170,408 research outputs found
Studio strutturale e conformazionale del copolimero esafluoropropilene-tetrafluoroetilene
No full textImproved 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
Constraining the third dredge-up via carbon stars in the Magellanic Clouds
No full textWe use the available data for Magellanic Cloud carbon stars to constrain the efficiency of the third dredge-up process in TP-AGB models. We show that star counts in LMC clusters provide quite stringent limits to the lifetime of the C-star phase, with a duration between 2 and 3 Myr for stars in the mass range from 1.5 to 2.8 Msun. Together with the luminosity functions of field C stars, this information allows us to re-calibrate the third dredge-up parameters log T_b_dred and lambda in TP-AGB models that include variable molecular opacities (Marigo 2002). Preliminary results are presented here
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
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
Asymptotic Giant Branch evolution at varying surface C/O ratio: effects of changes in molecular opacities
No full textWe investigate the effects of molecular opacities on the evolution of TP-AGB stars that experience the third dredge-up, i.e. with surface abundances of carbon and oxygen varying with time. To this aim, a routine is constructed to derive the molecular concentrations through dissociation equilibrium calculations, and estimate the opacities due to H2, H2O, OH, C2, CN, and CO for any given density, temperature and chemical composition of the gas. Then, synthetic TP-AGB models with dredge-up are calculated by either adopting the newly developed routine, or interpolating between fixed opacity tables for solar chemical composition. The comparison between the two cases shows that the change in the dominant opacity sources, as the C/O ratio grows from below to above unity, crucially affects the evolution of the effective temperature, i.e. causing a notable cooling of the carbon-rich models (with C/O>1). From the comparison with observational data, it turns out that TP-AGB models with variable molecular opacities are able to reproduce the observed range of effective temperatures, mass-loss rates, and wind expansion velocities of C-type giants in the solar neighbourhood, otherwise failed if assuming fixed molecular opacities for solar-scaled mixtures. Finally, we mention other possibly important evolutionary and observational effects that result from the adoption of the variable opacities, such as: i) significant shortening of the C-star phase due to the earlier onset of the super-wind; ii) consequent reduction of the carbon yields iii) reproduction of the observed range of near-infrared colours of C-stars
The red tail of carbon stars in the LMC: models meet 2MASS and DENIS observations
No full textCarbon stars are known to exhibit systematically redder near-infrared colours with respect to M-type stars. In the near-infrared colour-magnitude diagrams provided by the 2MASS and DENIS surveys, the LMC C-type stars draw a striking ``red tail'', well separated from the sequences of O-rich giants. So far, this conspicuous feature has been absent from any set of available isochrones, even the few existing ones that include the TP-AGB evolution of low- and intermediate-mass stars. To investigate such issue we simulate the complete 2MASS Ks vs. (J-Ks) data towards the LMC by means of a population synthesis approach, that relies on extended libraries of published stellar evolutionary tracks, including the TP-AGB phase. The simulations provide quite a detailed description of the several vertical ``fingers'' and inclined sequences seen in 2MASS data, due to both galactic foreground and LMC O-rich stars. Instead, as mentioned, the red tail of C-stars sets a major difficulty: we find that TP-AGB models with solar-scaled molecular opacities, the usual assumption of existing AGB calculations, do not succeed in reproducing this feature. Our tests indicate that the main reason for this failure should not be ascribed to empirical Teff-(J-K) transformations for C-type stars. Instead, the discrepancy is simply removed by adopting new evolutionary models that account for the changes in molecular opacities as AGB stars get enriched in carbon via the third dredge-up (Marigo \cite{Marigo02}). In fact, simulations that adopt these models are able to reproduce, for the first time, the red tail of C-stars in near-infrared CMDs. Finally, we point out that these simulations also provide useful indications about the efficiency of the third dredge-up process, and the pulsation modes of long-period variables
AGB evolution: the key role of the C/O ratio
No full textThe evolution of AGB stars is reviewed as a function of a crucial parameter: the photospheric C/O ratio
The TP-AGB phase: a new model
No full textThis study deals with the TP-AGB phase of low and intermediate-mass stars (0.7<=M/Msun_<=5). To this aim, a semi-analytical model is constructed. A representative set of TP-AGB evolutionary models is calculated for two classes of initial metallicity (Z=0.02 and Z=0.008). A detailed analysis is performed to estimate the changes in the surface chemical composition caused by (1) the inter-shell nucleosynthesis and convective dredge-up; (2) nuclear burning in the deepest layers of the convective envelope; and (3) mass loss by stellar wind. The evolution of the abundances of 13 chemical elements (H, ^3^He, ^4^He, ^12^C, ^13^C, ^14^N, ^15^N, ^16^O, ^17^O,^18^O, ^20^Ne, ^22^Ne, ^25^Mg) is followed. In particular, the formation of carbon stars is investigated. We use the observed luminosity function of carbon stars in the LMC as the constraint whose fulfillment determines the values of the parameters adopted in the model, namely: the minimum core mass for dredge-up M_c_^min^ and the efficiency of the third dredge-up λ. In this way, we derive a proper calibration which the reliability of the chemical analysis stands on. We calculate the stellar yields for both metallicities to provide new data for these key-ingredients in the process of chemical enrichment of the interstellar medium. The chemical composition of PNe is derived and compared to the latest experimental data both in the Galaxy and in the LMC, which leads to a partial agreement. Observed information on the correlation between luminosity and pulsational period of Mira and OH/IR variables is used to test further our results. Finally, we predict the initial-final mass relation and we compare it to the semi-empirically determined one for the solar neighbourhood. The agreement turns out to be satisfactory
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