types up to B3, and the ratio N-B/N-R. Furthermore, despite the net advantages offered by these new stellar models with diffusive mixing, the location of WR stars in the CMD encounters the same difficulties as with the classical models. We suggest that a distinct evolutionary scenario must be invoked for this type of stars (Deng et al. 1996b). Finally, the blue progenitor of SN 1987A is not matched by the models, even if clues can be found within the same evolutionary scheme for a possible way out
Probing the age of elliptical galaxies
No full textIn this paper we address the question whether age and metallicity effects can be disentangled with the aid of the broad-band colours and spectral indices from absorption feature strengths, so that the age of elliptical galaxies can be inferred. The observational data under examination are the indices H-beta and [MgFe], and the velocity dispersion Sigma for the sample of galaxies of Gonzales (1993), supplemented by the ultra-violet data, i.e. the colour (1550-V), of Burstein et al. (1988). The analysis is performed with the aid of chemo-spectro-photometric models of elliptical galaxies with infall of primordial gas (aimed at simulating the collapse phase of galaxy formation) and the occurrence of galactic winds. The galaxy models are from Tantalo et al. (1995). The study consists of four parts. In the first one, the aims are outlined and the key data are presented. In the second part, we summarize the main properties of the infall models that are relevant to our purposes. In the third part we present the detailed calculations of the spectral indices for single stellar populations and model galaxies. To this aim, we use the analytical relations of Worthey et al. (1994) who give index strengths as a function of stellar parameters. In the last part, we examine the age-metallicity problem. In contrast with previous interpretations of the H-beta and [MgFe] data as a sort of age sequence (Gonzales 1993), we find that the situation is more complicate when the space of the four variables H-beta, [MgFe], (1550-V), and Sigma is examined. Galaxies in the H-beta and [MgFe] plane do not follow a pure sequence either of age or metallicity. The observed (1550-V) colours are not compatible with young ages. Basically, all the galaxies in the sample are old objects (say as old as 13 divided by 15 Gyr) but have suffered from different histories of star formation. Specifically, it seems that some galaxies have exhausted the star forming activity at very early epochs with no significant later episodes. Others have continued to form stars for long periods of time. This is perhaps sustained by the analysis of the gradients in the H-beta and [MgFe] indices across the galaxies. There are galaxies with no age difference among the various regions. There are other galaxies in which large gradients in the mean age of the star forming activity between the central and the peripheral regions seem to exist. The nucleus turns out to be younger and more metal-rich than the outer regions. Finally, there are galaxies in which the nucleus is older but less metal-rich than the external regions. All this perhaps hints not only different histories of star formation but also different mechanisms of galaxy formation difficult to pin down at the present time. From the analysis of the H-beta, [MgFe], (1550-V), and Sigma space, and of the age and metallicity gradients in single galaxies, the suggestion is advanced that the overall duration of the star forming activity is inversely proportional to the velocity dispersion Sigma (and perhaps galactic mass)
MODELS WITH CONVECTIVE OVERSHOOT - TABLES OF ISOCHRONES
No full textIn this paper we briefly summarize the content of extensive tabulations of theoretical isochrones, integrated magnitudes and colours, and luminosity functions at varying age and chemical composition, to be presented elsewhere. These isochrones are based on stellar models incorporating the effect of convective overshoot
Stellar evolution with turbulent diffusion .1. A new formalism of mixing
No full textIn this paper we present a new formulation of diffusive mixing in stellar interiors aimed at casting light on the kind of mixing that should take place in the so-called overshoot regions surrounding fully convective zones. Key points of the analysis are the inclusion the concept of scale length most effective for mixing, by means of which the diffusion coefficient is formulated, and the inclusion of intermittence and stirring, two properties of turbulence known from laboratory fluid dynamics. The formalism is applied to follow the evolution of a 20 M(.) star with composition Z=0.008 and Y=0.25. Depending on the value of the diffusion coefficient holding in the overshoot region, the evolutionary behaviour of the test stars goes from the case of virtually no mixing (semiconvective like structures) to that of full mixing over there (standard overshoot models). Indeed, the efficiency of mixing in this region drives