The response of self-graviting protostellar discs to slow reduction in cooling timescale: the fragmentation boundary revisited

A number of previous studies of the fragmentation of self-gravitating protostellar discs have modeled radiative cooling with a cooling timescale (t_{cool}) parameterised as a simple multiple (beta_{cool}) of the local dynamical timescale. Such studies have delineated the `fragmentation boundary' in terms of a critical value of beta_{cool} (beta_{crit}), where the disc fragments if beta_{cool} < beta_{crit}. Such an approach however begs the question of how in reality a disc could ever be assembled with beta_{cool} < beta_{crit}. Here we adopt the more realistic approach of gradually reducing beta_{cool}, as might correspond to changes in thermal regime due to secular changes in the disc density profile. We find that when beta_{cool} is gradually reduced (on a timescale longer than t_{cool}), the disc is stabilised against fragmentation, compared with models in which beta_{cool} is reduced rapidly. We therefore conclude that a disc's ability to remain in a self-regulated, self-gravitating state (without fragmentation) is partly dependent on its thermal history, as well as its current cooling rate. Nevertheless, a slow reduction in t_{cool} appears only to lower the fragmentation boundary by about a factor two in t_{cool} and thus only permits maximum alpha values (parameterising the efficiency of angular momentum transfer in the disc) that are about a factor two higher than determined hitherto. Our results therefore do not undermine the notion of a fundamental upper limit to the heating rate that can be delivered by gravitational instabilities before the disc is subject to fragmentation. An important implication of this work, therefore, is that self-gravitating discs can enter into the regime of fragmentation via secular evolution and it is not necessary to invoke rapid (impulsive) events to trigger fragmentation.Comment: accepted for publication in MNRA

Analytical and experimental study of stratification and liquid-ullage coupling, 1 June 1964 - 31 May 1965

Closed-form solution for stratification of subcooled fluids in containers subjected to heating, and for liquid-ullage vapor couplin

Far infrared and submillimeter brightness temperatures of the giant planets

The brightness temperatures of Jupiter, Saturn, Uranus, and Neptune in the range 35 to 1000 micron. The effective temperatures derived from the measurements, supplemented by shorter wavelength Voyager data for Jupiter and Saturn, are 126.8 + or - 4.5 K, 93.4 + or - 3.3 K, 58.3 + or - 2.0 K, and 60.3 + or - 2.0 K, respectively. The implications of the measurements for bolometric output and for atmospheric structure and composition are discussed. The temperature spectrum of Jupiter shows a strong peak at approx. 350 microns followed by a deep valley at approx. 450 to 500 microns. Spectra derived from model atmospheres qualitatively reproduced these features but do not fit the data closely

Messaging, Marketing, and Identity: Content Analysis of Waterfowl Hunting Videos

Abstract Media has been documented to significantly impact children and adolescents’ behavior, attitudes, cultural norms, identity development, and normative behaviors. We conducted a content analysis of three waterfowl hunting videos to determine what images and words were being conveyed to hunters, and a mail survey of Illinois waterfowl hunters to test how age may influence viewing of waterfowl hunting media. In particular, we hypothesized that: 1) age will influence whether hunters have watched waterfowl hunting media, and 2) age will influence how often hunters watch them. Overall, 82% of waterfowl hunters had watched waterfowl hunting media. Age influenced whether hunters, and how often hunters, had watched them; younger hunters were more likely to have watched the media and watched them significantly more often. Media watching also influenced hunters’ expectations, their identity as a waterfowl hunter, and their perception of the number of birds they harvest when compared to others. Results suggest that media influences waterfowl hunters’ expectations, identity, and behavior, all of which play an important role in how we will manage for waterfowl hunters in the future

Aspects of the biology of the sclerotia of Sclerotium cepivorum : a thesis submitted for the requirements of the degree of Doctor of Philosophy, Lincoln University, Canterbury, New Zealand

