130,657 research outputs found
Probing self-gravitating protostellar discs using smoothed particle hydrodynamics and radiative transfer
No full textStars are likely to form with non-zero initial angular momentum, and will consequently
possess a substantial gaseous protostellar disc in the early phases of their evolution.
At this early stage, the disc mass is expected to be comparable to the mass of the
protostar. The disc’s self-gravity therefore plays an important role in the
subsequent evolution of the system, regulating the accretion of matter onto the
protostar, as well as being potentially capable of forming low mass stars and massive
planets by disc fragmentation. The protostellar disc may later evolve into a protoplanetary
disc, providing the feedstock for planet formation. Therefore, if the current
stellar populations and exoplanetary systems are to be understood, an understanding
of the evolution of protostellar discs is crucial, especially their earliest self-gravitating
phases. I have used various methods of numerical simulation to probe the physics of
self-gravitating protostellar discs and their constituents.
When constructing a model for self-gravitating protostellar discs, including detailed
thermodynamics and radiative transfer is essential. I have developed two distinct numerical
techniques for incorporating radiative transfer into Smoothed Particle Hydrodynamics
(SPH) simulations. The first allows the modelling of frequency-averaged
radiative transfer during the SPH simulation, in effect approximating radiative SPH
(RSPH) with only a marginal increase in runtime (around 6%). The second takes the
output from SPH simulations, and creates synthetic, wavelength-dependent telescope
images and spectra of SPH systems. This allows the direct construction of observables
from SPH simulations, providing, for the first time, a direct connection between the
output of SPH simulations and observations.
I have used these numerical methods to analyse, in detail, the local angular momentum
transport induced by self-gravity in protostellar discs, testing the robustness
of the “pseudo-viscous” analytical approximation for local disc stresses. I confirm that
semi-analytical disc modellers are justified in using the pseudo-viscous approximation
in some cases, but I also outline the limits in which non-local transport effects causes
the approximation to fail.
Also, I have investigated the evolution of protostellar discs when perturbed by a
secondary companion, in particular identifying whether such events will in general
trigger a) a disc fragmentation event, or b) a stellar outburst event. For case a), I found
no significant evidence that perturbation by a companion improves the possibility of
disc fragmentation in compact discs - in case b), I found that stellar outburst events do
indeed occur, but they are unlikely to be seen by observers due to their rare occurrence,
as well as due to self-obscuration effects
The nature of angular momentum transport in radiative self-gravitating protostellar discs
No full textSemi-analytic models of self-gravitating discs often approximate the angular momentum transport generated by the gravitational instability using the phenomenology of viscosity. This allows the employment of the standard viscous evolution equations, and gives promising results. It is, however, still not clear when such an approximation is appropriate.
This paper tests this approximation using high-resolution 3D smoothed particle hydrodynamics (SPH) simulations of self-gravitating protostellar discs with radiative transfer. The nature of angular momentum transport associated with the gravitational instability is characterized as a function of both the stellar mass and the disc-to-star mass ratio. The effective viscosity is calculated from the Reynolds and gravitational stresses in the disc. This is then compared to what would be expected if the effective viscosity were determined by assuming local thermodynamic equilibrium or, equivalently, that the local dissipation rate matches the local cooling rate.
In general, all the discs considered here settle into a self-regulated state where the heating generated by the gravitational instability is modulated by the local radiative cooling. It is found that low-mass discs can indeed be represented by a local α-parametrization, provided that the disc aspect ratio is small (H/r≤ 0.1) which is generally the case when the disc-to-star mass ratio q≲ 0.5. However, this result does not extend to discs with masses approaching that of the central object. These are subject to transient burst events and global wave transport, and the effective viscosity is not well modelled by assuming local thermodynamic equilibrium. In spite of these effects, it is shown that massive (compact) discs can remain stable and not fragment, evolving rapidly to reduce their disc-to-star mass ratios through stellar accretion and radial spreading
MeSH term explosion and author rank improve expert recommendations
No full textInformation overload is an often-cited phenomenon that reduces the productivity, efficiency and efficacy of scientists. One challenge for scientists is to find appropriate collaborators in their research. The literature describes various solutions to the problem of expertise location, but most current approaches do not appear to be very suitable for expert recommendations in biomedical research. In this study, we present the development and initial evaluation of a vector space model-based algorithm to calculate researcher similarity using four inputs: 1) MeSH terms of publications; 2) MeSH terms and author rank; 3) exploded MeSH terms; and 4) exploded MeSH terms and author rank. We developed and evaluated the algorithm using a data set of 17,525 authors and their 22,542 papers. On average, our algorithms correctly predicted 2.5 of the top 5/10 coauthors of individual scientists. Exploded MeSH and author rank outperformed all other algorithms in accuracy, followed closely by MeSH and author rank. Our results show that the accuracy of MeSH term-based matching can be enhanced with other metadata such as author rank
The dynamical fate of self-gravitating disc fragments after tidal downsizing
No full textDF and KR acknowledge support from STFC grant ST/J001422/1. DF also acknowledges support from the ECOGAL ERC Advanced Grant Programme. RJP acknowledges support from the Royal Astronomical Society in the form of a research fellowship.The gravitational instability model of planet/brown dwarf formation proposes that protostellar discs can fragment into objectswith masses above a few Jupiter masses at large semimajor axis. Tidal downsizing may reduce both the object mass and semimajor axis. However, most studies of tidal downsizing end when the protostellar disc disperses, while the system is embedded in its parent star-forming region. To compare disc fragment descendants with exoplanet and brown dwarf observations, the subsequent dynamical evolution must be explored. We carry out N-body integrations of fragment-fragment scattering in multi-object star systems, and star systems embedded in substructured clusters. In both cases, we use initial conditions generated by population synthesis models of tidal downsizing. The scattering simulations produce a wide range of eccentricities. The ejection rate is around 25 per cent. The ejecta mass distribution is similar to that for all objects, with a velocity dispersion consistent with those produced by full hydrodynamic simulations. The semimajor axis distribution after scattering extends to parsec scales. In the cluster simulations, 13 per cent of the objects are ejected from their planetary system, and around 10 per cent experience significant orbit modification. A small number of objects are recaptured on high-eccentricity, high-inclination orbits. The velocity distribution of ejecta is similar to that produced by fragment-fragment scattering. If fragment-fragment scattering and cluster stripping act together, then disc fragmentation should be efficient at producing free-floating substellar objects, and hence characterizing the free-floating planet population will provide strong constraints on the frequency of disc fragmentation.Peer reviewe
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
A computational and experimental approach linking disorder, high-pressure behaviour and mechanical properties in UiO Frameworks
No full textData underlying the publication: C. L. Hobday, R. J. Marshall, C. F. Murphie, J. Sotelo, T. Richards, D. R. Allan, T. Düren, F.-X. Coudert, R. S. Forgan, C. A. Morrison, S. A. Moggach, T. D. Bennett, Angew. Chem. Int. Ed. 2016, 55, 2401.##Files:##
* Datashare.zip: First principles ab-initio output filed for UiO-abdc and UiO-67.
* cifs.zip: crystallographic data for both UiO-67 and UiO-abdc.
* FINALUIODATA.xlsx: Unit cell dimensions and compression
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
No full textBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
Dispelling the Myths Behind First-author Citation Counts
No full textWe 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
Scholarly Communication and Publishing Lunch and Learn Talk #11: The ULS Open Access Author Fee Fund
No full textAt the May 2014 talk, you will learn about the ULS Open Access Author Fee Fund--what it is, why we do it, how it works, and how the program is going so far
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