1,720,958 research outputs found
Dust across galaxies
Full text linkIn this thesis, we investigate the evolution of dust mass and chemical composition in different galaxies by means of new detailed chemical evolution models which account for the presence of dust. We adopt updated prescriptions for dust formation in Asymptotic Giant Branch (AGB) stars and Type II SNe, as well as for dust accretion and destruction in the interstellar medium (ISM). We predict in detail the evolution of the abundances of single elements both in the dust and in gas phase of the ISM, and distinguish the contributions from different sources during the galactic time.
We study the dust evolution in galaxies of different morphological type, i.e. dwarf irregulars, spirals, Milky Way-type and ellipticals: our model has proven to be very useful to study various dust properties such as dust mass, dust-to-gas (DG) ratio and chemical composition. In our approach, the main difference between galaxies is the star formation history: in ellipticals it is assumed a very fast and intense star formation rate, and this rate decreases going towards spirals, irregulars and smaller galaxies.
First, we compare our model predictions for a typical dwarf irregular galaxy with chemical abundances measured in Damped Lyman Alpha (DLA) systems. After having reproduced the abundances of volatile elements S and Zn (unaffected by the presence of dust), we study the depletion patterns of refractory elements (Si and Fe), which tend to be incorporated in the dust phase. Our study suggests that Fe and Si undergo a different history of dust formation and evolution and that Fe is mainly incorporated into iron-rich solid nano-particles, which may form by dust growth in the ISM. We also provide a new method, based on DLA column density measurements and the ratio between volatile and refractory elements, to give for the first time an estimate of the chemical abundance ratios inside dust grains. In this way, we try to disentangle the main dust constituents and predict their evolution: in particular, we focus on the fraction between silicates and metallic particles and between pyroxenes and olivines.
Concerning the Milky Way, we present the evolution (in space and time) of the DG ratio in the context of the galactic habitable zone, defined as the region with highly enough metallicity to form planetary systems capable of sustaining life. In this study, we provide theoretical prescriptions of the DG ratio and metallicity for models of planetary systems formation.
Then we focus our study on high redshift elliptical galaxies, and we try to disentangle the responsible processes for the sudden appearance of metals and dust observed in those objects. The first metals and dust appear very early since they are both produced by short living massive stars (core-collapse SNe), on the time-scales of few tenths of million years. In their initial burst of star formation, the metallicity can attain almost a solar value after one hundred million years and the same is true for dust, to which also AGB stars contribute on time-scales equal or larger than 30 million years.
Finally, we study the cosmic dust rate (CDR) across the Universe by assuming that the cosmic dust abundance results from the contribution of galaxies of different morphological type averaged in a unitary volume of the Universe. These galaxies are assumed to evolve in number density according to their weight in the luminosity function at different redshifts and different cosmological scenarios. Parallel to the CDR we compute the cosmic star formation rate (CSFR) as well as the cosmic rate of metallicity. Our predictions are extreme important to understand the roles of dust production, accretion and destruction in the CDR evolution. Our best scenario predicts a dust rate peak between 2<z<3 and reproduces the observed CSFR. Eventually, we estimate the comoving dust density parameter Ωdust and we find a good agreement with data for z<0.5
Galactic habitable zone around M and FGK stars with chemical evolution models that include dust
No full textThe Galactic habitable zone is defined as the region with highly enough metallicity to form planetary systems in which Earth-like planets could be born and might be capable of sustaining life surviving to the destructive effects of nearby supernova explosion events. Galactic chemical evolution models can be useful tools for studying the galactic habitable zones in different systems. Our aim here is to find the Galactic habitable zone using chemical evolution models for the Milky Way disc, adopting the most recent prescriptions for the evolution of dust and for the probability of finding planetary systems around M and FGK stars. Moreover, for the first time, we will express those probabilities in terms of the dust-to-gas ratio of the ISM in the solar neighborhood as computed by detailed chemical evolution models. At a fixed Galactic time and Galactocentric distance we determine the number of M and FGK stars having Earths (but no gas giant planets) which survived supernova explosions, using the formalism of our Paper I. The probabilities of finding terrestrial planets but not gas giant planets around M stars deviate substantially from the ones around FGK stars for supersolar values of [Fe/H]. For both FGK and M stars the maximum number of stars hosting habitable planets is at 8 kpc from the Galactic Centre, if destructive effects by supernova explosions are taken into account. At the present time the total number of M stars with habitable planets are ≃ 10 times the number of FGK stars. Moreover, we provide a sixth order polynomial fit (and a linear one but more approximated) for the relation found with chemical evolution models in the solar neighborhood between the [Fe/H] abundances and the dust-to-gas ratio
The cosmic dust rate across the Universe
No full textWe investigate the evolution of interstellar dust in the Universe by means of chemical evolution models of galaxies of different morphological types, reproducing the main observed features of present-day galaxies. We adopt the most updated prescriptions for dust production from supernovae and asymptotic giant branch stars as well as for dust accretion and destruction processes. Then, we study the cosmic dust rate in the framework of three different cosmological scenarios for galaxy formation: (i) a pure luminosity scenario, (ii) a number density evolution scenario, as suggested by the classical hierarchical clustering scenario and (iii) an alternative scenario, in which both spirals and ellipticals are allowed to evolve in number on an observationally motivated basis. Our results give predictions about the evolution of the dust content in different galaxies as well as the cosmic dust rate as a function of redshift. Concerning the cosmic dust rate, the best scenario is the alternative one, which predicts a peak at 2 < z < 3 and reproduces the cosmic star formation rate. We compute the evolution of the comoving dust density parameter Ωdust and find agreement with data for z < 0.5 in the framework of DE and alternative scenarios. Finally, the evolution of the average cosmic metallicity is presented and it shows a quite fast increase in each scenario, reaching the solar value at the present time, although most of the heavy elements are incorporated into solid grains, and therefore not observable in the gas phase
Evolution of the Dust Composition in Damped Lyα Systems
No full textWe present a method for estimating the relative abundances of refractory elements in the interstellar dust of galaxies hosting damped Lyα (DLA) systems. The method requires gas-phase column densities of volatile and refractory elements, obtained from absorption-line spectroscopy, and interstellar abundances of the same elements, predicted by chemical evolution models of DLA galaxies. We applied this method to the sample of DLA systems with measurements of Mg, Si, S, Fe, and Zn column densities. We find that the dust abundance ratios (Si/Fe)d and (Mg/Fe)d decrease by almost two orders of magnitude in the metallicity range between ≃1/100 solar to roughly solar. This decrease is stronger than the well-known decline of α/Fe ratios with metallicity observed in metal-poor stars and galaxies, suggesting the existence of metallicity-dependent mechanisms of dust production. To cast light on these mechanisms we investigated the contributions of different stellar sources and interstellar processes to the galactic cycle of dust. We find that Type II SNe are important contributors to the dust composition at low metallicity ([Fe/H] < ‐0.6), whereas dust accretion in the interstellar medium appears to be important at higher metallicities, leading to a gradual rise of iron-rich particles, possibly in metal form. To further investigate the nature of the dust, we introduced an idealized model of dust grains based on a mixture of silicates (pyroxenes and olivines) and an iron-rich constituent. The model reproduces the evolutionary trends and suggests that olivines are dominant in silicates, in line with other studies of interstellar dust composition
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
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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