870 research outputs found
Chemical yields from low- and intermediate-mass stars
No full textWe present new sets of chemical yields from low- and intermediate-mass stars with 0.8 Msun <= M <= Mup ~ 5 Msun, and three choices of the metallicity, Z=0.02, Z=0.008, and Z=0.004 (Marigo 2000, in preparation). These are then compared with the yields calculated by other authors on the basis of different model prescriptions, and basic observational constraints which should be reproduced
Improved synthetic TP-AGB models
No full textWe present recent improvements and results of an extensive analysis of the TP-AGB phase performed by means of a synthetic model (Marigo 1998a, b; Marigo et al. 1998a, b). The improvements concern: i) the use of a homogeneous and accurate set of analytical relations (Wagenhuber & Groenewegen 1998); ii) a new treatment of envelope burning in the most massive TP-AGB stars (M > 3.5 Modot) to account for the possible break-down of the core mass-luminosity relation; iii) a better treatment of the third dredge-up to infer if and when the process takes place. Extensive calculations of synthetic TP-AGB models have been carried out over the mass range (0.8 Modot 5 Modot) and for three sets of initial metallicity (Z = 0.019, Z = 0.008, Z = 0.004). The formation of carbon stars is investigated addressing the following issues: a) the reproduction of the observed luminosity functions of carbons stars in both Magellanic Clouds, and b) the formation of very bright and optically obscured carbon stars
Can the third dredge-up extinguish hot-bottom burning in massive AGB stars?
No full textThe crucial importance of molecular opacities in modelling the evolution of AGB stars at varying surface C/O ratio has been highlighted in Marigo (2002). The inadequacy of solar-scaled opacities when applied to models of carbon stars has been shown, and hence the need for correctly coupling the molecular opacities to the current surface chemical composition of AGB stars. The aim of the present follow-up study is to investigate the effects of variable molecular opacities on the evolutionary properties of luminous AGB stars with massive envelopes, i.e. with initial masses from ≈3.5 Msun to 5-8 Msun, which are predicted to experience both the third dredge-up and hot-bottom burning. It is found that if the dredge-up of carbon is efficient enough to lead to an early transition from C/O 1, then hot-bottom burning may be weakened, extinguished, or even prevented. The physical conditions for this occurrence are analysed and a few theoretical and observational implications are discussed. Importantly, it is found that the inclusion of variable molecular opacities could significantly change the current predictions for the chemical yields contributed by intermediate-mass AGB stars, with M ≃ 3.5-4.0 Msun, that make as much as ~30-50% of all stars expected to undergo hot-bottom burning
Identification of a TGF-beta responsive element in the human elastin promoter
No full textIn a previous report (Marigo, V., Volpin, D., and Bressan, G. M. (1993) Biochim. Biophys. Acta 1172, 31-36) it was shown that the elastin promoter contains a region mediating transcriptional activation by TGF-β in aorta cells, but not in tendon fibroblasts from chick embryos. In this paper we have identified the sequence responsible for this effect by a combination of CAT assays with mutant constructs, DNase I footprinting and electrophoretic mobility shift assays. This TGF-β responsive element binds different nuclear proteins in chick embryo aorta and tendon cells. Whereas association of the aorta protein(s) to the element is necessary for TGF-β activation, binding of the tendon protein(s) has apparently no effect on promoter stimulation by the cytokine
Envelope burning over-luminosity: a challenge to synthetic TP-AGB models
No full textUntil recently synthetic AGB models had not taken into account the break-down of the core mass-luminosity (M_c - L) relation due to the occurrence of envelope burning in the most massive (M >~ 3.5 Msun for Pop. II and M >~ 4.5 Msun for Pop. I) and luminous (M_bol <~ -6) stars. Marigo et al. (1998) made the first attempt to consistently include the related over-luminosity effect (i.e. above the M_c-L relation) in synthetic TP-AGB calculations. The method couples complete envelope integrations with analytical prescriptions, these latter being presently updated with the highly detailed relations by Wagenhuber & Groenewegen (1998). In this paper the reliability of the solution scheme is tested by comparison with the results of complete evolutionary calculations for a 7 Msun AGB star undergoing envelope burning (Bloecker & Schoenberner 1991; Bloecker 1995). Indeed, the method proves to be valid as