652 research outputs found
Aromatic emission from the ionised mane of the Horsehead nebula
We study the evolution of the Aromatic Infrared Bands (AIBs) emitters across
the illuminated edge of the Horsehead nebula and especially their survival and
properties in the HII region. We present spectral mapping observations taken
with the Infrared Spectrograph (IRS) at wavelengths 5.2-38 microns. A strong
AIB at 11.3 microns is detected in the HII region, relative to the other AIBs
at 6.2, 7.7 and 8.6 microns. The intensity of this band appears to be
correlated with the intensity of the [NeII] at 12.8 microns and of Halpha,
which shows that the emitters of the 11.3 microns band are located in the
ionised gas. The survival of PAHs in the HII region could be due to the
moderate intensity of the radiation field (G0 about 100) and the lack of
photons with energy above about 25eV. The enhancement of the intensity of the
11.3 microns band in the HII region, relative to the other AIBs can be
explained by the presence of neutral PAHs. Our observations highlight a
transition region between ionised and neutral PAHs observed with ideal
conditions in our Galaxy. A scenario where PAHs can survive in HII regions and
be significantly neutral could explain the detection of a prominent 11.3
microns band in other Spitzer observations.Comment: 9 pages, 9 figures, accepted for publication in A&
Dust Emissivity Variations In the Milky Way
Dust properties appear to vary according to the environment in which the dust
evolves. Previous observational indications of these variations in the FIR and
submm spectral range are scarce and limited to specific regions of the sky. To
determine whether these results can be generalised to larger scales, we study
the evolution in dust emissivities from the FIR to mm wavelengths, in the
atomic and molecular ISM, along the Galactic plane towards the outer Galaxy. We
correlate the dust FIR to mm emission with the HI and CO emission. The study is
carried out using the DIRBE data from 100 to 240 mic, the Archeops data from
550 mic to 2.1 mm, and the WMAP data at 3.2 mm (W band), in regions with
Galactic latitude |b| < 30 deg, over the Galactic longitude range (75 deg < l <
198 deg) observed with Archeops. In all regions studied, the emissivity spectra
in both the atomic and molecular phases are steeper in the FIR (beta = 2.4)
than in the submm and mm (beta = 1.5). We find significant variations in the
spectral shape of the dust emissivity as a function of the dust temperature in
the molecular phase. Regions of similar dust temperature in the molecular and
atomic gas exhibit similar emissivity spectra. Regions where the dust is
significantly colder in the molecular phase show a significant increase in
emissivity for the range 100 - 550 mic. This result supports the hypothesis of
grain coagulation in these regions, confirming results obtained over small
fractions of the sky in previous studies and allowing us to expand these
results to the cold molecular environments in general of the outer MW. We note
that it is the first time that these effects have been demonstrated by direct
measurement of the emissivity, while previous studies were based only on
thermal arguments.Comment: 16 pages, 6 figures, accepted in A&
Exercise systolic pulmonary artery pressure>60mmHg: A questionable threshold for clinical decision. Our experience in 509 patients
A complement to the modern crystallographer's toolbox: Caged gadolinium complexes with versatile binding modes
A set of seven caged gadolinium complexes were used as vectors for introducing the chelated Gd3+ ion into protein crystals in order to provide strong anomalous scattering for de novo phasing. The complexes contained multidentate ligand molecules with different functional groups to provide a panel of possible interactions with the protein. An exhaustive crystallographic analysis showed them to be nondisruptive to the diffraction quality of the prepared derivative crystals, and as many as 50% of the derivatives allowed the determination of accurate phases, leading to high-quality experimental electron-density maps. At least two successful derivatives were identified for all tested proteins. Structure refinement showed that the complexes bind to the protein surface or solvent-accessible cavities, involving hydrogen bonds, electrostatic and CH-Ï interactions, explaining their versatile binding modes. Their high phasing power, complementary binding modes and ease of use make them highly suitable as a heavy-atom screen for high-throughput de novo structure determination, in combination with the SAD method. They can also provide a reliable tool for the development of new methods such as serial femtosecond crystallography. © 2014 International Union of Crystallography.Peer Reviewe
