10,147 research outputs found
The Role of Kinetic Energy Flux in the Convective Urca Process
The previous analysis of the convective Urca neutrino loss process in
degenerate, convective, quasi-static, carbon-burning cores by Barkat and
Wheeler omitted specific consideration of the role of the kinetic energy flux.
The arguments of Barkat and Wheeler that steady-state composition gradients
exist are correct, but chemical equilibrium does not result in net cooling.
Barkat and Wheeler included a "work" term that effectively removed energy from
the total energy budget that could only have come from the kinetic energy,
which must remain positive. Consideration of the kinetic energy in the
thermodynamics of the convective Urca process shows that the convective Urca
neutrinos reduce the rate of increase of entropy that would otherwise be
associated with the input of nuclear energy and slow down the convective
current, but, unlike the "thermal" Urca process do not reduce the entropy or
temperature.Comment: 16 pages, AAS LaTex, in press, Astrophysical Journal, September 20,
Vol 52
The Stagger-grid: A Grid of 3D Stellar Atmosphere Models - I. Methods and General Properties
We present the Stagger-grid, a comprehensive grid of time-dependent, 3D
hydrodynamic model atmospheres for late-type stars with realistic treatment of
radiative transfer, covering a wide range in stellar parameters. This grid of
3D models is intended for various applications like stellar spectroscopy,
asteroseismology and the study of stellar convection. In this introductory
paper, we describe the methods used for the computation of the grid and discuss
the general properties of the 3D models as well as their temporal and spatial
averages (). All our models were generated with the Stagger-code, using
realistic input physics for the equation of state (EOS) and for continuous and
line opacities. Our ~220 grid models range in Teff from 4000 to 7000K in steps
of 500K, in log g from 1.5 to 5.0 in steps of 0.5 dex, and [Fe/H] from -4.0 to
+0.5 in steps of 0.5 and 1.0 dex. We find a tight scaling relation between the
vertical velocity and the surface entropy jump, which itself correlates with
the constant entropy value of the adiabatic convection zone. The range in
intensity contrast is enhanced at lower metallicity. The granule size
correlates closely with the pressure scale height sampled at the depth of
maximum velocity. We compare the models with widely applied 1D models, as
well as with theoretical 1D hydrostatic models generated with the same EOS and
opacity tables as the 3D models, in order to isolate the effects of using
self-consistent and hydrodynamic modeling of convection, rather than the
classical mixing length theory approach. For the first time, we are able to
quantify systematically over a broad range of stellar parameters the
uncertainties of 1D models arising from the simplified treatment of physics, in
particular convective energy transport. In agreement with previous findings, we
find that the differences can be significant, especially for metal-poor stars.Comment: Accepted for publication in A&A, 31 pages, 29 figure
Explanatory models of mental disorders and treatment practices among traditional healers in Mpumulanga, South Africa
Objective: Key Words: Explanatory models; Traditional healers; Mental illness; South AfricaIn many traditional belief systems in Africa, including South Africa, mental health problems may be attributed to the influence of ancestors or to bewitchment. Traditional healers are viewed as having the expertise to address these causes. However, there is limited information on their explanatory models and consequent treatment practices. The present study examines traditional healers’ explanatory models (EMs) and treatment practices for psychotic and non-psychotic mental illnesses. Method: 4 focus group discussions (8 healers in each group) and 18 in-depth interviews were conducted. Four vignettes were presented (schizophrenia, depression, panic and somatization) and traditional healers’ views on the nature of the problem, cause, consequence, treatment and patient expectations were elicited. Results: Traditional healers held multiple explanatory models for psychotic and non-psychotic disorders. Psychotic illnesses appear to be the main exemplar of mental illness and were treated with traditional medicine, while nonpsychotic illnesses were not viewed as a mental illness at all. Additionally, traditional healers do not only use herbs and substances solely from “traditional” sources but rather have incorporated into their treatment practices modern ingredients that are potentially toxic. Conclusion: Interventions aimed at increasing the mental health literacy of traditional healers are essential. In addition, investigations of the effectiveness of traditional healer treatment for psychiatric disorders should be conducted
Resolvent Estimates in L^p for the Stokes Operator in Lipschitz Domains
We establish the resolvent estimates for the Stokes operator in
Lipschitz domains in , for . The result, in particular, implies that the Stokes operator in a
three-dimensional Lipschitz domain generates a bounded analytic semigroup in
for (3/2)-\varep < p< 3+\epsilon. This gives an affirmative answer to a
conjecture of M. Taylor.Comment: 28 page. Minor revision was made regarding the definition of the
Stokes operator in Lipschitz domain
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