82 research outputs found
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
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
Robust numerical methods for nonlocal (and local) equations of porous medium type. Part I: Theory
Abstract. We develop a unified and easy to use framework to study robust fully discrete numerical methods for nonlinear degenerate diffusion equations
âtu â LÏ,ÎŒ[Ï(u)] = f(x,t) in RN Ă (0,T),
where LÏ,ÎŒ is a general symmetric diffusion operator of L Ìevy type and Ï is
merely continuous and non-decreasing. We then use this theory to prove con-
vergence for many different numerical schemes. In the nonlocal case most of
the results are completely new. Our theory covers strongly degenerate Stefan
problems, the full range of porous medium equations, and for the first time
for nonlocal problems, also fast diffusion equations. Examples of diffusion op-
Ï,ÎŒ α
are the (fractional) Laplacians â and â(ââ)2 for α â (0,2),
erators L
discrete operators, and combinations. The observation that monotone finite difference operators are nonlocal L Ìevy operators, allows us to give a unified and compact nonlocal theory for both local and nonlocal, linear and nonlinear diffusion equations. The theory includes stability, compactness, and conver- gence of the methods under minimal assumptions â including assumptions that lead to very irregular solutions. As a byproduct, we prove the new and general existence result announced in [28]. We also present some numerical tests, but extensive testing is deferred to the companion paper [31] along with a more detailed discussion of the numerical methods included in our theory
On the evolutionary status of Be stars. I. Field Be stars near the Sun
A sample of 97 galactic field Be stars were studied by taking into account
the effects induced by the fast rotation on their fundamental parameters. All
program stars were observed in the BCD spectrophotometric system in order to
minimize the perturbations produced by the circumstellar environment on the
spectral photospheric signatures. This is one of the first attempts at
determining stellar masses and ages by simultaneously using model atmospheres
and evolutionary tracks, both calculated for rotating objects. The stellar ages
() normalized to the respective inferred time that each rotating star can
spend in the main sequence phase () reveal a mass-dependent
trend. This trend shows that: a) there are Be stars spread over the whole
interval 0 \la \tau/\tau\_{\rm MS} \la 1 of the main sequence evolutionary
phase; b) the distribution of points in the () diagram indicates that in massive stars (M \ga
12M\_{\odot}) the Be phenomenon is present at smaller
age ratios than for less massive stars (M \la 12M\_{\odot}). This
distribution can be due to: ) higher mass-loss rates in massive objets,
which can act to reduce the surface fast rotation; ) circulation time
scales to transport angular momentum from the core to the surface, which are
longer the lower the stellar mass.Comment: 18 pages, 6 figures, A&A, in pres
Observational indicators of the transition from fully convective stars to stars with radiative cores
We present a discussion of the similarities and key differences between the
transition onto (at the turn-on) and away from (at the turn-off) the main
sequence, the latter termed the Hertzsprung gap. Using a set of model
isochrones and adopting an initial mass function leads us to predict a dearth
of G-type stars for any star forming region. This is caused by the (relatively)
constant spectral type at which the transition from a fully convective star to
a star with a radiative core begins. We also present analysis of the details of
this transition in the ONC. In particular we show that a gap in the photometric
and spectral type distributions is centred on, and a change in the fractional
X-ray luminosity and rotation rate distribution occurs approximately at, the
position of a peak in radiative core size as a function of mass. Whilst
photometric signatures of this transition are lost at ages over ~20 Myrs, we
show that changes in fractional X-ray luminosity and magnetic field
configuration persist to older ages. Analysis of literature data show that the
mass at which the change in fractional X-ray luminosity is observed decreases
with age.Comment: 9 pages and 6 figures. Accepted for publication in MNRA
Effects of gravitational darkening on the determination of fundamental parameters in fast rotating B-type stars
In this paper we develop a calculation code to account for the effects
carried by fast rotation on the observed spectra of early-type stars. Stars are
assumed to be in rigid rotation and the grid of plane-parallel model
atmospheres used to represent the gravitational darkening are calculated by
means of a non-LTE approach. Attention is paid on the relation between the
apparent and parent non-rotating counterpart stellar fundamental parameters and
apparent and true vsini parameters as a function of the rotation rate
Omega/Omega_c, stellar mass and inclination angle. It is shown that omission of
gravitational darkening in the analysis of chemical abundances of CNO elements
can produce systematic overestimation or underestimation, depending on the
lines used, rotational rate and inclination angle. The proximity of Be stars to
the critical rotation is re-discussed by correcting not only the vsini of 130
Be stars, but also their effective temperature and gravity to account for
stellar rotationally induced geometrical distortion and for the concomitant
gravitational darkening effect. We concluded that the increase of the vsini
estimate is accompanied by an even higher value of the stellar equatorial
critical velocity, so that the most probable average rate of angular velocity
of Be stars attains Omega/Omega_c ~ 0.88.Comment: 20 pages, 16 figures. Submitted for publication in A&
Two classes of nonlocal Evolution Equations related by a shared Traveling Wave Problem
We consider reaction-diffusion equations and Korteweg-de Vries-Burgers (KdVB)
equations, i.e. scalar conservation laws with diffusive-dispersive
regularization. We review the existence of traveling wave solutions for these
two classes of evolution equations. For classical equations the traveling wave
problem (TWP) for a local KdVB equation can be identified with the TWP for a
reaction-diffusion equation. In this article we study this relationship for
these two classes of evolution equations with nonlocal diffusion/dispersion.
