1,721,274 research outputs found
LA DIRETTIVA CEE SULLA PROTEZIONE DEI LAVORATORI PROFESSIONALMENTE ESPOSTI A PIOMBO INORGANICO: ANALISI E COMMENTO
No full textConsiderazioni sulla direttiva CEE sulla protezione dei lavoratori esposti a piombo inorganico
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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
The HR diagram of massive stars : widening of the main sequence band and WR formation.
No full textPlausible physical causes of the observed main sequence band widening are first discussed separately and then their combined effect is evaluated. These are mass loss by stellar wind, overshooting from convective cores, effect of stellar winds on hydrostatic radii, effect of opacity in the CNO ionization zone. It is found that, while each term taken separately cannot produce stellar models matching the observational requirement, only a suitable combination of overshooting, mass loss by stellar wind all over the major stages of central nuclear burning, and a moderate opacity enhancement in the outer layers may lead theoretical models into satisfactory agreement with the observational data. The problem of formation, statistics, and distribution of WR stars across the galactic plane is discussed in the light of these new stellar models. It is found that the dependence of opacity rather than mass loss rate on the ambient metallicity plays the dominant role in accounting for the properties of WR stars
Integrated colors and ages of lmc clusters - the nature of the bimodal distribution of the (b-v) colors
No full textIntegrated (B-V)_0_ and (U-B)_0_ colours and synthetic HR diagrams of stellar clusters as a function of the age are obtained with the aid of stellar models that incorporate the effects of convective overshoot all over their major evolutionary phases. Effects of stochastic nature in the initial mass function caused by the finite number of stars per cluster, and effects of dispersion in the stellar ages are also taken into account. The resulting theoretical calibrating relationships are presented and some implications for real clusters of different age are outlined. In particular, we derive the age and colour distribution functions (number of clusters per age and colour range) for clusters of the LMC, and discuss the causes of the "gap" in the relation between (B-V) and cluster type by Searle et al. (1980) and/or in the V versus (B-V) diagram, or equivalently the bimodality of the colour distribution function. Over the past few years many suggestions have been advanced to explain the physical causes of the gap: an age gap and/or effects of cluster disruption, sudden reddening of the (B-V) color caused by the appearance for the first time in a cluster of stars in particular evolutionary phases (red and asymptotic giant branches, collapse of loops to a clump). None of these has been however tested on the basis of complete models of photometric synthesis. We find that no unique cause can be singled out, rather the observed colour distribution is the result of several concurrent factors. In brief, the colour distribution can be derived from models of photometric synthesis of star clusters provided the age distribution function and the colour speed are known. The age distribution is obtained from cluster counts per age range as originally proposed by Elson and Fall (1985). This is however modified to incorporate the novel ages derived from stellar models with convective overshoot. The observational age distribution obtained from this procedure is the end product of the history of cluster formation and disruption (by various dynamical effects). Finally, the colour speed is simply given by calculating from photometric models a colour age relation. We find that the colour distribution is primarily reflective of the age distribution function, which determines the ratio of the blue to red clusters, whereas the colour speed fixes the location and width of the gap
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 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
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