134 research outputs found
Frequency of Debris Disks around Solar-Type Stars: First Results from a Spitzer/MIPS Survey
We have searched for infrared excesses around a well defined sample of 69 FGK
main-sequence field stars. These stars were selected without regard to their
age, metallicity, or any previous detection of IR excess; they have a median
age of ~4 Gyr. We have detected 70 um excesses around 7 stars at the 3-sigma
confidence level. This extra emission is produced by cool material (< 100 K)
located beyond 10 AU, well outside the ``habitable zones'' of these systems and
consistent with the presence of Kuiper Belt analogs with ~100 times more
emitting surface area than in our own planetary system. Only one star, HD
69830, shows excess emission at 24 um, corresponding to dust with temperatures
> 300 K located inside of 1 AU. While debris disks with Ld/L* > 10^-3 are rare
around old FGK stars, we find that the disk frequency increases from 2+-2% for
Ld/L* > 10^-4 to 12+-5% for Ld/L* > 10^-5. This trend in the disk luminosity
distribution is consistent with the estimated dust in our solar system being
within an order of magnitude, greater or less, than the typical level around
similar nearby stars.Comment: 11 figure
Lithium Depletion in Pre-Mainsequence Solar-Like Stars
We examine the internal structure of solar-like stars in detail between 0.8
and 1.4M Sun and during pre-main sequence phase. Recent opacity computations of
OPAL along with a new hydrodynamical mixing process have been considered. We
also introduce up-to-date nuclear reaction rates and explore the impact of
accretion, mixing-length parameter, non-solar distributions among metals and
realistic rotation history. We compare models predictions of lithium depletion
to the content observations of the Sun and to 4 young clusters of
different metallicities and ages. We show that we can distinguish two phases in
lithium depletion: 1- a rapid nuclear destruction in the T-Tauri phase before
20 Myrs : this is independent of the mass used within our range but largely
dependent on the extension and temperature of the convective zone, 2- a second
phase where the destruction is slow and moderate and which is largely dependent
on the (magneto)hydrodynamic instability located at the base of the convective
zone.
In terms of composition, we show the interest on considering helium and
especially the mixture of heavy elements : carbon, oxygen, silicium and iron.
We outline the importance of O/Fe ratio. We note a reasonable agreement on
lithium depletion for the two best known cases, the Sun and the Hyades cluster
for solar-like stars. Other clusters suggest that processes which may partly
inhibit the predicted premainsequence depletion cannot be excluded, in
particular for stars below ~ 0.9M Sun. Finally we suggest different research
areas such as initial stellar models and more realistic atmospheres which could
contribute to a better understanding of this early phase of evolution and which
should become the object of subsequent research.Comment: Astrophysical Journal, in pres
Transit Timing Observations from Kepler: III. Confirmation of 4 Multiple Planet Systems by a Fourier-Domain Study of Anti-correlated Transit Timing Variations
We present a method to confirm the planetary nature of objects in systems
with multiple transiting exoplanet candidates. This method involves a
Fourier-Domain analysis of the deviations in the transit times from a constant
period that result from dynamical interactions within the system. The
combination of observed anti-correlations in the transit times and mass
constraints from dynamical stability allow us to claim the discovery of four
planetary systems Kepler-25, Kepler-26, Kepler-27, and Kepler-28, containing
eight planets and one additional planet candidate.Comment: Accepted to MNRA
Actin Polymerization Controls the Organization of WASH Domains at the Surface of Endosomes
Sorting of cargoes in endosomes occurs through their selective enrichment into sorting platforms, where transport intermediates are generated. The WASH complex, which directly binds to lipids, activates the Arp2/3 complex and hence actin polymerization onto such sorting platforms. Here, we analyzed the role of actin polymerization in the physiology of endosomal domains containing WASH using quantitative image analysis. Actin depolymerization is known to enlarge endosomes. Using a novel colocalization method that is insensitive to the heterogeneity of size and shape of endosomes, we further show that preventing the generation of branched actin networks induces endosomal accumulation of the WASH complex. Moreover, we found that actin depolymerization induces a dramatic decrease in the recovery of endosomal WASH after photobleaching. This result suggests a built-in turnover, where the actin network, i.e. the product of the WASH complex, contributes to the dynamic exchange of the WASH complex by promoting its detachment from endosomes. Our experiments also provide evidence for a role of actin polymerization in the lateral compartmentalization of endosomes: several WASH domains exist at the surface of enlarged endosomes, however, the WASH domains coalesce upon actin depolymerization or Arp2/3 depletion. Branched actin networks are thus involved in the regulation of the size of WASH domains. The potential role of this regulation in membrane scission are discussed
