3,255 research outputs found
Complementary use of TEM and APT for the investigation of steels nanostructured by severe plastic deformation
The properties of bulk nanostructured materials are often controlled by
atomic scale features like segregation along defects or composition gradients.
Here we discuss about the complimentary use of TEM and APT to obtain a full
description of nanostructures. The advantages and limitations of both
techniques are highlighted on the basis of experimental data collected in
severely deformed steels with a special emphasis on carbon spatial
distribution
High-order myopic coronagraphic phase diversity (COFFEE) for wave-front control in high-contrast imaging systems
The estimation and compensation of quasi-static aberrations is mandatory to
reach the ultimate performance of high-contrast imaging systems. COFFEE is a
focal plane wave-front sensing method that consists in the extension of phase
diversity to high-contrast imaging systems. Based on a Bayesian approach, it
estimates the quasi-static aberrations from two focal plane images recorded
from the scientific camera itself. In this paper, we present COFFEE's extension
which allows an estimation of low and high order aberrations with nanometric
precision for any coronagraphic device. The performance is evaluated by
realistic simulations, performed in the SPHERE instrument framework. We develop
a myopic estimation that allows us to take into account an imperfect knowledge
on the used diversity phase. Lastly, we evaluate COFFEE's performance in a
compensation process, to optimize the contrast on the detector, and show it
allows one to reach the 10^-6 contrast required by SPHERE at a few resolution
elements from the star. Notably, we present a non-linear energy minimization
method which can be used to reach very high contrast levels (better than 10^-7
in a SPHERE-like context)Comment: Accepted in Optics Expres
ALMA observations of cool dust in a low-metallicity starburst, SBS0335-052
We present Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 0 Band 7
observations of an extremely metal-poor dwarf starburst galaxy in the Local
Universe, SBS0335-052 (12+log(O/H)~7.2). With these observations, dust is
detected at 870micron (ALMA Band 7), but 87% of the flux in this band is due to
free-free emission from the starburst. We have compiled a spectral energy
distribution (SED) of SBS0335-052 that spans almost 6 orders of magnitude in
wavelength and fit it with a spherical dust shell heated by a single-age
stellar population; the best-fit model gives a dust mass of (3.8+/-0.6)x10^4
Msun. We have also constructed a SED including Herschel archival data for
IZw18, another low-metallicity dwarf starburst (12+log(O/H)=7.17), and fit it
with a similar model to obtain a dust mass of (3.4+/-1.0)x10^2 Msun. Compared
with their atomic gas mass, the dust mass of SBS0335-052 far exceeds the
prediction of a linear trend of dust-to-gas mass ratio with metallicity, while
IZw18 falls far below. We use gas scaling relations to assess a putative
missing gas component in both galaxies and find that the missing, possibly
molecular, gas in SBS0335-052 is a factor of 6 times higher than the value
inferred from the observed HI column density; in IZw18 the missing component is
4 times smaller. Ultimately, despite their similarly low metallicity, the
differences in gas and dust column densities in SBS0335-052 and IZw18 suggest
that metal abundance does not uniquely define star-formation processes. At some
level, self-shielding and the survival of molecules may depend just as much on
gas and dust column density as on metallicity. The effects of low metallicity
may at least be partially compensated for by large column densities in the
interstellar medium.Comment: 15 pages, 11 figures, accepted for publication in A&
The Embedded Super Star Cluster of SBS0335-052
We analyze the infrared (6-100 micron) spectral energy distribution of the
blue compact dwarf and metal-poor (Z=Z_solar/41) galaxy SBS0335-052. With the
help of DUSTY (Ivezic et al. 1999), a program that solves the radiation
transfer equations in a spherical environment, we evaluate that the infrared
(IR) emission of SBS0335-052 is produced by an embedded super-star cluster
(SSC) hidden under 10^5 M_solar of dust, causing 30 mag of visual extinction.