Complex membrane remodeling during virion assembly of the 30,000 years-old Mollivirus sibericum
International audienceCellular membranes ensure functional compartmentalization by dynamic fusion-fission remodeling and are often targeted by viruses during entry, replication, assembly and egress. Nucleocytoplasmic large DNA viruses (NCLDVs) can recruit host-derived open membrane precursors to form their inner viral membrane. Using complementary 3D-electron microscopy techniques including focused-ion beam scanning electron microscopy and electron tomography, we show that the giant Mollivirus sibericum utilizes the same strategy but also displays unique features. Indeed, assembly is specifically triggered by an open cisterna with a flat pole in its center and open curling ends that grow by recruitment of vesicles, never reported for NCLDVs. These vesicles, abundant in the viral factory (VF), are initially closed but open once in close proximity to the open curling ends of the growing viral membrane. The flat pole appears to play a central role during the entire virus assembly process. While additional capsid layers are assembled from it, it also shapes the growing cisterna into immature crescent-like virions and is located opposite to the membrane elongation and closure sites, thereby providing virions with a polarity. In the VF, DNA-associated filaments are abundant and DNA is packed within virions, prior to particle closure. Altogether, our results highlight the complexity of the interaction between giant viruses and their host. Mollivirus assembly relies on the general strategy of vesicle recruitment, opening and shaping by capsid layers similar to all NCLDVs studied until now. However, the specific features of its assembly suggests that the molecular mechanisms for cellular membrane remodeling and persistence are unique.ImportanceSince the first giant virus Mimivirus was identified, other giant representatives are isolated regularly around the World and appear to be unique in several aspects. They belong to at least four viral families and the ways they interact with their hosts remain poorly understood. We focused on Mollivirus sibericum, the sole representative of "Molliviridae" which was isolated from a 30,000 years-old permafrost sample, and exhibits spherical virions of complex composition. In particular, we show that (i) assembly is initiated by a unique structure containing a flat pole positioned at the center of an open cisterna; (ii) core packing involves another cisterna-like element seemingly pushing core proteins into particles being assembled; (iii) specific filamentous structures contain the viral genome before packaging. Altogether, our findings increase our understanding on how complex giant viruses interact with their host and provide the foundation for future studies to elucidate the molecular mechanisms of Mollivirus assembly
Magnetospectroscopy of epitaxial few-layer graphene
The inter-Landau level transitions observed in far-infrared transmission
experiments on few-layer graphene samples show a behaviour characteristic of
the linear dispersion expected in graphene. This behaviour persists in
relatively thick samples, and is qualitatively different from that of thin
samples of bulk graphite.Comment: Invited short review to appear in a special issue of Solid State
Communication
Optical performance of the JWST MIRI flight model: characterization of the point spread function at high-resolution
The Mid Infra Red Instrument (MIRI) is one of the four instruments onboard
the James Webb Space Telescope (JWST), providing imaging, coronagraphy and
spectroscopy over the 5-28 microns band. To verify the optical performance of
the instrument, extensive tests were performed at CEA on the flight model (FM)
of the Mid-InfraRed IMager (MIRIM) at cryogenic temperatures and in the
infrared. This paper reports on the point spread function (PSF) measurements at
5.6 microns, the shortest operating wavelength for imaging. At 5.6 microns the
PSF is not Nyquist-sampled, so we use am original technique that combines a
microscanning measurement strategy with a deconvolution algorithm to obtain an
over-resolved MIRIM PSF. The microscanning consists in a sub-pixel scan of a
point source on the focal plane. A data inversion method is used to reconstruct
PSF images that are over-resolved by a factor of 7 compared to the native
resolution of MIRI. We show that the FWHM of the high-resolution PSFs were
