238 research outputs found
The Lithium Depletion Boundary and the Age of the Young Open Cluster IC~2391
We have obtained new photometry and intermediate resolution ( \AA\ ) spectra of 19 of these objects
(14.9 17.5) in order to confirm cluster membership. We
identify 15 of our targets as likely cluster members based on their
photometry, spectral types, radial velocity, and H emission strengths.
Higher S/N spectra were obtained for 8 of these probable cluster members in
order to measure the strength of the lithium 6708 \AA\ doublet and thus obtain
an estimate of the cluster's age. One of these 8 stars has a definite lithium
detection and two other (fainter) stars have possible lithium detections. A
color-magnitude diagram for our program objects shows that the lithium
depletion boundary in IC~2391 is at =16.2. Using recent theoretical model
predictions, we derive an age for IC~2391 of 535 Myr. While this is
considerably older than the age most commonly attributed for this cluster
(35 Myr) this result for IC~2391 is comparable those recently derived for
the Pleiades and Alpha Persei clusters and can be explained by new models for
high mass stars that incorporate a modest amount of convective core
overshooting.Comment: ApJ Letters, acccepte
The Metallicity of the Pleiades
We have measured the abundances of Fe, Si, Ni, Ti, and Na in 20 Pleiads with
\teff values near solar and with low \vsini using high-resolution, high
signal-to-noise echelle spectra. We have validated our procedures by also
analyzing 10 field stars of a range of temperatures and metallicities that were
observed by \citet{Vale05}. Our result for the Pleiades is [Fe/H] =
(statistical and systematic). The average of published
measurements for the Pleiades is .Comment: accepted by Astron. J. for 2009-1
Risk for depression and neural responses to fearful facial expressions of emotion
BACKGROUND: Depression is associated with neural abnormalities in emotional processing. AIMS: This study explored whether these abnormalities underlie risk for depression. METHOD: We compared the neural responses of volunteers who were at high and low-risk for the development of depression (by virtue of high and low neuroticism scores; high-N group and low-N group respectively) during the presentation of fearful and happy faces using functional magnetic resonance imaging (fMRI). RESULTS: The high-N group demonstrated linear increases in response in the right fusiform gyrus and left middle temporal gyrus to expressions of increasing fear, whereas the low-N group demonstrated the opposite effect. The high-N group also displayed greater responses in the right amygdala, cerebellum, left middle frontal and bilateral parietal gyri to medium levels of fearful v. happy expressions. CONCLUSIONS: Risk for depression is associated with enhanced neural responses to fearful facial expressions similar to those observed in acute depression
Modeling the interactions of Alzheimer-related genes from the whole brain microarray data and diffusion tensor images of human brain
Genetic clustering on the hippocampal surface for genome-wide association studies
Imaging genetics aims to discover how variants in the human genome influence brain measures derived from images. Genome-wide association scans (GWAS) can screen the genome for common differences in our DNA that relate to brain measures. In small samples, GWAS has low power as individual gene effects are weak and one must also correct for multiple comparisons across the genome and the image. Here we extend recent work on genetic clustering of images, to analyze surface-based models of anatomy using GWAS. We performed spherical harmonic analysis of hippocampal surfaces, automatically extracted from brain MRI scans of 1254 subjects. We clustered hippocampal surface regions with common genetic influences by examining genetic correlations (rg) between the normalized deformation values at all pairs of surface points. Using genetic correlations to cluster surface measures, we were able to boost effect sizes for genetic associations, compared to clustering with traditional phenotypic correlations using Pearson's r
Time scales of Li evolution: a homogeneous analysis of open clusters from ZAMS to late-MS
We have performed a new and homogeneous analysis of all the Li data available
in the literature for main sequence stars (spectral-types from late F to K) in
open clusters. In the present paper we focus on a detailed investigation of MS
Li depletion and its time scales for stars in the 6350-5500 K effective
temperature range. For the first time, we were able to constrain the age at
which non-standard mixing processes, driving MS Li depletion, appear. We have
also shown that MS Li depletion is not a continuous process and cannot be
simply described by a t^(-alpha) law. We confirm that depletion becomes
ineffective beyond an age of 1-2 Gyr for the majority of the stars, leading to
a Li plateau at old ages. We compared the empirical scenario of Li as a
function of age with the predictions of three non-standard models. We found
that models including only gravity waves as main mixing process are not able to
fit the Li vs. age pattern and thus this kind of mixing can be excluded as the
predominant mechanism responsible for Li depletion. On the other hand, models
including slow mixing induced by rotation and angular momentum loss, and in
particular those including also diffusive processes not related to rotation,
can explain to some extent the empirical evidence. However, none of the
currently proposed models can fit the plateau at old ages.Comment: 20 pages, 10 figures A&A accepte
Keck Spectra of Brown Dwarf Candidates and a Precise Determination of the Lithium Depletion Boundary in the Alpha Persei Open Cluster
We have identified twenty-seven candidate very low mass members of the
relatively young Alpha Persei open cluster from a six square degree CCD imaging
survey. Based on their I magnitudes and the nominal age and distance to the
cluster, these objects should have masses less than 0.1 Msunif they are cluster
members. We have subsequently obtained intermediate resolution spectra of
seventeen of these objects using the Keck II telescope and LRIS spectrograph.
