1,721,028 research outputs found
Structure and Evolution of Protoplanetary Disks
Full text linkThe scope of this thesis is to investigate the structure and evolution of protoplanetary disk. High resolution observational techniques such as high resolution optical/infrared spectroscopy and infrared interferometry are well suited for this purpose. High resolution spectroscopy allow to resolve the velocity profile of disk emission lines and determine some important parameters such as the disk geometry and the physical conditions of the line emitting region. Infrared interferometry allows to spatially resolve and constraint the disk geometry within the planet forming region.
The work presented here aims at contributing to the comprehension of the disk structure and evolution at three different evolutionary stages: 1) the early phase when the system is still (partially) embedded in a remnant of the molecular cloud; 2) the so-called Class II phase (from the classification of Lada 1987). At this stage gas and dust evolve rapidly leading to drastic changes of the disk structure; 3) the transition phase from Class II to Class III when gas and dust are dissipated leaving, eventually, a planetary system.
During the early phases of disk evolution the star-disk- envelope system experience powerful instability which are related to rapid enhancement of the mass accretion rate on a timescale of few months. These events are recognizable as so-called FU Orionis outbursts, in which the optical brightness of the system can increase by 4 or more magnitudes. The mass accretion rate increases from 10-7-- 10-8 Myr to 10-3 --10-4 Myr. Statistical studies suggest that young low-mass stars experience several FU Orionis outburst.
In late 2003, the young star V1647 Orionis in the L1630 Ori cloud within the Orion B molecular cloud went into outburst. The outburst shares some properties of the FU Orionis outburst. Following spectro-photometric observations confirmed the nature of the outburst as a disk-instability event. We also find, for the first time, probe of a direct link between an accretion event and the ejection of an Herbig-Haro object (HH).
During the Class II phase dust coagulation and grain growth occur. This is the first step of planet formation. We applied high resolution optical spectroscopy and infrared interferometry to direct compare gas and dust emission from the disk surface of three protoplanetary disks. This study gives some insight on the relative distribution of gas and dust in disk and on the temporal evolution of the two components. A physical decoupling of gas and dust may occur leading to changes in the relative structure of the two (different scale height) and to rapid settling of dust on the disk midplane. This may increase the dust-to-gas mass ratio in the disk interior and, according to recent simulation, may trigger the formation of planetesimals via gravitational instability.
The transition phase from a Class II to a Class III system is characterized by various processes which dissipate the disk material. In particular, viscous accretion and photo-evaporation are very efficient in removing disk material and planet formation is likely in competition with disk dispersion. For this reason, a fundamental quantity is the mass accretion timescale, i.e. the time at which the disk accretion phase ceases. In turn, the time at which the disk accretion phase ceases is a strong constraint on the gas dissipation timescale, relevant for the formation of giant planets. We have observed a number of young stellar clusters of different age aimed at tracing the
evolution viscous accretion with time. The preliminary results show that the accretion seems to cease at similar age of the dust dissipation, i.e. within 5 -- 10 Myr
Long-lived Dust Rings around HD 169142
Full text linkRecent Atacama Large Millimeter/submillimeter Array (ALMA) observations of the protoplanetary disk around HD 169142 reveal a peculiar structure made of concentric dusty rings: a main ring at similar to 20 au, a triple system of rings at similar to 55-75 au in millimetric continuum emission, and a perturbed gas surface density from the (CO)-C-12,(CO)-C-13, and (CO)-O-18 (J = 2-1) surface brightness profile. In this Letter, we perform 3D numerical simulations and radiative transfer modeling exploring the possibility that two giant planets interacting with the disk and orbiting in resonant locking can be responsible for the origin of the observed dust inner rings structure. We find that in this configuration the dust structure is actually long lived while the gas mass of the disk is accreted onto the star and the giant planets, emptying the inner region. In addition, we also find that the innermost planet is located at the inner edge of the dust ring, and can accrete mass from the disk, generating a signature in the dust ring shape that can be observed in mm ALMA observations
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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