1,423 research outputs found
Seismic signature of envelope penetrative convection: the CoRoT star HD 52265
Aims: We aim at characterizing the inward transition from convective to
radiative energy transport at the base of the convective envelope of the
solar-like oscillator HD 52265 recently observed by the CoRoT satellite.
Methods: We investigated the origin of one specific feature found in the HD
52265 frequency spectrum. We modelled the star to derive the internal structure
and the oscillation frequencies that best match the observations and used a
seismic indicator sensitive to the properties of the base of the envelope
convection zone. Results: The seismic indicators clearly reveal that to best
represent the observed properties of HD 52265, models must include penetrative
convection below the outer convective envelope. The penetrative distance is
estimated to be , which corresponds to an extent over a distance
representing 6.0 per cents of the total stellar radius, significantly larger
than what is found for the Sun. The inner boundary of the extra-mixing region
is found at where is the stellar radius.
Conclusions: These results contribute to the tachocline characterization in
stars other than the Sun.Comment: 4 pages, 4 figures, accepted for publication in Astronomy &
Astrophysics Letter
Simulations of core convection in rotating A-type stars: Differential rotation and overshooting
We present the results of 3--D simulations of core convection within A-type
stars of 2 solar masses, at a range of rotation rates. We consider the inner
30% by radius of such stars, thereby encompassing the convective core and some
of the surrounding radiative envelope. We utilize our anelastic spherical
harmonic (ASH) code, which solves the compressible Navier-Stokes equations in
the anelastic approximation, to examine highly nonlinear flows that can span
multiple scale heights. The cores of these stars are found to rotate
differentially, with central cylindrical regions of strikingly slow rotation
achieved in our simulations of stars whose convective Rossby number (R_{oc}) is
less than unity. Such differential rotation results from the redistribution of
angular momentum by the nonlinear convection that strongly senses the overall
rotation of the star. Penetrative convective motions extend into the overlying
radiative zone, yielding a prolate shape (aligned with the rotation axis) to
the central region in which nearly adiabatic stratification is achieved. This
is further surrounded by a region of overshooting motions, the extent of which
is greater at the equator than at the poles, yielding an overall spherical
shape to the domain experiencing at least some convective mixing. We assess the
overshooting achieved as the stability of the radiative exterior is varied, and
the weak circulations that result in that exterior. The convective plumes serve
to excite gravity waves in the radiative envelope, ranging from localized
ripples of many scales to some remarkable global resonances.Comment: 48 pages, 16 figures, some color. Accepted to Astrophys. J. Color
figures compressed with appreciable loss of quality; a PDF of the paper with
better figures is available at
http://lcd-www.colorado.edu/~brownim/core_convectsep24.pd
Numerical constraints on the model of stochastic excitation of solar-type oscillations
Analyses of a 3D simulation of the upper layers of a solar convective
envelope provide constraints on the physical quantities which enter the
theoretical formulation of a stochastic excitation model of solar p modes, for
instance the convective velocities and the turbulent kinetic energy spectrum.
These constraints are then used to compute the acoustic excitation rate for
solar p modes, P. The resulting values are found ~5 times larger than the
values resulting from a computation in which convective velocities and entropy
fluctuations are obtained with a 1D solar envelope model built with the
time-dependent, nonlocal Gough (1977) extension of the mixing length
formulation for convection (GMLT). This difference is mainly due to the assumed
mean anisotropy properties of the velocity field in the excitation region. The
3D simulation suggests much larger horizontal velocities compared to vertical
ones than in the 1D GMLT solar model. The values of P obtained with the 3D
simulation constraints however are still too small compared with the values
inferred from solar observations. Improvements in the description of the
turbulent kinetic energy spectrum and its depth dependence yield further
increased theoretical values of P which bring them closer to the observations.