the extension of the intermediate fully convective shell developing at the onset of the shell H-burning, and in turn the path in the HR Diagram (HRD). Models with low efficiency of mixing burn helium in the core at high effective temperatures, models with intermediate efficiency perform extended loops in the HRD, finally models with high efficiency spend the whole core He-burning phase at low effective temperatures. In order to cast light on this important point of stellar structure, we test whether or not in the regions of the H-burning shell a convective layer can develop. More precisely, we examine whether the Schwarzschild or the Ledoux criterion ought to be adopted in this region. Furthermore, we test the response of stellar models to the kind of mixing supposed to occur in the H-burning shell regions. Finally, comparing the time scale of thermal dissipation to the evolutionary time scale, we get the conclusion that no mixing in this region should occur. The models with intermediate efficiency of mixing and no mixing at all in the shell H-burning regions are of particular interest as they possess at the same time evolutionary characteristics that are separately typical of models calculated with different schemes of mixing. In other words, the new models share the same properties of models with standard overshoot, namely a wider main sequence band, higher luminosity, and longer lifetimes than classical models, but they also possess extended loops that are the main signature of the classical (semiconvective) description of convection at the border of the core
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
Evolution of intermediate mass stars - The role of convective overshooting and stellar wind
No full textThe evolution of Pop I intermediate mass stars is followed from the zero-age main sequence until the early AGB in the presence of convective overshooting from the central cores and mass loss by stellar wind in late stages. Use is made of the determination of Kettner et al. (1982) for the C-12 (alpha, gamma) O-16 reaction rate. Several important consequences are shown to be possible if substantial overshooting occurs in the convective cores of real stars. The new evolutionary models have much larger convective cores and therefore larger He and CO cores than the classical ones. The evolutionary tracks in the H-R diagram run at higher luminosities, and cover a wider range of effective temperatures during the core H-burning phase. However, the extension of the blue loops during the core H-burning phase is strongly reduced. Finally, the mass limits which confine intermediate stars are significantly lowered by the effect of overshooting on critical masses for nondegenerative He and C ignition
Galactic chemical enrichment with new metallicity dependent stellar yields
No full textNew detailed stellar yields of several elemental species are derived for massive stars in a wide range of masses (from 6 to 120 M.) and metallicities (Z =0.0004, 0.004, 0.008, 0.02, 0.05). Our calculations are based on the Padova evolutionary tracks and take into account recent results on stellar evolution, such as overshooting and quiescent mass-loss, paying major attention to the effects of the initial chemical composition of the star. We finally include modern results on explosive nucleosynthesis in SN ae by Woosley & Weaver (1995). The issue of the chemical yields of Very Massive Objects (from 120 to 1000 M.) is also addressed. Our grid of stellar yields for massive stars is complementary to the results by Marigo et al. (1996, 1998) on the evolution and nucleosynthesis of low and intermediate mass stars, also based on the Padova evolutionary tracks. Altogether, they represent a complete set of stellar yields of unprecedented homogeneity and self-consistency. Our new stellar yields are inserted in a code for the chemical evolution of the Galactic disc with infall of primordial gas, according to the formulation first suggested by Talbot & Arnett (1971, 1973, 1975) and Chiosi (1980). As a first application, the code is used to develop a model of the chemical evolution of the Solar Vicinity, with a detailed comparison to the available observational constraints
SETTING THE CLOCK OF STELLAR MODELS
No full textThe study of old open clusters such as IC 4651 has brought into evidence that the age assigned to these clusters heavily depends on the type of stellar models in use, that is, standard models or models with a certain amount of convective overshoot from the core. While standard models lead to age estimates that reasonably agree among the various sources provided that the same physical assumptions (chemical composition, opacity, nuclear reaction rates, etc.) are adopted, models with convective overshoot give ages that are largely discrepant even if the same input physics is used (the efficiency of core overshoot in particular). This topic has been addressed by Nordstrom & Andersen analyzing the color-magnitude diagram of IC 4651 by