it is able to reproduce with remarkable accuracy several evolutionary features of the 7 Msun star (e.g. rate of brightening, luminosity evolution as a function of the core mass and envelope mass for different mass-loss prescriptions) as predicted by full AGB models. Basing on the new solution method, we present extensive synthetic TP-AGB calculations for stars with initial masses of 3.5, 4.0, 4.5, and 5.0 Msun, and three choices of the initial metallicity, i.e. Z=0.019, Z=0.008, and Z=0.004. Three values of the mixing-length parameter are used, i.e. alpha =1.68, 2.0, 2.5. We investigate the dependence of envelope burning on such stellar parameters (M, Z, and alpha ). The comparison between different cases gives hints on the interplay between envelope burning over-luminosity and mass loss, and related effects on TP-AGB lifetimes
The carbon star luminosity functions in the Magellanic Clouds
No full textWe address the question of reproducing the observed luminosity functions of carbons stars in both the LMC and SMC, characterized by quite different features (i.e. faint end and peak location). To this aim, we use recent synthetic TP-AGB evolutionary calculations for a dense grid of stellar masses in the range 0.8 Mo ~< M ~< 5 Mo and metallicities Z = 0.008 and Z = 0.004. These models rely on updated inputs, treating envelope burning and dregde-up in detail by means of envelope integrations (Marigo et al. 1996, 1998ab). The observed carbon star luminosity functions turn out to be strong calibrators of the dredge-up parameters, namely: the minimum temperature at the base of the convective envelope, Tbdred, and the efficiency, lambda. Both distributions are reproduced remarkably well. We also investigate the sensitiveness of the predictions to the star formation history (SFR). The peak location results essentially invariant with respect to the SFR, whereas the shape of the bright wing may give hints on the recent epochs
Evolution of planetary nebulae II. Population effects on the bright cut-off of the PNLF
No full textWe investigate the bright cut-off of the [OIII] 5007 planetary nebula luminosity function (PNLF), that has been suggested as a powerful extragalactic distance indicator in virtue of its observed invariance against populations effects. Theoretical PNLFs are constructed via Monte-Carlo simulations of populations of PNe, whose individual properties are described with the aid of recent PN synthetic models (Marigo et al. 2001, A&A, 378, 958), coupled to a detailed photoionisation code (CLOUDY). The basic dependences of the cut-off magnitude M* are then discussed. We find that : (i) In galaxies with recent or ongoing star formation, the modelled PNLF present M* values between -4 and -5, depending on model details. These are very close to the observationally-calibrated value for the LMC. (ii) In these galaxies, the PNLF cut-off is produced by PNe with progenitor masses of about 2.5 Msun, while less massive stars give origin to fainter PNe. As a consequence M* is expected to depend strongly on the age of the last burst of star formation, dimming by as much as 5 mag as we go from young to 10-Gyr old populations. (iii) Rather than on the initial metallicity of a stellar population, M* depends on the actual [O/H] of the observed PNe, a quantity that may differ significantly from the initial value (due to dredge-up episodes), especially in young and intermediate-age PN populations. (iv) Also the transition time from the end of AGB to the PN phase, and the nuclear-burning properties (i.e. H- or He-burning) of the central stars introduce non-negligible effects on M*. The strongest indication derived from the present calculations is a serious difficulty to explain the age-invariance of the cut-off brightness over an extended interval, say from 1 to 13 Gyr, that observations of PNLFs in galaxies of late-to-early type seem to suggest. We discuss the implications of our findings, also in relation to other interpretative pictures proposed in the past literature
TP-AGB stars with envelope burning
