251 research outputs found
Evolutionary variations of solar luminosity
The Theoretical arguments for a 30% increase in the solar luminosity over the past 4.7 billion years are reviewed. A scaling argument shows that this increase can be predicted without detailed numerical calculations. The magnitude of the increase is independent of nuclear reaction rates, as long as conversion of hydrogen to helium provides the basic energy source of the Sun. The effect of the solar luminosity increase on the terrestrial climate is briefly considered. It appears unlikely that an enhanced greenhouse effect, due to reduced gases (NH3, CH4), can account for the long-term paleoclimatic trends
contraction for bounded (non-integrable) solutions of degenerate parabolic equations
We obtain new contraction results for bounded entropy solutions of
Cauchy problems for degenerate parabolic equations. The equations we consider
have possibly strongly degenerate local or non-local diffusion terms. As
opposed to previous results, our results apply without any integrability
assumption on the %(the positive part of the difference of) solutions. They
take the form of partial Duhamel formulas and can be seen as quantitative
extensions of finite speed of propagation local contraction results for
scalar conservation laws. A key ingredient in the proofs is a new and
non-trivial construction of a subsolution of a fully non-linear (dual)
equation. Consequences of our results are maximum and comparison principles,
new a priori estimates, and in the non-local case, new existence and uniqueness
results
The evolution of rotating stars. 1: Method and exploratory calculations for a 7 solar mass star
A method was developed which allows us to study the evolution of rotating stars beyond the main sequence stage. Four different cases of redistribution of angular momentum in an evolving star are considered. Evolutionary sequences for a 7 solar mass star, rotating according to these different cases, were computed from the ZAMS to the double shell source stage. Each sequence was begun with a (typical) equatorial velocity of 210 km/sec. On the main sequence, the effects of rotation are of minor importance. As the core contracts during later stages, important effects arise in all physically plausible cases. The outer regions of the cores approach critical velocities and develop unstable angular velocity distributions. The effects of these instabilities should significantly alter the subsequent evolution
Influence of magnetic pressure on stellar structure: A Mechanism for solar variability
A physical mechanism is proposed that couples the Sun's dynamo magnetic field to its gravitational potential energy. The mechanism involves the isotropic field pressure resulting in a lifting force on the convective envelope, thereby raising its potential energy. Decay of the field due to solar activity allows the envelop to subside and releases this energy, which can augment the otherwise steady solar luminosity. Equations are developed and applied to the Sun for several field configurations. The best estimate model suggests that uniform luminosity variations as large as 0.02% for half a sunspot cycle may occur. Brief temporal variations or the rotation of spatial structures could allow larger excursions in the energy released
Thermal perturbation of the Sun
Thermal perturbations of the solar convection zone can be modeled (to the first order) by perturbing the mixing length parameter alpha (equal to the ratio of the mixing length to the pressure scale height) used in the standard mixing length theory of convection. Results of such an analysis are presented and discussed in relation to recent work by others
Uniqueness and properties of distributional solutions of nonlocal equations of porous medium type
We study the uniqueness, existence, and properties of bounded distributional
solutions of the initial value problem problem for the anomalous diffusion
equation . Here
can be any nonlocal symmetric degenerate elliptic operator including the
fractional Laplacian and numerical discretizations of this operator. The
function is only assumed to be continuous
and nondecreasing. The class of equations include nonlocal (generalized) porous
medium equations, fast diffusion equations, and Stefan problems. In addition to
very general uniqueness and existence results, we obtain -contraction and
a priori estimates. We also study local limits, continuous dependence, and
properties and convergence of a numerical approximation of our equations.Comment: To appear in "Advances in Mathematics
On distributional solutions of local and nonlocal problems of porous medium type
We present a theory of well-posedness and a priori estimates for bounded
distributional (or very weak) solutions of
where is merely continuous and
nondecreasing and is the generator of a general
symmetric L\'evy process. This means that can have
both local and nonlocal parts like e.g.
. New uniqueness results
for bounded distributional solutions of this problem and the corresponding
elliptic equation are presented and proven. A key role is played by a new
Liouville type result for . Existence and a priori
estimates are deduced from a numerical approximation, and energy type estimates
are also obtained.Comment: 6 pages. Minor revision. Added details to Step 2 of the proof of
Theorem 3.
