385 research outputs found
Accretion from debris disks onto white dwarfs : Fingering (thermohaline) instability and derived accretion rates
Recent observations of a large number of DA and DB white dwarfs show evidence
of debris disks, which are the remnants of old planetary systems. The infrared
excess detected with \emph{Spitzer} and the lines of heavy elements observed in
their atmospheres with high-resolution spectroscopy converge on the idea that
planetary material accretes onto these stars. Accretion rates have been derived
by several authors with the assumption of a steady state between accretion and
gravitational settling. The results are unrealistically different for DA and DB
white dwarfs. When heavy matter is accreted onto stars, it induces an inverse
-gradient that leads to fingering (thermohaline) convection. The aim of
this letter is to study the impact of this specific process on the derived
accretion rates in white dwarfs and on the difference between DA and DB. We
solve the diffusion equation for the accreted heavy elements with a
time-dependent method. The models we use have been obtained both with the IRAP
code, which computes static models, and the La Plata code, which computes
evolutionary sequences. Computations with pure gravitational settling are
compared with computations that include fingering convection. The most
important result is that fingering convection has very important effects on DAs
but is inefficient in DBs. When only gravitational settling is taken into
account, the time-dependent computations lead to a steady state, as postulated
by previous authors. When fingering convection is added, this steady state
occurs much later. The surprising difference found in the past for the
accretion rates derived for DA and DB white dwarfs disappears. The derived
accretion rates for DAs are increased when fingering convection is taken into
account, whereas those for DBs are not modified. More precise and developed
results will be given in a forthcoming paper
The new Toulouse-Geneva Stellar Evolution Code including radiative accelerations of heavy elements
Atomic diffusion has been recognized as an important process that has to be
considered in any computations of stellar models. In solar-type and cooler
stars, this process is dominated by gravitational settling, which is now
included in most stellar evolution codes. In hotter stars, radiative
accelerations compete with gravity and become the dominant ingredient in the
diffusion flux for most heavy elements. Introducing radiative accelerations
into the computations of stellar models modifies the internal element
distribution and may have major consequences on the stellar structure. Coupling
these processes with hydrodynamical stellar motions has important consequences
that need to be investigated in detail. We aim to include the computations of
radiative accelerations in a stellar evolution code (here the TGEC code) using
a simplified method (SVP) so that it may be coupled with sophisticated
macroscopic motions. We also compare the results with those of the Montreal
code in specific cases for validation and study the consequences of these
coupled processes on accurate models of A- and early-type stars. We implemented
radiative accelerations computations into the Toulouse-Geneva stellar evolution
code following the semi-analytical prescription proposed by Alecian and
LeBlanc. This allows more rapid computations than the full description used in
the Montreal code. We present results for A-type stellar models computed with
this updated version of TGEC and compare them with similar published models
obtained with the Montreal evolution code. We discuss the consequences for the
coupling with macroscopic motions, including thermohaline convection.Comment: 12 pages, 13 figures, published in A&
Fingering Convection in Red Giants Revisited
Fingering (thermohaline) convection has been invoked for several years as a
possible extra-mixing which could occur in Red Giant stars due to the
modification of the chemical composition induced by nuclear reactions in the
hydrogen burning zone. Recent studies show however that this mixing is not
sufficient to account for the needed surface abundances. A new prescription for
fingering convection, based on 3D numerical simulations has recently been
proposed (BGS). The resulting mixing coefficient is larger than the ones
previously given in the literature. We compute models using this new
coefficient and compare them to previous studies. We use the LPCODE stellar
evolution code with the GNA generalized version of the mixing length theory to
compute Red Giant models and we introduce fingering convection using the BGS
prescription. The results show that, although the fingering zone now reaches
the outer dynamical convective zone, the efficiency of the mixing is not enough
to account for the observations. The fingering mixing coefficient should be
increased by two orders of magnitude for the needed surface abundances to be
reached. We confirm that fingering convection cannot be the mixing process
needed to account for surface abundances in RGB stars.Comment: Accepted for publication in Astronomy and Astrophysic
Lithium in field Am and normal A-F-type stars
Preliminary abundances of lithium and a few other elements have been obtained
for 31 field Am stars with good Hipparcos parallaxes, as well as for 36 normal
A and F stars. Radial and projected rotational velocities were determined as
well. We examine the Li abundance as a function of the stellar parameters: for
normal stars, it is clearly bimodal for Teff < 7500 K, while Am-Fm stars are
all somewhat Li-deficient in this range. The most Li-deficient stars - either
Am or normal - tend to be at least slightly evolved, but the reverse is not
true.Comment: 4 pages, 2 figures, poster presented at the conference "Element
stratification in stars, 40 years of atomic diffusion", eds. G. Alecian, O.
Richard and S. Vauclair, EAS Publication Series, in pres
Models of Metal Poor Stars with Gravitational Settling and Radiative Accelerations: I. Evolution and Abundance Anomalies
Evolutionary models have been calculated for Pop II stars of 0.5 to
1.0 from the pre-main-sequence to the lower part of the giant branch.