No full textIn this paper we focus on the TP-AGB evolution of intermediate-mass stars experiencing envelope burning (M = 4/5Msun). Our model of the TP-AGB phase is suitably designed to follow the peculiar behaviour of these stars, to which the simple analytical treatment valid in the low-mass range can no longer be applied. The approach we have adopted is a semi-analytical one as it combines analytical relationships derived from complete models of TP-AGB stars with sole envelope models in which the physical structure is calculated from the photosphere down to the core. The solution for the envelope models stands on an original numerical method which allows to treat major aspects of envelope burning. The method secures that, during the quiescent inter-pulse periods, fundamental quantities such as the effective temperature, the surface luminosity, the physical structure of the deepest and hottest layers of the envelope, and the related energy generation from nuclear burning, are not input parameters but the consequence of envelope model calculations. This minimizes the use of analytical relations, thus giving our results greater homogeneity and accuracy. Moreover, we would like to draw the attention on the general validity of our algorithm which can be applied also to the case of low-mass stars, in which envelope burning does not occur. Our efforts are directed to analyse the effects produced by envelope burning, such as: i) the energy contribution which may drive significant deviations from the standard core mass-luminosity relationship; and ii) the changes in the surface chemical composition due to nuclear burning via the CNO cycle. Evolutionary models for stars with initial mass of 4.0, 4.5, 5.0 Msun and two choices of the initial chemical composition ([Y=0.28, Z=0.02] and [Y=0.25, Z=0.008]) are calculated from the first thermal pulse till the complete ejection of the envelope. We find that massive TP-AGB stars can rapidly reach high luminosities (-6 > M_bol > -7), without exceeding, however, the classical limit to the AGB luminosity of M_bol =~ -7.1 corresponding to the Chandrasekhar value of the core mass. No carbon stars brighter than M_bol ~ -6.5 are predicted to form (the alternative of a possible transition from M-star to C-star during the final pulses is also explored), in agreement with observations which indicate that most of the very luminous AGB stars are oxygen-rich. Finally, new chemical yields from stars in the mass range 4 /5 Msun are presented, so as to extend the sets of stellar yields from low-mass stars already calculated by Marigo et al. (1996). For each CNO element we give both the secondary and the primary components
The Initial-Final Mass Relation of White Dwarfs: A Tool to Calibrate the Third Dredge-Up
No full textThe initial mass-final mass relationship (IFMR) of white dwarfs (WD) represents a crucial benchmark for stellar evolution models, especially for the efficiency of mixing episodes and mass loss during the asymptotic giant branch (AGB) phase. In this study, we argue that this relation offers the opportunity to constrain the third dredge-up (3DU), with important consequences for chemical yields. The results are discussed in light of recent studies that have identified a kink in the IFMR for initial masses close to 2M⊙. Adopting a physically-sound approach in which the efficiency λ of the 3DU varies as a function of core and envelope masses, we calibrate λ in solar-metallicity TP-AGB models in order to reproduce the final masses of their WD progeny, over the range of initial masses 0.9≤Mi/M⊙≤6. In particular, we find that in low-mass stars with 1.4≲Mi/M⊙≲2.0 the efficiency is small, λ≤0.3, it steeply rises to about λ≃0.65 in intermediate-mass stars with 2.0≤Mi/M⊙≤4.0, and then it drops in massive TP-AGB stars with 4.0≲Mi/M⊙≲6.0. Our study also suggests that a second kink may show up in the IFMR at the transition between the most massive carbon stars and those that are dominated by hot-bottom burning
Zero-metallicity stars II. Evolution of very massive objects with mass loss
No full textWe present evolutionary models of zero-metallicity very massive objects, with initial masses in the range 120 Msun-1000 Msun, covering their quiescent evolution up to central carbon ignition. In the attempt of exploring the possible occurrence of mass loss by stellar winds, calculations are carried out with recently-developed formalisms for the mass-loss rates driven by radiation pressure (Kudritzki 2002) and stellar rotation (Maeder & Meynet 2000). The study completes the previous analysis by Marigo et al. (2001) on the constant-mass evolution of primordial stars. Our results indicate that radiation pressure (assuming a minimum metallicity Z = 10-4x Zsun) is not an efficient driving force of mass loss, except for very massive stars with M >~ 750 Msun. On the other hand, stellar rotation might play a crucial role in triggering powerful stellar winds, once the Omega Gamma -limit is approached. However, this critical condition of intense mass loss can be maintained just for short, as the loss of angular momentum due to mass ejection quickly leads to the spinning down of the star. As by-product to the present work, the wind chemical yields from massive zero-metallicity stars are presented. The helium and metal enrichments, and the resulting Delta Y/Delta Z ratio are briefly discussed
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