1,720,986 research outputs found
Chemical evolution of damped Lyα systems
No full textHigh redshift DLA systems suggest that the relative abundances of elements might be roughly solar, although with absolute abundances of more than two orders of magnitude below solar. The result comes from observations of the [SII/ZnII] ratio, which is a reliable diagnostic of the true abundance, and from DLA absorbers with small dust depletion and negligible HII contamination. In particular, in two DLA systems nitrogen is detected and at remarkably high levels (Vladilo et al. 1995 (Proc. ESO Workshop on QSO absorption lines, ed. G. Meylan, Springer Verlag, 103 ), Molaro et al. (1996A&A...308....1M), Green et al. 1995 (Proc. ESO Workshop on QSO absorption lines, ed. G. Meylan, Springer Verlag, 85), Kulkarni et al. (1996MNRAS.279..197K)). Here we compare the predictions from chemical evolution models of galaxies of different morphological type with the abundances and abundance ratios derived for such systems. We conclude that solar ratios and relatively high nitrogen abundances can be obtained in the framework of a chemical evolution model assuming short but intense bursts of star formation, which in turn trigger enriched galactic winds, and a primary origin for nitrogen in massive stars. Such a model is the most successful in describing the chemical abundances of dwarf irregular galaxies and in particular of the peculiar galaxy IZw18. Thus, solar ratios at very low absolute abundances, if confirmed, seem to favour dwarf galaxies rather than spirals as the progenitors of at least some of the DLA systems
A new galactic chemical evolution model with dust: Results for dwarf irregular galaxies and DLA systems
No full textWe present a galactic chemical evolution model which adopts updated prescriptions for all the main processes governing the dust cycle. We follow in detail the evolution of the abundances of several chemical species (C, O, S, Si, Fe and Zn) in the gas and dust of a typical dwarf irregular galaxy. The dwarf irregular galaxy is assumed to evolve with a low but continuous level of star formation and experience galactic winds triggered by supernova (SN) explosions. We predict the evolution of the gas to dust ratio in such a galaxy and discuss critically the main processes involving dust, such as dust production by asymptotic giant branch stars and Type II SNe, destruction and accretion (gas condensation in clouds). We then apply our model to damped Lyman α (DLA) systems which are believed to be dwarf irregulars, as witnessed by their abundance patterns. Our main conclusions are the following. (i) We can reproduce the observed gas to dust ratio in dwarf galaxies. (ii) We find that the process of dust accretion plays a fundamental role in the evolution of dust and in certain cases it becomes the dominant process in the dust cycle. On the other hand, dust destruction seems to be a negligible process in irregulars. (iii) Concerning DLA systems, we show that the observed gas-phase abundances of silicon, normalized to volatile elements (zinc and sulfur), are in agreement with our model. (iv) The abundances of iron and silicon in DLA systems suggest that the two elements undergo a different history of dust formation and evolution. Our work casts light on the nature of iron-rich dust: the observed depletion pattern of iron is well reproduced only when an additional source of iron dust is considered. Here we explore the possibility of a contribution from Type Ia SNe as well as an efficient accretion of iron nanoparticles
Chemical evolution of dwarf irregular and blue compact galaxies
No full textAims: Dwarf irregular and blue compact galaxies are very interesting objects since they are relatively simple and unevolved. We aim at deriving the formation and chemical evolution history of late-type dwarf galaxies and at comparing it with DLA systems.
Methods: We present new models for the chemical evolution of these galaxies by assuming different regimes of star formation (bursting and continuous) and different kinds of galactic winds (normal and metal-enhanced). The dark-to-baryonic mass ratio is assumed to be ten in these models. The chemical evolution model follows the evolution of He, C, N, O, S, Si, and Fe in detail. We have collected the most recent data on these galaxies and compared them with our model's results. We also collected data for damped-Lyman α-systems.
