1,720,959 research outputs found
Line-driven ablation of circumstellar discs IV. The role of disc ablation in massive star formation and its contribution to the stellar upper mass limit
No full text© 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. Radiative feedback from luminous, massive stars during their formation is a key process in moderating accretion on to the stellar object. In the prior papers in this series, we showed that one form such feedback takes is UV line-driven disc ablation. Extending on this study, we now constrain the strength of this effect in the parameter range of star and disc properties appropriate to forming massive stars. Simulations show that ablation rate depends strongly on stellar parameters, but that this dependence can be parameterized as a nearly constant, fixed enhancement over the wind mass-loss rate, allowing us to predict the rate of disc ablation for massive (proto)stars as a function of stellar mass and metallicity. By comparing this to predicted accretion rates, we conclude that ablation is a strong feedback effect for very massive (proto)stars which should be considered in future studies of massive star formation.sponsorship: We would like to thank S. Owocki and J. Sundqvist for discussions related to the physics of line-driven stellarwinds and their behaviour at sub-solar metallicities. We also would like to thank the anonymous referee for questions and comments clarifying the discussion in this paper. This study was conducted within the Emmy Noether research group on 'Accretion Flows and Feedback in Realistic Models of Massive Star Formation' funded by the German Research Foundation under grant no. KU 2849/3-1. Simulations were carried out using the computational resources available through the bwHPC initiative for high-performance computing in Baden-Wurttemberg, Germany. (German Research Foundation, KU|2849/3-1)status: Publishe
Radiative ablation of disks around massive stars
No full textOwocki, StanleyHot, massive stars (spectral types O and B) have extreme luminosities (104 −
106L ) that drive strong stellar winds through UV line-scattering. Some massive stars
also have disks, formed by either decretion from the star (as in the rapidly rotating
“Classical Be stars”), or accretion during the star’s formation. Extending the winddeveloped
Sobolev methods for line radiative transfer, this dissertation examines the
role of stellar radiation in driving (ablating) material away from these circumstellar
disks.
A key result is that the observed month to year decay of optically thin Be
disks can be explained by line-driven ablation without, as was done in previous work,
appealing to anomalously strong viscous diffusion. Moreover, the higher luminosity of
O stars leads to ablation of optically thin disks on dynamical timescales of order a day,
providing a natural explanation for the lack of observed Oe stars. In addition to the
destruction of Be disks, this dissertation also introduces a model for their formation
via “Pulsationally Driven Orbital Mass Ejection”. This “PDOME” model couples
observationally inferred non-radial pulsation modes and rapid stellar rotation to launch
material into orbiting Keplerian disks of Be-like densities.
In contrast to such Be decretion disks, star-forming accretion disks are much
denser and so are generally optically thick to continuum processes like electron scattering.
To circumvent the computational challenges associated with long-characteristic
radiation hydrodynamics through optically thick media, we develop an approximate
method for treating optically thick continuum absorption in the limit of geometrically
thin disks. The comparison of ablation with and without continuum absorption shows that accounting for disk optical thickness leads to less than a 50% reduction in ablation
rate, implying that ablation rate is largely independent of disk mass, and depends
mainly on stellar properties like luminosity.
Finally, as a side problem, we discuss the role of “thin-shell mixing” in reducing
X-rays from colliding wind binaries. Laminar, adiabatic shocks produce well
understood X-ray emission, but the emission from radiatively cooled shocks is more
complex due to thin-shell instabilities. The parameter study conducted here systematically
varies colliding wind binary shock densities to determine scaling relations for
this emission. A key result is that, in the limit of strongly radiatively cooled shocks,
emission is reduced by a fixed factor ∼ 50 from analytic scalings that ignore thin-shell
structure.University of Delaware, Department of Physics and AstronomyPh.D
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
- …