We have also obtained near-IR photometry for many of the stars. Our primary
goal was to determine the location of the "lithium depletion boundary" and
hence to derive a precise age for the cluster. We detect lithium with
equivalent widths greater than or equal to 0.4 \AA in five of the program
objects. We have constructed a color-magnitude diagram for the faint end of the
Alpha Persei main sequence. These data allow us to accurately determine the
Alpha Persei single-star lithium depletion boundary at M(I) = 11.47, M(Bol)
= 11.42, (R-I) = 2.12, spectral type M6.0. By reference to theoretical
evolutionary models, this converts fairly directly into an age for the Alpha
Persei cluster of 90 10 Myr. At this age, the two faintest of our
spectroscopically confirmed members should be sub-stellar (i.e., brown dwarfs)
according to theoretical models.Comment: Accepted Ap
Partial Volume Segmentation of Brain MRI Scans of any Resolution and Contrast
Partial voluming (PV) is arguably the last crucial unsolved problem in
Bayesian segmentation of brain MRI with probabilistic atlases. PV occurs when
voxels contain multiple tissue classes, giving rise to image intensities that
may not be representative of any one of the underlying classes. PV is
particularly problematic for segmentation when there is a large resolution gap
between the atlas and the test scan, e.g., when segmenting clinical scans with
thick slices, or when using a high-resolution atlas. In this work, we present
PV-SynthSeg, a convolutional neural network (CNN) that tackles this problem by
directly learning a mapping between (possibly multi-modal) low resolution (LR)
scans and underlying high resolution (HR) segmentations. PV-SynthSeg simulates
LR images from HR label maps with a generative model of PV, and can be trained
to segment scans of any desired target contrast and resolution, even for
previously unseen modalities where neither images nor segmentations are
available at training. PV-SynthSeg does not require any preprocessing, and runs
in seconds. We demonstrate the accuracy and flexibility of the method with
extensive experiments on three datasets and 2,680 scans. The code is available
at https://github.com/BBillot/SynthSeg.Comment: accepted for MICCAI 202
Astragalus Polysaccharides Attenuate Postburn Sepsis via Inhibiting Negative Immunoregulation of CD4+CD25high T Cells
BACKGROUND: Astragalus polysaccharides (APS) isolated from one of the Chinese herbs, Astragalus mongholicus, are known to have a variety of immunomodulatory activities. However, it is not yet clear whether APS can exert an effect on the immune functions of regulatory T cells (Tregs). This study was carried out to investigate the effect of APS on the immune function of peripheral blood Tregs in postburn sepsis. METHODOLOGY/PRINCIPAL FINDINGS: BALB/C mice were randomly divided into six groups as follows: sham burn group, burn control (burn without infection animals) group, burn plus P. aeruginosa group, burn plus P. aeruginosa with APS (50 mg/kg) treatment group, burn plus P. aeruginosa with APS (100 mg/kg) treatment group, and burn plus P. aeruginosa with APS (200 mg/kg) treatment group, and they were sacrificed on postburn day 1, 3, 5, and 7, respectively, with seven animals at each time point. Magnetic microbeads were used to isolate peripheral blood Tregs and CD4(+) T cells. Phenotypes were analyzed by flow cytometry, and cytokine levels were determined with ELISA. In the burn plus P. aeruginosa group, forkhead/winged helix transcription factor p3 (Foxp3) expression on CD4(+)CD25(+)Tregs were strongly enhanced in comparison to the sham group, and the capacity of CD4(+)CD25(+)Tregs to produce interleukin (IL)-10 was markedly increased. Administration of APS to inhibit CD4(+)CD25(+)Tregs could significantly decrease expression of Foxp3 on CD4(+)CD25(+)Tregs, and IL-10 production in burned mice with P. aeruginosa infection. At the same time, proliferative activity and expression of IL-2 and IL-2Rα on CD4(+) T cells were restored. In contrast, anti-Toll-like receptor 4 (TLR4) antibody could block the effect of APS on Tregs immune function. CONCLUSION: APS might suppress CD4(+)CD25(+)Treg activity, at least in part, via binding TLR4 on Tregs and trigger a shift of Th2 to Th1 with activation of CD4(+) T cells in burned mice with P. aeruginosa infection
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
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