The fungus, Sclerotium cepivorum Berk. is the causal agent of onion white rot disease and reproduces solely by means of sclerotia that persist in the soil between susceptible crops. Sclerotia are, therefore, a vulnerable part of the lifecycle and disease control measures are being targeted towards them. These methods depend on a thorough knowledge of sclerotial biology. Three key aspects of sclerotial biology were investigated in the course of this research programme; the factors that affect survival of sclerotia in soil, the phenomenon of sclerotial dormancy and the relationship between inoculum levels in the soil and subsequent disease incidence. The influence of soil type, location and sclerotial condition on survival of sclerotia was examined under field conditions. There was no significant difference in sclerotial survival in the two soil types tested (Patumahoe clay loam and Wakanui silt loam). Experiments were conducted at two locations (Auckland and Lincoln), which differed substantially in both average soil temperature and rainfall; Auckland having a warmer wetter climate. Location significantly affected sclerotial survival over time, with fewer sclerotia surviving at Auckland. The condition of the sclerotia, related to structural damage caused by desiccation and re-wetting in the field, was an important influence on survival. Most sclerotia (40 - 60%) decayed after just two months in soil, after which numbers remained relatively stable for up to 2 years. During the growing season, sclerotia became progressively more contaminated by other fungi and exhibited signs of desiccation. The sudden drop in viability over the first few months in soil was attributed to damage caused by adverse environmental conditions and subsequent attack by microbes. A study was made of the dormancy requirements of naturally-produced sclerotia in New Zealand soils. Dormancy was tested in sclerotia from 5 isolates buried in one of two different soils (Patumahoe clay loam and Wakanui silt loam). Two established methods were modified to test for dormancy in New Zealand conditions; one based in the laboratory and one in the field. In the laboratory-based experiment, < 40% of sclerotia germinated after 15 weeks in soil. Soil type did not significantly affect the length of dormancy in this study. The low rates of germination observed resulted in no significant differences among the isolates in the length of dormancy. This method was prone to problems caused by contamination by other fungi and is not recommended for use in New Zealand conditions. In the field trial, 47 - 72% of sclerotia had germinated or decayed after 4 months in soil, depending on the isolate. Sclerotia required 6 months in soil before high rates of germination occurred (89 - 98%). In the field trial, isolates differed in the amount of time required in soil before dormancy began to break (3 - 5 months). In both experiments described above, dormancy lasted several months longer than reported in the literature. Sclerotia collected from onions in a commercial field also exhibited sclerotial dormancy. This has implications for the use of germination stimulants (e.g. diallyl disuphide, DADS) in disease control in New Zealand. With dormancy lasting up to 6 months in some circumstances, a spring treatment of a germination stimulant would seem more appropriate than an autumn treatment following an outbreak of white rot. This is in line with overseas recommendations and should be practical as DADS can be applied while a cover crop is in place without any adverse effects. It may be necessary to delay planting until 8 weeks after the last DADS treatment, to ensure that seedlings are not exposed to a burst of germinating sclerotia. Eleven field trials were conducted over three years. The aim of these was to describe the relationship between inoculum density (ID) in the soil at the time of planting and incidence of white rot at harvest. Soil samples were taken from each trial site and the numbers of sclerotia present in them determined. A novel method was developed to separate sclerotia from soil. This technique used the magnetic properties of haematite, an iron ore common in the soils of the Pukekohe region, to divide soil particles from sclerotia. Disease incidence, soil temperature and moisture were then recorded through the growing season. Curve-fitting was performed to determine the pattern of disease progress. In the first growing season (1998 / 99) data from 57% of the quadrats were sucessfully fitted to the gompertz model, one of the sigmoid curves. In the second year (1999 / 2000), low ID (and disease incidence) meant that only 26% of quadrats could be fitted to the gompertz model. In the third year, the pattern of disease was unlike that of the previous two seasons, with lower soil temperatures probably the cause of a later onset of white rot and continuous increase in disease incidence until harvest. In previous years, numbers of new infections had decreased in November - December. In 2000 / 01, the rate of new infections continued to increase into December. Data collected during the third year was fitted to exponential, log and linear models, with all data fitting one of these models. There was a positive correlation in year one between inoculum density and time to maximum rate of disease increase (TMR), with high ID resulting in an earlier disease onset by approximately 4 weeks. A positive, but not statistically significant, trend was also observed between ID and both maximum rate of disease increase (MR) and the point of inflection in the curve (LI). No correlations between ID and these parameters were identified in 1999, due probably to the low levels of inoculum and disease observed. TMR, LI and MR were not calculated for the 2000 data, as these could not be fitted to the gompertz model

Local dissipation effects in two-dimensional quantum Josephson junction arrays with magnetic field

We study the quantum phase transitions in two-dimensional arrays of Josephson-couples junctions with short range Josephson couplings (given by the Josephson energy) and the charging energy. We map the problem onto the solvable quantum generalization of the spherical model that improves over the mean-field theory method. The arrays are placed on the top of a two-dimensional electron gas separated by an insulator. We include effects of the local dissipation in the presence of an external magnetic flux f in square lattice for several rational fluxes f=0,1/2,1/3,1/4 and 1/6. We also have examined the T=0 superconducting-insulator phase boundary as function of a dissipation alpha for two different geometry of the lattice: square and triangular. We have found critical value of the dissipation parameter independent on geometry of the lattice and presence magnetic field.Comment: accepted to PR

Area versus Length Distribution for Closed Random Walks

Using a connection between the $q$-oscillator algebra and the coefficients of the high temperature expansion of the frustrated Gaussian spin model, we derive an exact formula for the number of closed random walks of given length and area, on a hypercubic lattice, in the limit of infinite number of dimensions. The formula is investigated in detail, and asymptotic behaviours are evaluated. The area distribution in the limit of long loops is computed. As a byproduct, we obtain also an infinite set of new, nontrivial identities.Comment: 17 page

Magnetic Field Effect on the Pseudogap Temperature within Precursor Superconductivity

We determine the magnetic field dependence of the pseudogap closing temperature T* within a precursor superconductivity scenario. Detailed calculations with an anisotropic attractive Hubbard model account for a recently determined experimental relation in BSCCO between the pseudogap closing field and the pseudogap temperature at zero field, as well as for the weak initial dependence of T* at low fields. Our results indicate that the available experimental data are fully compatible with a superconducting origin of the pseudogap in cuprate superconductors.Comment: 4 pages, 3 figure

Syntax for free: representing syntax with binding using parametricity

We show that, in a parametric model of polymorphism, the type ∀ α. ((α → α) → α) → (α → α → α) → α is isomorphic to closed de Bruijn terms. That is, the type of closed higher-order abstract syntax terms is isomorphic to a concrete representation. To demonstrate the proof we have constructed a model of parametric polymorphism inside the Coq proof assistant. The proof of the theorem requires parametricity over Kripke relations. We also investigate some variants of this representation