TGFβ pathway limits dedifferentiation following WNT and MAPK pathway activation to suppress intestinal tumourigenesis
Recent studies have suggested increased plasticity of differentiated cells within the intestine to act both as intestinal stem cells (ISCs) and tumour-initiating cells. However, little is known of the processes that regulate this plasticity. Our previous work has shown that activating mutations of Kras or the NF-κB pathway can drive dedifferentiation of intestinal cells lacking Apc. To investigate this process further, we profiled both cells undergoing dedifferentiation in vitro and tumours generated from these cells in vivo by gene expression analysis. Remarkably, no clear differences were observed in the tumours; however, during dedifferentiation in vitro we found a marked upregulation of TGFβ signalling, a pathway commonly mutated in colorectal cancer (CRC). Genetic inactivation of TGFβ type 1 receptor (Tgfbr1/Alk5) enhanced the ability of KrasG12D/+ mutation to drive dedifferentiation and markedly accelerated tumourigenesis. Mechanistically this is associated with a marked activation of MAPK signalling. Tumourigenesis from differentiated compartments is potently inhibited by MEK inhibition. Taken together, we show that tumours arising in differentiated compartments will be exposed to different suppressive signals, for example, TGFβ and blockade of these makes tumourigenesis more efficient from this compartment
Aging behavior and modeling studies of unsaturated polyester resin and unsaturated polyester resin-based blends
This chapter deals with various cases of degradation of unsaturated polyester resin (UPR)-based materials (composites with polyester matrices and polyester interpenetrated networks). The consequences of degradation mainly on mechanical and engineering properties are presented from a structure-relationships point of view. Two main kinds of mechanisms are presented, namely physical (chemicals and water penetration) and chemical mechanisms (hydrolysis, radiolysis, photolysis and photooxidation, and thermal oxidation) together with experimental trackers, existing kinetic models, and some of their available parameters. It seems in particular that the lifetime prediction of UPR-based materials submitted to chemical aging remains an open issue due to the nonideality of networks
Type I interferon: friend or foe?
Although the role of type I interferon (IFN) in the protection against viral infections has been known and studied for decades, its role in other immunologically relevant scenarios, including bacterial infections, shock, autoimmunity, and cancer, is less well defined and potentially much more complicated
Helioseismology and Solar Abundances
Helioseismology has allowed us to study the structure of the Sun in
unprecedented detail. One of the triumphs of the theory of stellar evolution
was that helioseismic studies had shown that the structure of solar models is
very similar to that of the Sun. However, this agreement has been spoiled by
recent revisions of the solar heavy-element abundances. Heavy element
abundances determine the opacity of the stellar material and hence, are an
important input to stellar model calculations. The models with the new, low
abundances do not satisfy helioseismic constraints. We review here how
heavy-element abundances affect solar models, how these models are tested with
helioseismology, and the impact of the new abundances on standard solar models.
We also discuss the attempts made to improve the agreement of the low-abundance
models with the Sun and discuss how helioseismology is being used to determine
the solar heavy-element abundance. A review of current literature shows that
attempts to improve agreement between solar models with low heavy-element
abundances and seismic inference have been unsuccessful so far. The
low-metallicity models that have the least disagreement with seismic data
require changing all input physics to stellar models beyond their acceptable
ranges. Seismic determinations of the solar heavy-element abundance yield
results that are consistent with the older, higher values of the solar
abundance, and hence, no major changes to the inputs to solar models are
required to make higher-metallicity solar models consistent with helioseismic
data.Comment: To appear in Physics Reports. Large file (1.6M PDF, 3.4M PS), 27
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Etude des effets protecteurs vasculaire et cérébral de l'activation des récepteurs nucléaires PPAR-alpha au cours de l'ischémie cérébrale
LILLE2-BU Santé-Recherche (593502101) / SudocSudocFranceF
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