1,434 research outputs found
The X-ray Position and Optical Counterpart of the Accretion-Powered Millisecond Pulsar XTE J1814-338
We report the precise optical and X-ray localization of the 3.2 ms
accretion-powered X-ray pulsar XTE J1814-338 with data from the Chandra X-Ray
Observatory as well as optical observations conducted during the 2003 June
discovery outburst. Optical imaging of the field during the outburst of this
soft X-ray transient reveals an R = 18 star at the X-ray position. This star is
absent (R > 20) from an archival 1989 image of the field and brightened during
the 2003 outburst, and we therefore identify it as the optical counterpart of
XTE J1814-338. The best source position derived from optical astrometry is R.A.
= 18h13m39.s04, Dec.= -33d46m22.3s (J2000). The featureless X-ray spectrum of
the pulsar in outburst is best fit by an absorbed power-law (with photon index
= 1.41 +- 0.06) plus blackbody (with kT = 0.95 +- 0.13 keV) model, where the
blackbody component contributes approximately 10% of the source flux. The
optical broad-band spectrum shows evidence for an excess of infrared emission
with respect to an X-ray heated accretion disk model, suggesting a significant
contribution from the secondary or from a synchrotron-emitting region. A
follow-up observation performed when XTE J1814-338 was in quiescence reveals no
counterpart to a limiting magnitude of R = 23.3. This suggests that the
secondary is an M3 V or later-type star, and therefore very unlikely to be
responsible for the soft excess, making synchroton emission a more reasonable
candidate.Comment: Accepted for publication in ApJ. 6 pages; 3 figure
A Multiplicity Census of Young Stars in Chamaeleon I
We present the results of a multiplicity survey of 126 stars spanning ~0.1-3
solar masses in the ~2-Myr-old Chamaeleon I star-forming region, based on
adaptive optics imaging with the ESO Very Large Telescope. Our observations
have revealed 30 binaries and 6 triples, of which 19 and 4, respectively, are
new discoveries. The overall multiplicity fraction we find for Cha I (~30%) is
similar to those reported for other dispersed young associations, but
significantly higher than seen in denser clusters and the field, for comparable
samples. Both the frequency and the maximum separation of Cha I binaries
decline with decreasing mass, while the mass ratios approach unity; conversely,
tighter pairs are more likely to be equal mass. We confirm that brown dwarf
companions to stars are rare, even at young ages at wide separations. Based on
follow-up spectroscopy of two low-mass substellar companion candidates, we
conclude that both are likely background stars. The overall multiplicity
fraction in Cha I is in rough agreement with numerical simulations of cloud
collapse and fragmentation, but its observed mass dependence is less steep than
predicted. The paucity of higher-order multiples, in particular, provides a
stringent constraint on the simulations, and seems to indicate a low level of
turbulence in the prestellar cores in Cha I.Comment: Accepted for publication in Ap
The dynamical evolution of the circumstellar gas around low-and intermediate-mass stars I: the AGB
We have investigated the dynamical interaction of low- and-intermediate mass
stars (from 1 to 5 Msun) with their interstellar medium (ISM). In this first
paper, we examine the structures generated by the stellar winds during the
Asymptotic Giant Branch (AGB) phase, using a numerical code and the wind
history predicted by stellar evolution. The influence of the external ISM is
also taken into account. We find that the wind variations associated with the
thermal pulses lead to the formation of transient shells with an average
lifetime of 20,000 yr, and consequently do not remain recorded in the density
or velocity structure of the gas. The formation of shells that survive at the
end of the AGB occurs via two main processes: shocks between the shells formed
by two consecutive enhancements of the mass-loss or via continuous accumulation
of the material ejected by the star in the interaction region with the ISM. Our
models show that the mass of the circumstellar envelope increases appreciably
due to the ISM material swept up by the wind (up to 70 % for the 1 Msun stellar
model). We also point out the importance of the ISM on the deceleration and
compression of the external shells. According to our simulations, large regions
(up to 2.5 pc) of neutral gas surrounding the molecular envelopes of AGB stars
are expected. These large regions of gas are formed from the mass-loss
experienced by the star during the AGB evolution.Comment: 43 pages, 15 figures. Accepted for publication in the Astrophysical
Journa
Isometries of infinite dimensional Hilbert geometries
In this paper we extend two classical results concerning the isometries of strictly convex Hilbert geometries, and the characterisation of the isometry groups of Hilbert geometries on finite dimensional simplices, to infinite dimensions. The proofs rely on a mix of geometric and functional analytic methods
Knots in the outer shells of the planetary nebulae IC 2553 and NGC 5882
We present images and high-resolution spectra of the planetary nebulae IC
2553 and NGC 5882. Spatio-kinematic modeling of the nebulae shows that they are
composed of a markedly elongated inner shell, and of a less aspherical outer
shell expanding at a considerably higher velocity than the inner one. Embedded
in the outer shells of both nebulae are found several low-ionization knots. In
IC 2553, the knots show a point-symmetric distribution with respect to the
central star: one possible explanation for their formation is that they are the
survivors of pre-existing point-symmetric condensations in the AGB wind, a fact
