571 research outputs found
New Constraints on Hidden Photons using Very High Energy Gamma-Rays from the Crab Nebula
Extensions of the standard model of particle physics, in particular those
based on string theory, often predict a new U(1) gauge symmetry in a hidden
sector. The corresponding gauge boson, called hidden photon, naturally
interacts with the ordinary photon via gauge kinetic mixing, leading to photon
- hidden photon oscillations. In this framework, one expects photon
disappearance as a function of the mass of the hidden photon and the mixing
angle, loosely constrained from theory. Several experiments have been carried
out or are planned to constrain the mass-mixing plane.
In this contribution we derive new constraints on the hidden photon
parameters, using very high energy gamma-rays detected from the Crab Nebula,
whose broad-band spectral characteristics are well understood. The very high
energy gamma-ray observations offer the possibility to provide bounds in a
broad mass range at a previously unexplored energy and distance scale. Using
existing data that were taken with several Cherenkov telescopes, we discuss our
results in the context of current constraints and consider the possibilities of
using astrophysical data to search for hidden photon signatures.Comment: Proceedings of the "Heidelberg International Symposium on High Energy
Gamma-Ray Astronomy", Heidelberg, Germany, July 7-11, 2008, submitted to AIP
Conference Proceedings. 4 pages, 2 figures, 1 tabl
Exact Insulating and Conducting Ground States of a Periodic Anderson Model in Three Dimensions
We present a class of exact ground states of a three-dimensional periodic
Anderson model at 3/4 filling. Hopping and hybridization of d and f electrons
extend over the unit cell of a general Bravais lattice. Employing novel
composite operators combined with 55 matching conditions the Hamiltonian is
cast into positive semidefinite form. A product wave function in position space
allows one to identify stability regions of an insulating and a conducting
ground state. The metallic phase is a non-Fermi liquid with one dispersing and
one flat band.Comment: 4 pages, 3 figure
PhAST : pharmacophore alignment search tool
We developed the Pharmacophore Alignment Search Tool (PhAST), a text-based technique for rapid hit and lead structure searching in large compound databases. For each molecule, a two-dimensional graph of potential pharmacophoric points (PPPs) is created, which has an identical topology as the original molecule with implicit hydrogen atoms. Each vertex is coloured by a symbol representing the corresponding PPP. The vertices of the graph are canonically labelled. The symbols associated with the vertices are combined to a so-called PhAST-Sequence beginning with the vertex with the lowest canonical label. Due to the canonical labelling the created PhAST-Sequence is characteristic for each molecule. For similarity assessment, PhAST-Sequences are compared using the sequence identity in their global pairwise alignment. The alignment score lies between 0 (no similarity) and 1 (identical PhAST-Sequences). In order to use global pairwise sequence alignment, a score matrix for pharmacophoric symbols was developed and gap penalties were optimized. PhAST performed comparably and sometimes superior to other similarity search tools (CATS2D, MOE pharmacophore quadruples) in retrospective virtual screenings using the COBRA collection of drugs and lead structures. Most importantly, the PhAST alignment technique allows for the computation of significance estimates that help prioritize a virtual hit list
Time-dependent absorption of very high-energy gamma-rays from the Galactic center by pair-production
Very high energy (VHE) gamma-rays have been detected from the direction of
the Galactic center. The H.E.S.S. Cherenkov telescopes have located this
gamma-ray source with a preliminary position uncertainty of 8.5" per axis (6"
statistic + 6" sytematic per axis). Within the uncertainty region several
possible counterpart candidates exist: the Super Massive Black Hole Sgr A*, the
Pulsar Wind Nebula candidate G359.95-0.04, the Low Mass X-Ray Binary-system
J174540.0-290031, the stellar cluster IRS 13, as well as self-annihilating dark
matter. It is experimentally very challenging to further improve the positional
accuracy in this energy range and therefore, it may not be possible to clearly
associate one of the counterpart candidates with the VHE-source. Here, we
present a new method to investigate a possible link of the VHE-source with the
near environment of Sgr A* (within approximately 1000 Schwarzschild radii).
