1,721,137 research outputs found
The Initial-Final Mass Relation of White Dwarfs: A Tool to Calibrate the Third Dredge-Up
Full text linkThe 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
The third dredge-up and the carbon star luminosity functions in the Magellanic Clouds
Full text linkWe investigate the formation of carbon stars as a function of the stellar mass and parent metallicity. Theoretical modelling is based on an improved scheme for treating the third dredge-up in synthetic calculations of thermally pulsing asymptotic giant branch (TP-AGB) stars. In this approach, the usual criterion (based on a constant minimum core mass for the occurrence of dredge-up, M_c() min) is replaced by one on the minimum temperature at the base of the convective envelope, T_b() dred, at the stage of the post-flash luminosity maximum. Envelope integrations then allow determination of M_c() min as a function of stellar mass, metallicity, and pulse strength (see Wood 1981), thus inferring if and when dredge-up first occurs. Moreover, the final possible shut down of the process is predicted. Extensive grids of TP-AGB models were computed by Marigo (1998a,b) using this scheme. In this paper, we present and discuss the calibration of the two dredge-up parameters (i.e. efficiency lambda and T_b() dred) aimed at reproducing the carbon star luminosity function (CSLF) in the LMC, using TP-AGB models with original metallicity Z=0.008. In addition to this, the effects of different input quantities on the theoretical CSLF are analyzed. It turns out that the faint tail is almost insensitive to the history of star formation rate (SFR) in the parent galaxy, in contrast to the bright wing which may be more affected by the details of the recent history. Actually, we find that the faint end of the CSLF is essentially determined by the temperature parameter T_b() dred. Once the faint end is reproduced, the peak location is a stringent calibrator of the efficiency parameter lambda . The best fit to the observed CSLF in the LMC is obtained with lambda =0.50, log T_b() dred = 6.4, and a constant SFR up to an age of about 5 x 10(8) yr. This recent drop of the SFR is invoked to remove a slight excess of bright carbon stars otherwise predicted. A good fit to the observed CSLF in the SMC is then easily derived from the Z=0.004 models, with a single choice of parameters lambda =0.65, log T_b() dred = 6.4, and a constant SFR over the entire significant age interval. The result for lambda is consistent with the theoretical expectation that the third dredge-up is more efficient at lower metallicities
The TP-AGB phase: a new model
Full text linkThis 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
TP-AGB stars with envelope burning
Full text linkIn 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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