This connection is especially useful, if the TW equation is not studied
directly, but the existence of a TWS is proven using one of the evolution
equations instead. Finally, we present three models from fluid dynamics and
discuss the TWP via its link to associated reaction-diffusion equations
Rotational velocities of A-type stars. III. Velocity distributions
Aim - In this work, a sample of vsini of B9 to F2-type main sequence single
stars has been built from highly homogeneous vsini parameters determined for a
large sample cleansed from objects presenting the Am and Ap phenomenon as well
as from all known binaries. The aim is to study the distributions of rotational
velocities in the mass range of A-type stars for normal single objects.
Methods - Robust statistical methods are used to rectify the vsini
distributions from the projection effect and the error distribution. The
equatorial velocity distributions are obtained for an amount of about 1100
stars divided in six groups defined by the spectral type, under the assumption
of randomly orientated rotational axes.
Results - We show that late B and early A-type main-sequence stars have
genuine bimodal distributions of true equatorial rotational velocities due
probably to phenomena of angular momentum loss and redistribution the star
underwent before reaching the main sequence. A striking lack of slow rotators
is noticed among intermediate and late A-type stars. The bimodal-like shape of
their true equatorial rotational velocity distributions could be due to
evolutionary effects.Comment: 16 pages, 10 figures, accepted in A&
Diagnoses to unravel secular hydrodynamical processes in rotating main sequence stars
(Abridged) We present a detailed analysis of the main physical processes
responsible for the transport of angular momentum and chemical species in the
radiative regions of rotating stars. We focus on cases where meridional
circulation and shear-induced turbulence only are included in the simulations.
Our analysis is based on a 2-D representation of the secular hydrodynamics,
which is treated using expansions in spherical harmonics. We present a full
reconstruction of the meridional circulation and of the associated fluctuations
of temperature and mean molecular weight along with diagnosis for the transport
of angular momentum, heat and chemicals. In the present paper these tools are
used to validate the analysis of two main sequence stellar models of 1.5 and 20
Msun for which the hydrodynamics has been previously extensively studied in the
literature. We obtain a clear visualization and a precise estimation of the
different terms entering the angular momentum and heat transport equations in
radiative zones. This enables us to corroborate the main results obtained over
the past decade by Zahn, Maeder, and collaborators concerning the secular
hydrodynamics of such objects. We focus on the meridional circulation driven by
angular momentum losses and structural readjustements. We confirm
quantitatively for the first time through detailed computations and separation
of the various components that the advection of entropy by this circulation is
very well balanced by the barotropic effects and the thermal relaxation during
most of the main sequence evolution. This enables us to derive simplifications
for the thermal relaxation on this phase. The meridional currents in turn
advect heat and generate temperature fluctuations that induce differential
rotation through thermal wind thus closing the transport loop.Comment: 16 pages, 18 figures. Accepted for publication in A&
Physical exercise augmented cognitive behaviour therapy for older adults with generalised anxiety disorder (PEXACOG): a feasibility study for a randomized controlled trial
Background
Generalised anxiety disorder (GAD) is a frequent and severe disorder among older adults. For older adults with GAD the effect of the recommended treatment, cognitive behaviour therapy (CBT), is reduced. Physical exercise (PE) may enhance the effect of CBT by improving cognitive function and increasing levels of brain-derived neurotrophic factor (BDNF), a predictor of the effect of CBT in patients with anxiety. The aim of the study was to assess the feasibility of a randomized controlled trial (RCT) investigating treatment effect of the combination of CBT and PE for GAD in a sample of older adults, including procedures for assessment and treatment.
Methods
Four participants aged 62â70 years (Mâ=â65.5, SDâ=â3.2) with a primary diagnosis of GAD were included. Participants received 15 weeks of PE in combination with 10 weeks of CBT. Participants completed self-report measures, and clinical, biological, physiological and neuropsychological tests at pre-, interim- and post-treatment.
Results
Procedures, protocols, and results are presented. One participant dropped out during treatment. For the three participants completing, the total adherence to PE and CBT was 80% and 100%, respectively. An independent assessor concluded that the completers no longer fulfilled the criteria for GAD after treatment. Changes in self-report measures suggest symptom reduction related to anxiety and worry. The sample is considered representative for the target population.
Conclusions
The results indicate that combining CBT and PE for older adults with GAD is feasible, and that the procedures and tests are suitable and manageable for the current sample.publishedVersio
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