Results: Our results show that in order to reproduce all the properties of these galaxies, including the spread in the chemical abundances, the star formation should have proceeded in bursts and the number of bursts should be not more than ten in each galaxy, and that metal-enhanced galactic winds are required. The presence of metal-enhanced galactic winds can by itself reproduce the observed mass-metallicity relation, although an increasing efficiency of star formation and/or number and/or duration of bursts can also reproduce such a relation equally well.
Conclusions: Metal-enhanced winds, together with an increasing amount of star formation with galactic mass, are required to explain most of the properties of these galaxies. Normal galactic winds, where all the gas is lost at the same rate, do not reproduce the features of these galaxies. On the other hand, a global increase in the amount of star formation (increasing efficiency, number of bursts, or burst duration) with galactic mass is able by itself to reproduce the mass-metallicity relation even without winds, but without metal-enhanced winds is not able to explain many other constraints. We suggest that these galaxies should have suffered a different number of bursts varying from two to ten and that the efficiency of metal-enhanced winds should not have been too high (λmw ~ 1). We predict for these galaxies present-time type Ia SN rates from 0.00084 and 0.0023 per century. Finally, by comparing the abundance patterns of damped Lyman-α objects with our models, we conclude that they are very likely the progenitors of the current dwarf irregulars
Statistical Properties of Habitable Zones in Stellar Binary Systems
Full text linkObservations of exoplanets and protoplanetary disks show that binary stellar systems can host planets in stable orbits. Given the high binary fraction among stars, the contribution of binary systems to Galactic habitability should be quantified. Therefore, we have designed a suite of Monte Carlo experiments aimed at generating large (up to 106) samples of binary systems. For each system randomly extracted we calculate the intersection between the radiative habitable zones (HZs) and the regions of dynamical stability using published empirical formulations that account for the dynamical and radiative parameters of both stars of the system. We also consider constraints on planetary formation in binary systems. We find that the habitability properties of circumstellar and circumbinary regions are quite different and complementary with respect to the binary system parameters. Circumbinary HZs are, generally, rare (≃4%) in the global population of binary systems, even if they are common for stellar separations ≲0.2 au. Conversely, circumstellar HZs are frequent (≥80%) in the global population but are rare for stellar separations ≲1 au. These results are robust against variations of poorly constrained binary system parameters. We derive ranges of stellar separations and stellar masses for which HZs in binary systems can be wider than the HZs around single stars; the widening can be particularly strong (up to one order of magnitude) for circumstellar regions around M-type secondary stars. The comparison of our statistical predictions with observational surveys shows the impact of selection effects on the habitability properties of detected exoplanets in binary systems
Silicon depletion in damped Ly α systems. The S/Zn method
No full textilicates are an important component of interstellar dust that has been poorly investigated in high redshift galaxies. As a preliminary step to studying silicates at high redshift, we survey silicon depletions in damped Ly α (DLA) systems. Silicon depletion is mild in the Galactic interstellar medium (ISM) and is expected to be weaker in most DLA systems, so we introduce a method for improving the accuracy of DLA depletion measurements. We compare abundance ratios measured in the gas with calculations of total abundance ratios of gas and dust predicted by models of galactic chemical evolution tailored for DLA systems. To tune the model parameters, we use the dust-free observational diagram S/Zn versus Zn/H, and we also compare the look back time estimated from the absorption redshift with the evolutionary time predicted by the model. By applying our method to a large set of DLA column densities, we succeeded in measuring the depletion of silicon in 74 systems. For comparison, we also measure iron and magnesium depletions (105 and 10 systems, respectively) with the same method. The mean depletion of silicon that we derive, ⟨ δSi ⟩ ≃ -0.27 ± 0.16 dex, is surprisingly close to that of iron, ⟨ δFe ⟩ ≃ -0.42 ± 0.28 dex, despite iron being much more depleted than silicon in the Galactic ISM. Silicon depletion in DLA systems does not correlate with metallicity, at variance with iron depletion, for which we confirm a rise with [Fe/H] found in previous work. Magnesium depletion seems to behave more in accordance with silicon than with iron. The different behaviors of the silicon and iron depletions suggests a complex history of dust production at the early stages of galactic chemical evolution
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
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