which would imply a quite peculiar mass-loss geometry from the giant
progenitor. In the case of NGC 5882, the lack of symmetry in the distribution
of the observed low-ionization structures makes it possible that they are the
result of in situ instabilities.Comment: 20 pages including 1 table and 6 figures. ApJ accepted. Also
available at http://andromeda.roque.ing.iac.es/~sanchez/ingpub/index2000.htm
Characterising resting-state functional connectivity in a large sample of adults with ADHD
AbstractAttention-deficit/hyperactivity disorder (ADHD) is a common childhood psychiatric disorder that often persists into adulthood. While several studies have identified altered functional connectivity in brain networks during rest in children with ADHD, few studies have been performed on adults with ADHD. Existing studies have generally investigated small samples. We therefore investigated aberrant functional connectivity in a large sample of adult patients with childhood-onset ADHD, using a data-driven, whole-brain approach. Adults with a clinical ADHD diagnosis (N=99) and healthy, adult comparison subjects (N=113) underwent a 9-minute resting-state fMRI session in a 1.5T MRI scanner. After elaborate preprocessing including a thorough head-motion correction procedure, group independent component analysis (ICA) was applied from which we identified six networks of interest: cerebellum, executive control, left and right frontoparietal and two default-mode networks. Participant-level network maps were obtained using dual-regression and tested for differences between patients with ADHD and controls using permutation testing. Patients showed significantly stronger connectivity in the anterior cingulate gyrus of the executive control network. Trends were also observed for stronger connectivity in the cerebellum network in ADHD patients compared to controls. However, there was considerable overlap in connectivity values between patients and controls, leading to relatively low effect sizes despite the large sample size. These effect sizes were slightly larger when testing for correlations between hyperactivity/impulsivity symptoms and connectivity strength in the executive control and cerebellum networks. This study provides important insights for studies on the neurobiology of adult ADHD; it shows that resting-state functional connectivity differences between adult patients and controls exist, but have smaller effect sizes than existing literature suggested
Remodeling of central metabolism in invasive breast cancer compared to normal breast tissue - a GC-TOFMS based metabolomics study
BACKGROUND: Changes in energy metabolism of the cells are common to many kinds of tumors and are considered a hallmark of cancer. Gas chromatography followed by time-of-flight mass spectrometry (GC-TOFMS) is a well-suited technique to investigate the small molecules in the central metabolic pathways. However, the metabolic changes between invasive carcinoma and normal breast tissues were not investigated in a large cohort of breast cancer samples so far. RESULTS: A cohort of 271 breast cancer and 98 normal tissue samples was investigated using GC-TOFMS-based metabolomics. A total number of 468 metabolite peaks could be detected; out of these 368 (79%) were significantly changed between cancer and normal tissues (p80%. Two-metabolite classifiers, constructed as ratios of the tumor and normal tissues markers, separated cancer from normal tissues with high sensitivity and specificity. Specifically, the cytidine-5-monophosphate / pentadecanoic acid metabolic ratio was the most significant discriminator between cancer and normal tissues and allowed detection of cancer with a sensitivity of 94.8% and a specificity of 93.9%. CONCLUSIONS: For the first time, a comprehensive metabolic map of breast cancer was constructed by GC-TOF analysis of a large cohort of breast cancer and normal tissues. Furthermore, our results demonstrate that spectrometry-based approaches have the potential to contribute to the analysis of biopsies or clinical tissue samples complementary to histopathology
About the suitability of different numerical methods to reproduce model wind turbine measurements in a wind tunnel with a high blockage ratio
In the present paper, numerical and experimental investigations of a model
wind turbine with a diameter of 3.0âŻm are described. The study has
three objectives. The first one is the provision of validation data. The
second one is to estimate the influence of the wind tunnel walls by comparing
measurements to simulated results with and without wind tunnel walls. The
last objective is the comparison and evaluation of methods of high fidelity,
namely computational fluid dynamics, and medium fidelity, namely lifting-line
free vortex wake. The experiments were carried out in the large wind tunnel
of the TU Berlin where a blockage ratio of 40âŻ% occurs. With the lifting-line free vortex wake code QBlade, the turbine was simulated under far
field conditions at the TU Berlin. Unsteady Reynolds-averaged NavierâStokes
simulations of the wind turbine, including wind tunnel walls and under far
field conditions, were performed at the University of Stuttgart with the
computational fluid dynamics code FLOWer.
Comparisons among the experiment, the lifting-line free vortex wake code and
the computational fluid dynamics code include on-blade velocity and angle of
attack. Comparisons of flow fields are drawn between the experiment and the
computational fluid dynamics code. Bending moments are compared among the
simulations.
A good accordance was achieved for the on-blade velocity and the angle of
attack, whereas deviations occur for the flow fields and the bending moments
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