1,720,979 research outputs found
Nitrogen evolution in the halo, thick disc, thin disc and bulge of the Galaxy
Full text linkWe study the evolution of nitrogen (N) in the Galactic halo, thick disc, thin disc, and bulge by comparing detailed chemical evolution models with recent observations. The models used in this work have already been constrained to explain the abundance patterns of α-elements and the metallicity distribution functions of halo, disc, and bulge stars; here, we adopt them to investigate the origin and evolution of N in the different Galactic components. First, we consider different sets of yields and study the importance of the various channels proposed for N production. Secondly, we apply the reference models to study the evolution of both the Galactic discs and bulge. We conclude that: i) primary N produced by rotating massive stars is required to reproduce the plateau in log(N/O) and [N/Fe] ratios at low metallicity, as well as the secondary and primary production from low- and intermediate-mass stars to reproduce the data of the thin disc; ii) the parallel model can provide a good explanation of the evolution of N abundance in the thick and thin discs, and we confirm that the thick disc has evolved much faster than the thin disc, in agreement with the results from the abundance patterns of other chemical elements; and iii) finally, we present new model predictions for N evolution in the Galactic bulge, and we show that the observations in bulge stars can be explained if massive stars rotate fast during the earliest phases of Galactic evolution, in agreement with findings from the abundance pattern of carbon
Abundance gradients along the Galactic disc from chemical evolution models
Full text linkIn this paper, we study the formation and chemical evolution of the Milky Way disc with particular focus on the abundance patterns ([α/Fe] versus [Fe/H]) at different Galactocentric distances, the present-time abundance gradients along the disc, and the time evolution of abundance gradients. We consider the chemical evolution models for the Galactic disc developed by Grisoni et al. for the solar neighbourhood, both the two-infall and the one-infall ones, and we extend our analysis to the other Galactocentric distances. In particular, we examine the processes that mainly influence the formation of the abundance gradients: the inside-out scenario, a variable star formation efficiency, and radial gas flows. We compare our model results with recent abundance patterns obtained along the Galactic disc from the APOGEE survey and with abundance gradients observed from Cepheids, open clusters, H II regions, and PNe. We conclude that the inside-out scenario is a key ingredient but cannot be the only one to explain abundance patterns at different Galactocentric distances and abundance gradients. Further ingredients, such as radial gas flows and variable star formation efficiency, are needed to reproduce the observed features in the thin disc. The evolution of abundance gradients with time is also shown, although firm conclusions cannot still be drawn
Heavy element evolution in the inner regions of the Milky Way
Full text linkWe present results for the evolution of the abundances of heavy elements (O, Mg, Al, Si, K, Ca, Cr, Mn, Ni, and Fe) in the inner Galactic regions (RGC ≤ 4 kpc). We adopt a detailed chemical evolution model already tested for the Galactic bulge and compare the results with Apache Point Observatory Galactic Evolution Experiment data. We start with a set of yields from the literature that are considered the best to reproduce the abundance patterns in the solar vicinity. We find that, in general, the predicted trends nicely reproduce the data but in some cases either the trend or the absolute values of the predicted abundances need to be corrected, even by large factors, in order to reach the best agreement. We suggest how the current stellar yields should be modified to reproduce the data and we discuss whether such corrections are reasonable in the light of the current knowledge of stellar nucleosynthesis. However, we also critically discuss the observations. Our results suggest that Si, Ca, Cr, and Ni are the elements for which the required corrections are the smallest, while for Mg and Al moderate modifications are necessary. On the other hand, O and K need the largest corrections to reproduce the observed patterns, a conclusion already reached for solar vicinity abundance patterns, with the exception of oxygen. For Mn, we apply corrections already suggested in previous works
Chemical evolution of the Milky Way: constraints on the formation of the thick and thin discs
Full text linkWe study the evolution of Milky Way thick and thin discs in the light of the most recent observational data. In particular, we analyse abundance gradients of O, N, Fe, and Mg along the thin disc as well as the [Mg/Fe] versus [Fe/H] relations and the metallicity distribution functions at different Galactocentric distances. We run several models starting from the two-infall paradigm, assuming that the thick and thin discs formed by means of two different infall episodes, and we explore several physical parameters, such as radial gas flows, variable efficiency of star formation, different times for the maximum infall on to the disc, different distributions of the total surface mass density of the thick disc, and enriched gas infall. Our best model suggests that radial gas flows and variable efficiency of star formation should be acting together with the inside-out mechanism for the thin disc formation. The time-scale for maximum infall on to the thin disc, which determines the gap between the formation of the two discs, should be tmax ≃ 3.25 Gyr. The thick disc should have an exponential, small-scale length density profile and gas infall on the inner thin disc should be enriched. We also compute the evolution of Gaia-Enceladus system and study the effects of possible interactions with the thick and thin discs. We conclude that the gas lost by Enceladus or even part of it could have been responsible for the formation of the thick disc but not the thin disc
