887 research outputs found
Effect of local treatments of convection upon the solar p-mode excitation rates
We compute, for several solar models, the rates P at which the solar radial p
modes are expected to be excited. The solar models are computed with two
different local treatments of convection : the classical mixing-length theory
(MLT hereafter) and Canuto, Goldmann and Mazzitelli(1996, CGM hereafter)'s
formulation. For one set of solar models (EMLT and ECGM models), the atmosphere
is gray and assumes Eddington's approximation. For a second set of models (KMLT
and KCGM models), the atmosphere is built using a T(tau) law which has been
obtained from a Kurucz's model atmosphere computed with the same local
treatment of convection. The mixing-length parameter in the model atmosphere is
chosen so as to provide a good agreement between synthetic and observed Balmer
line profiles, while the mixing-length parameter in the interior model is
calibrated so that the model reproduces the solar radius at solar age. For the
MLT treatment, the rates P do depend significantly on the properties of the
atmosphere. On the other hand, for the CGM treatment, differences in P between
the ECGM and the KCGM models are very small compared to the error bars attached
to the seismic measurements. The excitation rates P for modes from the EMLT
model are significantly under-estimated compared with the solar seismic
constraints. The KMLT model results in intermediate values for P and shows also
an important discontinuity in the temperature gradient and the convective
velocity. On the other hand, the KCGM model and the ECGM model yield values for
P closer to the seismic data than the EMLT and KMLT models. We conclude that
the solar p-mode excitation rates provide valuable constraints and according to
the present investigation cleary favor the CGM treatment with respect to the
MLT.Comment: 4 pages, 3 figures, proceedings of the SOHO14/GONG 2004 workshop
"Helio- and Asteroseismology: Towards a Golden Future" from July 12-16 2004
at New Haven CT (USA
The Interaction Of Multiple Convection Zones In A-type Stars
A-type stars have a complex internal structure with the possibility of
multiple convection zones. If not sufficiently separated, such zones will
interact through the convectively stable regions that lie between them. It is
therefore of interest to ask whether the typical conditions that exist within
such stars are such that these convections zones can ever be considered as
disjoint.
In this paper we present results from numerical simulations that help in
understanding how increasing the distance between the convectively unstable
regions are likely to interact through the stable region that separates them.
This has profound implications for mixing and transport within these stars.Comment: 9 pages, 15 figures, Preprint accepted for publication in MNRA
Influence of local treatments of convection upon solar p mode excitation rates
We compute the rates P at which acoustic energy is injected into the solar
radial p modes for several solar models. The solar models are computed with two
different local treatments of convection: the classical mixing-length theory
(MLT hereafter) and Canuto et al (1996)'s formulation (CGM hereafter). Among
the models investigated here, our best models reproduce both the solar radius
and the solar luminosity at solar age and the observed Balmer line profiles.
For the MLT treatment, the rates P do depend significantly on the properties of
the atmosphere whereas for the CGM's treatment the dependence of P on the
properties of the atmosphere is found smaller than the error bars attached to
the seismic measurements. The excitation rates P for modes associated with the
MLT models are significantly underestimated compared with the solar seismic
constraints. The CGM models yield values for P closer to the seismic data than
the MLT models. We conclude that the solar p-mode excitation rates provide
valuable constraints and according to the present investigation clearly favor
the CGM treatment with respect to the MLT, although neither of them yields
values of P as close to the observations as recently found for 3D numerical
simulations.Comment: 11 pages, 7 figures, accepted for publication in Astronomy &
Astrophysic
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Predictors of Stunting, Wasting and Underweight among Tanzanian Children Born to HIV-Infected Women.
Children born to human immunodeficiency virus (HIV)-infected women are susceptible to undernutrition, but modifiable risk factors and the time course of the development of undernutrition have not been well characterized. The objective of this study was to identify maternal, socioeconomic and child characteristics that are associated with stunting, wasting and underweight among Tanzanian children born to HIV-infected mothers, followed from 6 weeks of age for 24 months. Maternal and socioeconomic characteristics were recorded during pregnancy, data pertaining to the infant's birth were collected immediately after delivery, morbidity histories and anthropometric measurements were performed monthly. Multivariate Cox proportional hazards methods were used to assess the association between potential predictors and the time to first episode of stunting, wasting and underweight. A total of 2387 infants (54.0% male) were enrolled and followed for a median duration of 21.2 months. The respective prevalence of prematurity (<37 weeks) and low birth weight (<2500 g) was 15.2% and 7.0%; 11.3% of infants were HIV-positive at 6 weeks. Median time to first episode of stunting, wasting and underweight was 8.7, 7.2 and 7.0 months, respectively. Low maternal education, few household possessions, low infant birth weight, child HIV infection and male sex were all independent predictors of stunting, wasting and underweight. In addition, preterm infants were more likely to become wasted and underweight, whereas those with a low Apgar score at birth were more likely to become stunted. Interventions to improve maternal education and nutritional status, reduce mother-to-child transmission of HIV, and increase birth weight may lower the risk of undernutrition among children born to HIV-infected women
Angle dependence of Andreev scattering at semiconductor-superconductor interfaces
We study the angle dependence of the Andreev scattering at a
semiconductor-superconductor interface, generalizing the one-dimensional theory
of Blonder, Tinkham and Klapwijk. An increase of the momentum parallel to the
interface leads to suppression of the probability of Andreev reflection and
increase of the probability of normal reflection. We show that in the presence
of a Fermi velocity mismatch between the semiconductor and the superconductor
the angles of incidence and transmission are related according to the
well-known Snell's law in optics. As a consequence there is a critical angle of
incidence above which only normal reflection exists. For two and
three-dimensional interfaces a lower excess current compared to ballistic
transport with perpendicular incidence is found. Thus, the one-dimensional BTK
model overestimates the barrier strength for two and three-dimensional
interfaces.Comment: 8 pages including 3 figures (revised, 6 references added
Developing an online positive psychology application for people with bipolar disorder:‘How expectations of consumers and professionals turned into an intervention.’