Collaboration, a Creative Catalyst
An insight into developing the "Warehouse" project through Producer Mediation, Band Influence, and Facilitation for Musical Practice
HÞyde og kroppsbilde- en studie av unge kvinner som spiller hÄndball
English abstract: Purpose: the purpose of this study is to look into how the participants of the study feel about their body image in relation to their height, and if handball can prove tall girls with an enclave or social network. Going through literature on body-image, sport, health and public health it became apparent that many studies have been done based on weight and BMI. Height on the other hand, is a factor which is hardly examined at all in the literature on body-image, especially regarding adolescent girls.
Method: The methodology of this study is based upon grounded theory, and is used through the entire research process. 12 girls participating in a national âtall girlâ project conducted by the Norwegian Handball Association (NHF) were interviewed about their body image, seen in relation to their height and the fact they are all handball players.
Results: The results from the empirical data show a clear distinction between talking about body image and talking about height. The participants were talking more about âgeneral issuesâ regarding their body image such as: what they liked or disliked about their bodies, media impact and if they would change something on their body or not. When they were talking about their height, they talked more specific about benefits and disadvantages being tall in everyday life and on the handball court. The girls distinguish between being tall in everyday life and being tall at the handball court. Playing handball was important to them in order to feel confident and proud of their height, however, this did not translate into everyday life. The findings in this study shows there are several factors having an impact on the participants and how they view themselves. Being participants in NHFâs âtall girlâs projectâ has helped them to take advantage of their height in the handball court and for some it has even raised their level of confidence as tall girls in everyday life. However, even though handball does provide a suitable enclave for the tall girls, handball is only temporarily able to elleviate the pressure on girls to conform to gender stereotypes of the female body as tiny and petite.Norsk sammendrag: Hensikt: Hensikten med denne studien, har vĂŠrt Ă„ se pĂ„ hvordan deltakerne i denne studien ser pĂ„ sitt kroppsbilde i forhold til hĂžyden sin, og om hĂ„ndball kan gi jenter som er hĂžyere enn gjennomsnittlig, et sosialt nettverk Ă„ lene seg pĂ„. Etter Ă„ ha sett pĂ„ litteratur om kroppsbilde, sport, helse og folkehelse, har det kommet klart frem at mange studier har blitt gjort pĂ„ bakgrunn av vekt og BMI- relaterte utfordringer. HĂžyde derimot er en faktor som ikke har vĂŠrt undersĂžkt i sĂŠrlig grad i sammenheng med kroppsbilde, hvert fall ikke sett i sammenheng med unge jenter.
Metode: I denne studien er Grounded Theory brukt som gjennomgÄende metode. 12 jenter som allerede er deltakere i Norges HÄndballforbund sitt «hÞye jenter» prosjekt har blitt intervjuet angÄende deres kroppsbilde, sett i sammenheng med hÞyden deres og at de alle spiller hÄndball.
Resultat: Resultatet fra de empiriske dataene, viser at jentene skiller klart mellom Ä prate om kroppsbilde generelt og hÞyde. Deltakerne pratet mer om «generelle problemer» nÄr de pratet om kroppsbilde som: hva de eller ikke liker med kroppene sine, medias innvirkning og om de ville forandret noe pÄ kroppene sine eller ikke. NÄr de snakket om hÞyden sin, kommenterte de mer om fordeler og ulemper med Ä vÊre hÞg i hverdagen og pÄ hÄndballbanen. Jentene skilte veldig pÄ Ä vÊre hÞy i hverdagen og det Ä vÊre hÞg pÄ hÄndballbanen. Funnene som ble gjort viser at det er mange ulike faktorer som spiller inn pÄ hvordan deltakerne ser pÄ seg selv. Det Ä vÊre deltaker i NHF sitt prosjekt har bidratt til at mange har fÄtt utbytte fordelene av Ä vÊre hÞy pÄ hÄndballbanen, i tillegg til at prosjektet har bidratt med at noen har fÄtt bedre selvtillit som hÞy jente i hverdagen. Selv om hÄndball gir de hÞge jentene et sosialt nettverk de kan lene seg pÄ, kan det bare midlertidig ta vekk presset jentene fÞler. For dem er det en utfordring Ä passe inn i det typiske kjÞnnsstyrte bilde laget av kvinnekroppen som liten og nett
A model for the X-ray nova A0620-00
The model involves a white dwarf accreting mass from a late-type subgiant companion. The transient behavior of the X-ray source is explained by the instability to mass loss of the companion (as in Algol-type binaries). The brightening, spectrum, and decay timescale of the optical counterpart are explained in terms of re-emission of X-radiation intercepted by the subgiant. It was concluded that A0620-00 can provide an excellent test case for numerical models of stellar atmospheres irradiated by an external X-ray flux
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