Anthony-Twarog et al. They pointed out that the age given by Mazzei & Pigatto based on the overshoot models by Bertelli et al. is about a factor of 3 lower than the value estimated by Maeder and by Maeder & Meynet. Current stellar models indicate that the age of this cluster lies in the range 1.3 x 10(9) to 4 x 10(9) yr, and that the turnoff mass falls in the range 1.5-1.7 M., depending on the adopted color excess, distance modulus, and type of models (classical versus overshoot). In this paper first we show that a fundamental inconsistency affects the models with core overshoot and corresponding isochrones calculated by Maeder & Meynet, and second we re-derive the color excess, distance modulus, and age of IC 4651 with the aid of new models for both the classical and the overshoot mixing scheme. Specifically, we find that the core H-burning lifetime of the stars in this mass range published by Maeder & Meynet exceeds the maximum nuclear lifetime allowed by the structure of their own models. The maximumm lifetime is simply given by the ratio of the energy liberated by the available fuel to the minimum luminosity of the stars in this phase. This inconsistency does not occur during the central H-burning phase of stars of higher mass, whereas it is likely present during the core He-burning phase of stars of any mass. The reason of this is not understood. The study of the color-magnitude diagram of IC 4651 with the aid of the new model calculations for both standard and overshoot models by Fagotto and Alongi et al., respectively, leads to the following results. With the standard models, we get a color excess E(B-V) = 0.16, a distance modulus (m - M)0 = 9.6, and an age of 1.4 x 10(9) yr. With the overshoot models, we obtain a color excess E(B-V) = 0.14, a distance modulus (m - M)0 = 9.5, and an age of 2.0 x 10(9) yr. Reliable ages of star clusters are of the greatest importance to trace back the history of star formation in the Galactic disk
Evolution of massive stars under new mass-loss rates for RSG: is the mystery of the missing blue gap solved?
No full textIn this paper we present new models of massive stars based on recent advancements in the theory of diffusive mixing and a new empirical formulation of the mass-loss rates of red supergiant stars. The study is articulated in two main parts. First, by means of a simple diffusive algorithm, we amalgamate the results of complex studies on non local convection (overshooting region) by Xiong (1989) and Grossman (1996), and apply them to model the structure and evolution of massive stars in occurrence of mass loss by stellar winds according to the popular relationship by de Jager et al. (1988). Stars with initial mass in the range 6 to 120 M. and initial chemical composition [Z=0.008, Y=0.25] and [Z=0.020, Y=0.28] are followed from the zero age main sequence till core He-exhaustion. Particular attention is paid to the 20 M. star as the prototype of the evolution of massive stars in the luminosity (mass) interval in which both blue and red supergiants occur in the HR diagram (HRD). The models confirm that, in the evolution of a massive star with mass loss, the dimension of the H-exhausted core and the efficiency of intermediate mixing strongly affect the evolution during the subsequent core He-burning phase, the extension of the blue loops in particular. However, despite the new mixing prescription, also these models share the same problems of older models in literature as far as the interpretation of the observational distribution of stars across the HRD is concerned. In the second part, we examine possible causes of the failure and find that the rate of mass loss for the red supergiant stages implied by the de Jager et al. (1988) relationship under-estimates the observational values by a large factor. Revising the whole problem, we adopt the recent formulation by Feast (1991) based on infrared data, and take also into account the possibility that the dust to gas ratio varies with the stellar luminosity. Stellar models are then calculated with the new prescription for the mass-loss rates during the red supergiant stages in addition to the new diffusive algorithm. The models now possess very extended loops in the HRD and are able to match the distribution of stars across the HRD from the earliest to the latest spectral types both in the Milky Way, LMC and SMC. During the loop phase the surface abundance of helium is significantly enhanced with respect to the original value as suggested by observational data for blue supergiant stars. Finally, because the surface H-abundance can be lowered to the limit adopted to start the Wolf-Rayet phase (WNL type), we suggest that a new channel is possible for the formation of low luminosity WNL stars, i.e. by progenitors whose mass can be as low as 20 M., that have evolved horizontally across the HRD following the blue-red-blue scheme and suffering large mass loss during the red supergiant stages
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