INTRODUCTION: In Bipolar Disorder (BD), people report a lower quality of life and lower levels of well-being than the general population. Additionally, patients with bipolar disorder have unmet needs which are closely linked to elements of positive psychology. OBJECTIVES: The current study aimed to gain insight from patients with BD and care professionals about their thoughts of online Positive Psychology Interventions (PPI) to develop an app containing PPI’s for people with BD. METHODS: The study is conducted in accordance with the CeHRes roadmap principles. Data were collected by focus groups, questionnaires, rapid prototyping and online feedback from the participants. Three focus groups meetings (FGM) were held with consumers (8) and professionals (5). RESULTS: The FGM reveals a need for positive psychology interventions to cover some of the unmet needs that can be applied in an app in addition to the guidelines-advised treatment. Patients and professionals expect that PPIs in the current treatment in BD can meet some of the needs that are currently still unmet, specifically offering hope, increasing self-esteem, expressing feelings, acceptation and preventing social isolation. The process of contextual inquiry and value specification is helpful to guide this process. CONCLUSIONS: The consensus on the different topics about the use of positive psychology intervention shows that both consumers and professionals underline the importance of applying PPI’s in BD. The use during subsyndrome and mild depressive episodes seem the most beneficial periods for patients with BD. A more extended study has to be conducted to confirm if these findings are more generalizable DISCLOSURE: No significant relationships
Turbulent convection: comparing the moment equations to numerical simulations
The non-local hydrodynamic moment equations for compressible convection are
compared to numerical simulations. Convective and radiative flux typically
deviate less than 20% from the 3D simulations, while mean thermodynamic
quantities are accurate to at least 2% for the cases we have investigated. The
moment equations are solved in minutes rather than days on standard
workstations. We conclude that this convection model has the potential to
considerably improve the modelling of convection zones in stellar envelopes and
cores, in particular of A and F stars.Comment: 10 pages (6 pages of text including figure captions + 4 figures),
Latex 2e with AAS Latex 5.0 macros, accepted for publication in ApJ
Electronic States in Diffused Quantum Wells
In the present study we calculate the energy values and the spatial
distributions of the bound electronic states in some diffused quantum wells.
The calculations are performed within the virtual crystal approximation, spin dependent empirical tight-binding model and the surface Green
function matching method. A good agreement is found between our results and
experimental data obtained for AlGaAs/GaAs quantum wells with thermally induced
changes in the profile at the interfaces. Our calculations show that for
diffusion lengths {\AA} the transition (C3-HH3) is not
sensitive to the diffusion length, but the transitions (C1-HH1), (C1-LH1),
(C2-HH2) and (C2-LH2) display large "blue shifts" as L_{D} increases. For
diffusion lengths {\AA} the transitions (C1-HH1) and (C1-LH1)
are less sensitive to the L_{D} changes than the (C3-HH3) transition. The
observed dependence is explained in terms of the bound states spatial
distributions.Comment: ReVTeX file, 7pp., no macros, 4 figures available on the reques
Models of infrared spectra of Sakurai's Object (V4334 Sgr) in 1997
Theoretical spectral energy distributions computed for a grid of
hydrogen-deficient and carbon-rich model atmospheres have been compared with
the observed infrared (1--2.5 m) spectra of V4334 Sgr (Sakurai's Object)
on 1997 April 21 and July 13. The comparison yields an effective temperature of
\Tef = 5500 200 K for the April date and \Tef = 5250 200 K for
July. The observed spectra are well fitted by Asplund et al. (1999) abundances,
except that the carbon abundance is higher by 0.3 dex. Hot dust produces
significant excess continuum at the long wavelength ends of the 1997 spectra.
\keywords{Stars: individual: V4334 Sgr (Sakurai's Object) -- Stars: AGB and
post-AGB evolution -- Stars: model atmospheres -- Stars: energy distributions
-- Stars: effective temperatures}Comment: 6 pages, 7 eps figs, accepted for A
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