2,434 research outputs found
Quince Duncan Moodie
No full textLiterary Encyclopedia entry on Costa Rican author Quince Duncan Moodi
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
Also By The Same Author: AKTiveAuthor, a Citation Graph Approach to Name Disambiguation
No full textThe desire for definitive data and the semantic web drive for inference over heterogeneous data sources requires co-reference resolution to be performed on those data. In particular, name disambiguation is required to allow accurate publication lists, citation counts and impact measures to be determined. This paper describes a graph-based approach to author disambiguation on large-scale citation networks. Using self-citation, co-authorship and document source analyses, AKTiveAuthor clusters papers, achieving precision of 0.997 and recall of 0.818 over a test group of eight surname clusters
Southern Thailand: from conflict to negotiations?
No full textSummary: In this Analysis, University of Leeds professor Duncan McCargo argues that the recent Malaysian-backed Southern Thai peace initiative has now run into some serious problems. He argues that despite its various shortcomings the initiative is still worthy of support, since it has gained far more traction that any previous attempts to address the decade-long insurgency. Thailand needs to maintain focus on the southern conflict despite its current preoccupation with a national-level political crisis that threatens to topple the government of Yingluck Shinawatra.
Key findings
The conflict in Southern Thailand is one of Asia’s most serious insurgencies, with over 6,000 dead over the last 10 years.
The Malaysian government sponsored negotiations represents the best hope for reaching a political settlement and bringing peace to the region.
However, both sides need to show greater commitment to the negotiations, introducing new structures and procedures
The Family Story of Alissa Duncan
No full textThe Family History of
Alissa A. Duncan
24 April 2016
Alissa Anne Duncan authored this family history as part of the course requirements for HIST 550/700 Your Family in History offered online in Spring 2016 and was submitted to the Pittsburg State University Digital Commons. Please contact the author directly with any questions or comments: [email protected]
David Duncan Wallace Papers - Accession 333
No full textThe David Duncan Wallace Papers consist of microfiche copies of the original David D. Wallace family papers, 1866-1951, SCHS 1233.00 held at the South Carolina Historical Society. David Duncan Wallace (1874-1951) was a Professor of History at Wofford College from 1899 through 1947 and was the author of the three volume set titled, History of South Carolina published in 1934. He is considered one of the foremost historians in State. The papers consist of his correspondence, research notes, clippings, and published and unpublished manuscripts related to his publications and areas of research. Also, included is some ephemera and other items.https://digitalcommons.winthrop.edu/manuscriptcollection_findingaids/1417/thumbnail.jp
Preservation assessment of the collections at the Oregon State Library, Salem, Oregon: October 6, 7 & 8, 2009
No full textReport -- Appendix A. Documentation Images -- Appendix B. Resources -- Appendix C. Recommended Books, Collection Policy Resources, Organizations for Reference & Vendors for Supplies -- Appendix D. Cost Estimates.prepared by Lisa Duncan, Art Conservator, LLC.Title from PDF title page (viewed on February 8, 2023).This archived document is maintained by the State Library of Oregon as part of the Oregon Documents Depository Program. It is for informational purposes and may not be suitable for legal purposes.Includes bibliographical references.Mode of access: Internet from the Oregon Government Publications Collection.Text in English
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
Letter from H. Scott Duncan to Hayao (Sam) Chuman
No full textA letter from H. Scott Duncan Associate Executive Secretary for Finance for the American Friends Service Committee, to Hayao (Sam) Chuman. The letter thanks Hayao for his donation.The Chuman (Hayao "Sam" and Toshiko) Papers documents the World War II experiences of Hayao "Sam" and Toshiko Chuman, who were Kibei Nisei born in the United States but grew up and completed school in Japan, and then returned to the U.S. prior to the war. It chronicles the Chuman's incarceration from the Santa Anita Assembly Center, through Jerome, Rohwer, Tule Lake camps, and the Santa Fe and Crystal City internment camps as well as their struggle for restoring their U.S. citizenships in the 1960s. The digital collection consists of mostly textual material, including correspondence, affidavits, incarceration camp records, lease agreements, financial documents, receipts, pamphlets, and booklets
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
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