Rosseland opacities and radiative accelerations were calculated taking into
account the concentration variations of 28 chemical species, including all
species contributing to Rosseland opacities in the OPAL tables. The effects of
radiative accelerations, thermal diffusion and gravitational settling are
included. While models were calculated both for Z=0.00017 and 0.0017, we
concentrate on models with Z=0.00017 in this paper. These are the first Pop II
models calculated taking radiative acceleration into account. It is shown that,
at least in a 0.8 star, it is a better approximation not to let Fe
diffuse than to calculate its gravitational settling without including the
effects of . In the absence of any turbulence outside of
convection zones, the effects of atomic diffusion are large mainly for stars
more massive than 0.7. Overabundances are expected in some stars with
\teff \ge 6000K. Most chemical species heavier than CNO are affected. At 12
Gyr, overabundance factors may reach 10 in some cases (e.g. for Al or Ni) while
others are limited to 3 (e.g. for Fe). The calculated surface abundances are
compared to recent observations of abundances in globular clusters as well as
to observations of Li in halo stars. It is shown that, as in the case of Pop I
stars, additional turbulence appears to be present.Comment: 40 pages, 17 color figures, to appear in The Astrophysical Journal,
April 2002 (paper with original high resolution figures can be found at
http://www.cerca.umontreal.ca/~richer/Fichiersps/popII_1.ps
Cultural adaptation and psychometric validation of the Portuguese breakthrough pain assessment tool with cancer patients
Breakthrough cancer pain (BTcP) is a transient exacerbation of pain that occurs over persistent, stable, and adequately controlled cancer background pain. It is prevalent and bears severe consequences to patients' quality-of-life. The effective management of BTcP depends on fast and reliable (re)assessment. The Breakthrough pain Assessment Tool (BAT) is one of the most concise and reliable self-report instruments adapted to clinical contexts so far, showing good psychometric qualities in the United Kingdom, the Netherlands, and South Korea. As to promote the effective management of BTcP in Portuguese-speaking communities this study, first aimed to culturally adapt and validate the Portuguese version of the BAT (BAT-Pt). Second, and most importantly, it sought to provide novel evidence on its criterion validity by investigating its association with measures of psychological distress, which has not been yet investigated. The BAT was translated into European Portuguese, using the back-translation method, and culturally adapted. Its psychometric properties (factor structure, internal consistency, construct and criterion validity) were analyzed in a cross-sectional multicenter study, with a sample of 65 cancer patients (49.2% women) recruited from eight hospitals in mainland Portugal (a priori power analysis determined a minimum sample of 50). Health professionals collected patients' clinical information, assessed their functional disability (ECOG Performance Status) and the adequacy of pain control. In addition to the Portuguese version of the BAT (BAT_Pt), patients completed the Portuguese versions of the Brief Pain Inventory, the Hospital Anxiety and Depression Scale, a Distress Thermometer and answered questions about the adequacy of pain control. The BAT-Pt was very well accepted by experts and patients. As hypothesized, a Principal Axis Factor Analysis revealed two underlying factors accounting for 55.2% of the variance: (1) Pain Severity and Impact of BTcP and (2) Duration of BTcP and Medication Inefficacy. Two items (on episode frequency and medication efficacy) were analyzed separately given their lower/cross loadings. The BAT-Pt showed good internal consistency overall (?=0.79) and for each sub-scale, namely, Pain Severity and Impact of BTcP (n=5 items; ?=0.86) and Duration of BTcP and Medication Inefficacy (n=2 items; rsb=0.62).info:eu-repo/semantics/acceptedVersio
Asteroseismology of the planet-hosting star mu Arae. II. Seismic analysis
As most exoplanets host stars, HD 160691 (alias mu Ara) presents a
metallicity excess in its spectrum compared to stars without detected planets.
This excess may be primordial, in which case the star would be completely
overmetallic, or it may be due to accretion in the early phases of planetary
formation, in which case it would be overmetallic only in its outer layers. As
discussed in a previous paper (Bazot and Vauclair 2004), seismology can help
choosing between the two scenarios. This star was observed during eight nights
with the spectrograph HARPS at La Silla Observatory. Forty three p-modes have
been identified (Bouchy et al. 2005). In the present paper, we discuss the
modelisation of this star. We computed stellar models iterated to present the
same observable parameters (luminosity, effective temperature, outer chemical
composition) while the internal structure was different according to the two
extreme assumptions : original overmetallicity or accretion. We show that in
any case the seismic constraints lead to models in complete agreement with the
external parameters deduced from spectroscopy and from the Hipparcos parallax
(L and Teff). We discuss the tests which may lead to a choice between the two
typical scenarios. We show that the ``small separation'' seem to give a better
fit for the accretion case than for the overmetallic case, but in spite of the
very good data the uncertainties are still too large to conclude. We discuss
the observations which would be needed to go further and solve this question.Comment: 16 pages, 8 figures, accepted in A&
Physical processes leading to surface inhomogeneities: the case of rotation
In this lecture I discuss the bulk surface heterogeneity of rotating stars,
namely gravity darkening. I especially detail the derivation of the omega-model
of Espinosa Lara & Rieutord (2011), which gives the gravity darkening in
early-type stars. I also discuss the problem of deriving gravity darkening in
stars owning a convective envelope and in those that are members of a binary
system.Comment: 23 pages, 11 figure, Lecture given to the school on the cartography
of the Sun and the stars (May 2014 in Besan\c{c}on), to appear in LNP, Neiner
and Rozelot edts V2: typos correcte
Element Segregation in Low Metallicity Stars and the Primordial Lithium Abundance
Observational constraints on the primordial lithium abundance are important
for the evaluation of the baryonic density of the universe. Its precise
determination however suffers from uncertainties due to the possible depletion
of this element inside the stars. Here we present and discuss new results on
the lithium abundances in pop II stars obtained with most recent stellar models
including the best available physics. We show that it is possible to account
for the general behavior of lithium observed in pop II stars without any free
parameters. Macroscopic motions inside the stars are needed but it is not
necessary to specify their exact nature to interpret the observational data.
This constatation allows to derive a parameter-free value for the lithium
primordial abundance : log(Lip) = 2.35 +/- .10 in the log(H) = 12 scale.Comment: 5 pages, 3 figures, to be published in Ap.
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