It is also found that the source of excitation arising from the advection of
the turbulent fluctuations of entropy by the turbulent movements contributes ~
65-75 % to the excitation and therefore remains dominant over the Reynolds
stress contribution. The derived theoretical values of P obtained with the 3D
simulation constraints remain smaller by a factor ~3 compared with the solar
observations. This shows that the stochastic excitation model still needs to be
improved.Comment: 11 pages, 9 figures, accepted for publication in A&
Comparison of the prognostic value of measures of the tumor inflammatory cell infiltrate and tumor-associated stroma in patients with primary operable colorectal cancer
The aim of the present study was to compare the clinical utility of two measures of the
inflammatory cell infiltrate - a H&E-based assessment of the generalised inflammatory cell
infiltrate (the Klintrup-Mäkinen (KM) grade), and an immunohistochemistry-based
assessment of combined CD3+ and CD8+ T-cell density (the “Immunoscore”), in conjunction
with assessment of the tumor stroma percentage (TSP) in patients undergoing resection of
stage I-III colorectal cancer (CRC). 246 patients were identified from a prospectively
maintained database of CRC resections in a single surgical unit. Assessment of KM grade
and TSP was performed using full H&E sections. CD3+ and CD8+ T-cell density was
assessed on full sections and the Immunoscore calculated. KM grade and Immunoscore were
strongly associated (P<0.001). KM grade stratified cancer-specific survival (CSS) from 88%
to 66% (P=0.002) and Immunoscore from 93% to 61% (P<0.001). Immunoscore further
stratified survival of patients independent of KM grade from 94% (high KM, Im4) to 60%
(low KM, Im0/1). Furthermore, TSP stratified survival of patients with a weak inflammatory
cell infiltrate (low KM: from 75% to 47%; Im0/1: from 71% to 38%, both P<0.001) but not
those with a strong inflammatory infiltrate. On multivariate analysis, only Immunoscore (HR
0.44, P<0.001) and TSP (HR 2.04, P<0.001) were independently associated with CSS. These
results suggest that the prognostic value of an immunohistochemistry-based assessment of the
inflammatory cell infiltrate is superior to H&E-based assessment in patients undergoing
resection of stage I-III CRC. Furthermore, assessment of the tumor-associated stroma, using
TSP, further improves prediction of outcome
Articulation therapy for children with cleft palate using visual articulatory models and ultrasound biofeedback
Visual biofeedback tools, such as Electropalatography (EPG), are recommended for assessing and treating speech sound disorders (SSDs) associated with Cleft Palate (CP). However, EPG is not suitable for all clients, due to dependencies on stable dentition and timing of palatal repair. Ultrasound is becoming increasingly popular for its use in treating SSDs, with no reports on its dependency on structure of the vocal tract. However its clinical application in the CP population remains to be tested. We compared Visual Articulatory Models (VAMs) with Ultrasound for the treatment of SSDs in two children with repaired submucous CP. Both children received two blocks of therapy each with eight sessions, with the first block using VAMs and the second using ultrasound. Results showed that both children improved overall, with more improvement found in the first block of therapy using VAMs
Open issues in probing interiors of solar-like oscillating main sequence stars: 2. Diversity in the HR diagram
We review some major open issues in the current modelling of low and
intermediate mass, main sequence stars based on seismological studies. The
solar case was discussed in a companion paper, here several issues specific to
other stars than the Sun are illustrated with a few stars observed with CoRoT
and expectations from Kepler data.Comment: GONG 2010 - SoHO 24, A new era of seismology of the Sun and
solar-like stars, To be published in the Journal of Physics: Conference
Series (JPCS
No evidence of a significant role for CTLA-4 in multiple sclerosis
Variation in the cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) gene plays a significant role in determining susceptibility to autoimmune thyroid disease and type 1 diabetes. Its role in multiple sclerosis is more controversial. In order to explore this logical candidate more thoroughly, we genotyped 771 multiple sclerosis trio families from the United Kingdom for the 3? untranslated region variable number tandem repeat, the CT60 single nucleotide polymorphism (SNP) and five haplotype-tagging SNPs. No individual marker or common haplotype showed evidence of association with disease. These data suggest that any effect of CTLA-4 on multiple sclerosis susceptibility is likely to be very small
Constraining mixing processes in stellar cores using asteroseismology. Impact of semiconvection in low-mass stars
The overall evolution of low-mass stars is heavily influenced by the
processes occurring in the stellar interior. In particular, mixing processes in
convectively unstable zones and overshooting regions affect the resulting
observables and main sequence lifetime. We study the effects of different
convective boundary definitions and mixing prescriptions in convective cores of
low-mass stars, to discriminate the existence, size, and evolutionary stage of
the central mixed zone by means of asteroseismology. We implemented the Ledoux
criterion for convection in our stellar evolution code, together with a
time-dependent diffusive approach for mixing of elements when semiconvective
zones are present. We compared models with masses ranging from 1 M* to 2 M*
computed with two different criteria for convective boundary definition and
including different mixing prescriptions within and beyond the formal limits of
the convective regions. Using calculations of adiabatic oscillations
frequencies for a large set of models, we developed an asteroseismic diagnosis
using only l=0 and l=1 modes based on the ratios of small to large separations
r01 and r10 defined by Roxburgh & Vorontsov (2003). These variables are almost
linear in the expected observable frequency range, and we show that their slope
depends simultaneously on the central hydrogen content, the extent of the
convective core, and the amplitude of the sound-speed discontinuity at the core
boundary. By considering about 25 modes and an accuracy in the frequency
determinations as expected from the CoRoT and Kepler missions, the technique we
propose allows us to detect the presence of a convective core and to
discriminate the different sizes of the homogeneously mixed central region
without the need of a strong a priori for the stellar mass.Comment: 13 pages, 9 figures, accepted for publication in A&
Abnormal long wave dispersion phenomena in a slightly compressible elastic plate with non-classical boundary conditions
A two parameter asymptotic analysis is employed to investigate some unusual long wave dispersion phenomena in respect of symmetric motion in a nearly incompressible elastic plate. The plate is not subject to the usual classical traction free boundary conditions, but rather has its faces fixed, precluding any displacement on the boundary. The abnormal long wave behaviour results in the derivation of non-local approximations for symmetric motion, giving frequency as a function of wave number. Motivated by these approximations, the asymptotic forms of displacement components established and long wave asymptotic integration is carried out
- …