1,721,025 research outputs found
HST and ground-based analysis of multiple stellar populations in globular clusters
Full text linkIn the last decades spectroscopic and photometric evidence has proven that globular clusters host multiple stellar populations characterized by different
chemical abundances.
The aim of my thesis is to give a contribution in the analysis of multiple stellar populations in Galactic globular clusters, using observations collected both with ground-based telescopes and with the Hubble Space Telescope.
The first part of the thesis presents an overview regarding the topic of multiple stellar populations in Galactic globular clusters, from the first photometric and spectroscopic discoveries to the various theories developed to describe the formation and evolution of different stellar generations in a cluster.
Then a description of my work regarding multiple stellar populations in the globular clusters NGC 6121, NGC 6397, and NGC 6752 is given. Using ground-based FORS2@VLT observations of external regions of these three globular clusters, we identified multiple stellar populations in the main sequence of NGC 6121 and NGC 6752. This work has been possible thanks to high-precision photometry and
appropriate combinations of colors and magnitudes. We presented the radial distribution of the two stellar populations hosted by each globular cluster, combining our result for the external regions with the fraction of first and second stellar generation measured in the central regions using Hubble Space Telescope data. Both for NGC 6121 and for NGC 6752, we found that the radial distribution of the number ratio of the blue main sequence to the red main sequence is
almost flat inside ~17 arcmin from the center of each cluster. Hydrodynamical and N-body simulations for the formation and evolution of multiple stellar populations predict that second generation stars form in the inner regions of the cluster and are initially more concentrated than first generation stars. The subsequent long-term dynamical evolution, driven by two-body relaxation, gradually erases the initial differences in the first and second generation spatial distributions; this is the reason why, for long relaxation-time systems (e.g. omega Cen), second generation stars are concentrated in the globular cluster inner regions and retain some memory of the initial predicted spatial segregation. The relaxation-times of the globular clusters NGC 6121 and NGC 6752 are quite short (less then 1 Gyr), and therefore the two populations hosted by each cluster are today well-mixed, as demonstrated by their flat distribution.
In this work, we have also given an estimate of how much the second generation is enriched in helium with respect to the first generation, finding a mild (Delta Y ~ 0.02) difference between the two sequences for both the clusters.
I am a Co-I of the Hubble Space Telescope Treasury programme ``UV Legacy Survey of Galactic Globular Clusters: Shedding Light on Their Populations and Formation'' (GO-13297, PI: G. Piotto). The second part of the thesis is based on this project. The aim of the Hubble Space Telescope treasury programme GO-13297 is the characterization and analysis of multiple stellar populations in a
sample containing 56 globular clusters, using UV/blue and optical filters.
Using the Hubble Space Telescope treasury programme data, we analyzed the multiple stellar populations of the metal-rich globular cluster NGC 6352. The combination of UV/blue and optical observations has made possible to split and follow the two stellar populations across all the evolutionary sequences of all color-magnitude diagrams. We estimated the enrichment in helium of the second stellar generation, finding Delta Y~0.03. Moreover, we developed an innovative technique to put an upper limit on the relative age between the two stellar generations, with a careful discussion of the impact on relative age of all possible sources of uncertainty in the cluster parameters, such as the error on \Delta Y or variations of metallicity and alpha-enhancement. Considering all these uncertainties together, we found that the two stellar populations of NGC 6352 are coeval within ~300 Myr. This result will be useful to constraint theoretical models of formation and evolution of multiple stellar populations in globular clusters.
Finally, in the last part of the thesis, I summarize the results presented in this work and present a list of works that I would like to realize in future
A PSF-based approach to Kepler/K2 data - I. Variability within the K2 Campaign 0 star clusters M 35 and NGC 2158
No full textKepler and K2 data analysis reported in the literature is mostly based on aperture photometry. Because of Kepler's large, undersampled pixels and the presence of nearby sources, aperture photometry is not always the ideal way to obtain high-precision photometry, and, because of this, the data set has not been fully exploited so far. We present a new method that builds on our experience with undersampled HST images. The method involves a point-spread function (PSF) neighbour-subtraction and was specifically developed to exploit the huge potential offered by the K2 ‘super-stamps’ covering the core of dense star clusters. Our test-bed targets were the NGC 2158 and M 35 regions observed during the K2 Campaign 0. We present our PSF modelling and demonstrate that, by using a high-angular-resolution input star list from the Asiago Schmidt telescope as the basis for PSF neighbour subtraction, we are able to reach magnitudes as faint as KP ≃ 24 with a photometric precision of 10 per cent over 6.5 h, even in the densest regions. At the bright end, our photometric precision reaches ∼30 parts per million. Our method leads to a considerable level of improvement at the faint magnitudes (KP ≳ 15.5) with respect to the classical aperture photometry. This improvement is more significant in crowded regions. We also extracted raw light curves of ∼60 000 stars and detrended them for systematic effects induced by spacecraft motion and other artefacts that harms K2 photometric precision. We present a list of 2133 variables
A PSF-based approach to Kepler/K2data – III. Search for exoplanets and variable stars within the open cluster M 67 (NGC 2682)
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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
Variable stars in one open cluster within the Kepler/K2-Campaign-5 field: M 67 (NGC 2682)
Full text linkThe binary populations of eight globular clusters in the outer halo of the Milky Way
No full textWe analyse colour-magnitude diagrams of eight globular clusters (GCs) in the outer Galactic halo. Images were taken with the Wide Field Channel of the Advanced Camera for Survey and the Ultraviolet and Visual Channel of the Wide Field Camera 3 on board of the Hubble Space Telescope. We have determined the fraction of binary stars along the main sequence and combined results with those of a recent paper where some of us have performed a similar analysis on 59 Galactic GCs. In total, binaries have been now studied homogeneously in 67 GCs.We studied the radial and luminosity distributions of the binary systems, the distribution of their mass ratios and investigated univariate relations with several parameters of the host GCs. We confirm the anticorrelation between the binary fraction and the luminosity of the host cluster, and find that low-luminosity clusters can host a large population in excess of ~40 per cent in the cluster core. However, our results do not support a significant corre..
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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