5-10% wider than that obtained with Zemax simulations. The main cause was
identified as an out-of-specification tilt of the M4 mirror. After correction,
two additional test campaigns were carried out, and we show that the shape of
the PSF is conform to expectations. The FWHM of the PSFs are 0.18-0.20 arcsec,
in agreement with simulations. 56.1-59.2% of the total encircled energy
(normalized to a 5 arcsec radius) is contained within the first dark Airy ring,
over the whole field of view. At longer wavelengths (7.7-25.5 microns), this
percentage is 57-68%. MIRIM is thus compliant with the optical quality
requirements. This characterization of the MIRIM PSF, as well as the
deconvolution method presented here, are of particular importance, not only for
the verification of the optical quality and the MIRI calibration, but also for
scientific applications.Comment: 13 pages, submitted to SPIE Proceedings vol. 7731, Space Telescopes
and Instrumentation 2010: Optical, Infrared, and Millimeter Wav
Plasma and Warm Dust in the Collisional Ring Galaxy VIIZw466 from VLA and ISO Observations
We present the first mid-infrared (Mid-IR) (m) and radio
continuum (20,~6 and 3.6 cm) observations of the star-forming
collisional ring galaxy VII Zw 466 and its host group made with the Infrared
Space Observatory and the NRAO Very Large Array. A search was also made for CO
line emission in two of the galaxies with the Onsala 20m radio telescope and
upper limits were placed on the mass of molecular gas in those galaxies. The
ring galaxy is believed to owe its morphology to a slightly off-center
collision between an `intruder' galaxy and a disk. An off-center collision is
predicted to generate a radially expanding density wave in the disk which
should show large azimuthal variations in overdensity, and have observational
consequences. The radio continuum emission shows the largest asymmetry,
exhibiting a crescent-shaped distribution consistent with either the trapping
of cosmic-ray particles in the target disk, or an enhanced supernova rate in
the compressed region. On the other hand, the ISO observations (especially
those made at m) show a more scattered distribution, with
emission centers associated with powerful star formation sites distributed more
uniformly around the ring. Low-signal to noise observations at
m show possible emission inside the ring, with little emission
directly associated with the \ion{H}{2} regions. The observations emphasize the
complex relationship between the generation of radio emission and the
development of star formation even in relatively simple and well understood
collisional scenarios.Comment: Accepted for publication in The Astrophysical Journal, 23 pages + 6
PS figure
A Spherical Model for "Starless" Cores of Magnetic Molecular Clouds and Dynamical Effects of Dust Grains
In the standard picture of isolated star formation, dense ``starless'' cores
are formed out of magnetic molecular clouds due to ambipolar diffusion. Under
the simplest spherical geometry, I demonstrate that ``starless'' cores formed
this way naturally exhibit a large scale inward motion, whose size and speed
are comparable to those detected recently by Taffala et al. and Williams et al.
in ``starless'' core L1544. My model clouds have a relatively low mass (of
order 10 ) and low field strength (of order 10 G) to begin with.
They evolve into a density profile with a central plateau surrounded by a
power-law envelope, as found previously. The density in the envelope decreases
with radius more steeply than those found by Mouschovias and collaborators for
the more strongly magnetized, disk-like clouds.
At high enough densities, dust grains become dynamically important by greatly
enhancing the coupling between magnetic field and the neutral cloud matter. The
trapping of magnetic flux associated with the enhanced coupling leads, in the
spherical geometry, to a rapid assemblage of mass by the central protostar,
which exacerbates the so-called ``luminosity problem'' in star formation.Comment: 27 pages, 4 figures, accepted by Ap
Feedback control of spin systems
The feedback stabilization problem for ensembles of coupled spin 1/2 systems
is discussed from a control theoretic perspective. The noninvasive nature of
the bulk measurement allows for a fully unitary and deterministic closed loop.
The Lyapunov-based feedback design presented does not require spins that are
selectively addressable. With this method, it is possible to obtain control
inputs also for difficult tasks, like suppressing undesired couplings in
identical spin systems.Comment: 16 pages, 15 figure
- âŠ