This implies that one cannot detect any stellar emission from the 2x10^6
M_solar stellar cluster even at near-infrared (NIR) wavelengths. The derived
grain size distribution departs markedly from the widely accepted size
distribution inferred for dust in our galaxy (the so-called MRN distribution,
Mathis et al. 1977), but resembles what is seen around AGNs, namely an absence
of PAH and smaller grains, and grains that grow to larger sizes (around 1
micron). The fact that a significant amount of dust is present in such a
low-metallicity galaxy, hiding from UV and optical view most of the star
formation activity in the galaxy, and that the dust size distribution cannot be
reproduced by a standard galactic law, should be borne in mind when
interpreting the spectrum of primeval galaxies.Comment: 32 pages, 3 figures,accepted for publication in A
CEA Bolometer Arrays: the First Year in Space
The CEA/LETI and CEA/SAp started the development of far-infrared filled bolometer arrays for space applications
over a decade ago. The unique design of these detectors makes possible the assembling of large focal planes
comprising thousands of bolometers running at 300 mK with very low power dissipation. Ten arrays of 16x16
pixels were thoroughly tested on the ground, and integrated in the Herschel/PACS instrument before launch in
May 2009. These detectors have been successfully commissioned and are now operating in their nominal environment
at the second Lagrangian point of the Earth-Sun system. In this paper we briefly explain the functioning
of CEA bolometer arrays, and we present the properties of the detectors focusing on their noise characteristics,
the effect of cosmic rays on the signal, the repeatability of the measurements, and the stability of the system
Performances of Herschel/PACS bolometer arrays and future developments at CEA
The European Space Agency is building a space telescope to observe the Universe in the Far-IR and sub-millimeter regime of the electromagnetic spectrum. The scientific payload is composed of three instruments. One of them, PACS, is equipped with a novel type of bolometer arrays developed by CEA in the late 90's. We briefly present the PACS Photometer and the architecture of CEA filled bolometer arrays. We accessed the physics of the detectors and thoroughly measured their performances by developing a pragmatic calibration procedure. The Photometer is now calibrated and integrated on the focal plane of the observatory. The launch is scheduled for April 2009. Meanwhile, CEA is working on adapting PACS bolometers to longer wavelength for wide-field ground-based telescopes, and for the future cold-telescope space mission SPICA
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Parasitic helminths induce fetal-like reversion in the intestinal stem cell niche.
Epithelial surfaces form critical barriers to the outside world and are continuously renewed by adult stem cells1. Whereas dynamics of epithelial stem cells during homeostasis are increasingly well understood, how stem cells are redirected from a tissue-maintenance program to initiate repair after injury remains unclear. Here we examined infection by Heligmosomoides polygyrus, a co-evolved pathosymbiont of mice, to assess the epithelial response to disruption of the mucosal barrier. H. polygyrus disrupts tissue integrity by penetrating the duodenal mucosa, where it develops while surrounded by a multicellular granulomatous infiltrate2. Crypts overlying larvae-associated granulomas did not express intestinal stem cell markers, including Lgr53, in spite of continued epithelial proliferation. Granuloma-associated Lgr5- crypt epithelium activated an interferon-gamma (IFN-γ)-dependent transcriptional program, highlighted by Sca-1 expression, and IFN-γ-producing immune cells were found in granulomas. A similar epithelial response accompanied systemic activation of immune cells, intestinal irradiation, or ablation of Lgr5+ intestinal stem cells. When cultured in vitro, granuloma-associated crypt cells formed spheroids similar to those formed by fetal epithelium, and a sub-population of H. polygyrus-induced cells activated a fetal-like transcriptional program, demonstrating that adult intestinal tissues can repurpose aspects of fetal development. Therefore, re-initiation of the developmental program represents a fundamental mechanism by which the intestinal crypt can remodel itself to sustain function after injury
The Nature of Radio Continuum Emission in the Dwarf Starburst Galaxy NGC 625
We present new multi-frequency radio continuum imaging of the dwarf starburst
galaxy NGC 625 obtained with the Very Large Array. Data at 20, 6, and 3.6 cm
reveal global continuum emission dominated by free-free emission, with only
mild synchrotron components. Each of the major HII regions is detected; the
individual spectral indices are thermal for the youngest regions (showing
strongest H Alpha emission) and nonthermal for the oldest. We do not detect any
sources that appear to be associated with deeply embedded, dense, young
clusters, though we have discovered one low-luminosity, obscured source that
has no luminous optical counterpart and which resides in the region of highest
optical extinction. Since NGC 625 is a Wolf-Rayet galaxy with strong recent
star formation, these radio properties suggest that the youngest star formation
complexes have not yet evolved to the point where their thermal spectra are
significantly contaminated by synchrotron emission. The nonthermal components
are associated with regions of older star formation that have smaller ionized
gas components. These results imply a range of ages of the HII regions and
radio components that agrees with our previous resolved stellar population
analysis, where an extended burst of star formation has pervaded the disk of
NGC 625 over the last ~ 50 Myr. We compare the nature of radio continuum
emission in selected nearby dwarf starburst and Wolf-Rayet galaxies,
demonstrating that thermal radio continuum emission appears to be more common
in these systems than in typical HII galaxies with less recent star formation
and more evolved stellar clusters.Comment: ApJ, in press; 27 pages, 5 figures. Full-resolution version may be
obtained at http://www.astro.umn.edu/~cannon/n625.vla.p
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