Fluorine in the solar neighbourhood : modelling the Galactic thick and thin discs
Full text linkWe investigate the evolution of the abundance of fluorine in the Milky Way thick and thin discs by means of detailed chemical evolution models compared with recent observational data. The chemical evolution models adopted here have already been shown to fit the observed abundance patterns of CNO and alpha-elements as well as the metallicity distribution functions for the Galactic thick and thin disc stars. We apply them here to the study of the origin and evolution of fluorine, which is still a matter of debate. First, we study the importance of the various sites proposed for the production of fluorine. Then, we apply the reference models to follow the evolution of the two different Galactic components. We conclude that rotating massive stars are important producers of F and they can set a plateau in F abundance below [Fe/H]=-0.5 dex, though its existence for [Fe/H]<-1 has yet to be confirmed by extensive observations of halo stars. In order to reproduce the F abundance increase in the discs at late times, instead, a contribution from lower mass stars - single asymptotic giant branch stars and/or novae - is required. The dichotomy between the thick and thin discs is more evident in the [F/O] versus [O/H] plot than in the [F/Fe] versus [Fe/H] one, and we confirm that the thick disc has evolved much faster than the thin disc, in agreement with findings from the abundance patterns of other chemical elements
Modelling the chemical evolution of Zr, La, Ce, and Eu in the Galactic discs and bulge
Full text linkWe study the chemical evolution of Zr, La, Ce, and Eu in the Milky Way discs and bulge by means of chemical evolution models compared with spectroscopic data. We consider detailed chemical evolution models for the Galactic thick disc, thin disc, and bulge, which have been already tested to reproduce the observed [α/Fe] versus [Fe/H] diagrams and metallicity distribution functions for the three different components, and we apply them to follow the evolution of neutron capture elements. In the [Eu/Fe] versus [Fe/H] diagram, we observe and predict three distinct sequences corresponding to the thick disc, thin disc, and bulge, similar to what happens for the α-elements. We can nicely reproduce the three sequences by assuming different time-scales of formation and star formation efficiencies for the three different components, with the thin disc forming on a longer time-scale of formation with respect to the thick disc and bulge. On the other hand, in the [X/Fe] versus [Fe/H] diagrams for Zr, La, and Ce, the three populations are mixed and also from the model point of view there is an overlapping between the predictions for the different Galactic components, but the observed behaviour can be also reproduced by assuming different star formation histories in the three components. In conclusions, it is straightforward to see how different star formation histories can lead to different abundance patterns and also looking at the abundance patterns of neutron capture elements can help in constraining the history of formation and evolution of the major Galactic 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
Evolution of lithium in the Milky Way halo, discs, and bulge
Full text linkIn this work, we study the Galactic evolution of lithium by means of chemical evolution models in the light of the most recent spectroscopic data from Galactic stellar surveys. We consider detailed chemical evolution models for the Milky Way halo, discs, and bulge, and we compare our model predictions with the most recent spectroscopic data for these different Galactic components. In particular, we focus on the decrease of lithium at high metallicity observed by the AMBRE Project, the Gaia-ESO Survey, and other spectroscopic surveys, which still remains unexplained by theoretical models. We analyse the various lithium producers and confirm that novae are the main source of lithium in the Galaxy, in agreement with other previous studies. Moreover, we show that, by assuming that the fraction of binary systems giving rise to novae is lower at higher metallicity, we can suggest a novel explanation to the lithium decline at super-solar metallicities: the aforementioned assumption is based on independent constraints on the nova system birth rate, which have been recently proposed in the literature. As regards the thick disc, it is less lithium enhanced due to the shorter time-scale of formation and higher star formation efficiency with respect to the thin disc; therefore, we have a faster evolution and the 'reverse knee' in the A(Li) versus [Fe/H] relation is shifted towards higher metallicities. Finally, we present our predictions about lithium evolution in the Galactic bulge, which, however, still need further data to be confirmed or disproved
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
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