This method uses the time- and energy-dependent effect of absorption of
gamma-rays by pair-production (in the following named pair-eclipse) with
low-energy photons of stars closely orbiting the SMBH Sgr A*.Comment: 4 pages, 6 figures, Published in Proceedings of the 4th International
Meeting on High Energy Gamma-Ray Astronomy (Gamma 08), Heidelber
High spectral resolution imaging of the dynamical atmosphere of the red supergiant Antares in the CO first overtone lines with VLTI/AMBER
We present high spectral resolution aperture-synthesis imaging of the red
supergiant Antares (alpha Sco) in individual CO first overtone lines with
VLTI/AMBER. The reconstructed images reveal that the star appears differently
in the blue wing, line center, and red wing and shows an asymmetrically
extended component. The appearance of the star within the CO lines changes
drastically within one year, implying a significant change in the velocity
field in the atmosphere. Our modeling suggests an outer atmosphere (MOLsphere)
extending to 1.2--1.4 stellar radii with CO column densities of
(0.5--1)x10^{20} cm^{-2} and a temperature of ~2000 K. While the velocity field
in 2009 is characterized by strong upwelling motions at 20--30 km/s, it changed
to strong downdrafts in 2010. On the other hand, the AMBER data in the
continuum show only a slight deviation from limb-darkened disks and only
marginal time variations. We derive a limb-darkened disk diameter of
37.38+/-0.06 mas and a power-law-type limb-darkening parameter of
(8.7+/-1.6)x10^{-2} (2009) and 37.31+/-0.09 mas and (1.5+/-0.2)x10^{-1} (2010).
We also obtain Teff = 3660+/-120 K and log L/Lsun = 4.88+/-0.23, which suggests
a mass of 15+/-5 Msun with an age of 11-15 Myr. This age is consistent with the
recently estimated age for the Upper Scorpius OB association. The properties of
the outer atmosphere of Antares are similar to those of another well-studied
red supergiant, Betelgeuse. The density of the extended outer atmosphere of
Antares and Betelgeuse is higher than predicted by the current 3-D convection
simulations by at least six orders of magnitude, implying that convection alone
cannot explain the formation of the extended outer atmosphere.Comment: 18 pages, 16 figures, accepted for publication in Astronomy and
Astrophysics, short discussion on the age of Antares and the Upper Scorpius
OB association added, movies of the reconstructed images available at
http://www.mpifr-bonn.mpg.de/staff/kohnaka
Probing the accretion-ejection connection with VLTI/AMBER: High spectral resolution observations of the Herbig Ae star HD163296
Accretion and ejection are tightly connected and represent the fundamental
mechanisms regulating star formation. However, the exact physical processes
involved are not yet fully understood. We present high angular and spectral
resolution observations of the Br Gamma emitting region in the Herbig Ae star
HD163296 (MWC275) in order to probe the origin of this line and constrain the
physical processes taking place at sub-AU scales in the circumstellar region.
By means of VLTI-AMBER observations at high spectral resolution (R~12000), we
studied interferometric visibilities, wavelength-differential phases, and
closure phases across the Br Gamma line of HD163296. To constrain the physical
origin of the Br Gamma line in Herbig Ae stars, all the interferometric
observables were compared with the predictions of a line radiative transfer
disc wind model. The measured visibilities clearly increase within the Br Gamma
line, indicating that the Br Gamma emitting region is more compact than the
continuum. By fitting a geometric Gaussian model to the continuum-corrected Br
Gamma visibilities, we derived a compact radius of the Br Gamma emitting region
of ~0.07+/-0.02AU (Gaussian half width at half maximum; or a ring-fit radius of
~0.08+/-0.02AU). To interpret the observations, we developed a
magneto-centrifugally driven disc wind model. Our best disc wind model is able
to reproduce, within the errors, all the interferometric observables and it
predicts a launching region with an outer radius of ~0.04AU. However, the
intensity distribution of the entire disc wind emitting region extends up to
~0.16AU. Our observations, along with a detailed modelling of the Br Gamma
emitting region, suggest that most of the Br Gamma emission in HD163296
originates from a disc wind with a launching region that is over five times
more compact than previous estimates of the continuum dust rim radius.Comment: Accepted for publication in A&
Si-DLC coatings optimized for low wear and low friction applications.
Diamond-like carbon (DLC) has excellent properties for the use as tribological coating such as high hardness, high wear resistance and a low friction coefficient. Nanolayer systems of hydrogenated diamond-like carbon (a-C:H) and silicon doped hydrogenated diamond-like carbon (Si-DLC or a-C:H:Si) have been studied. The objectives of this work are to develop layer combinations which allow combining low abrasive wear, low friction properties, and improved temperature stability for the coatings.A literature survey on sputtering, plasma enhanced chemical vapor deposition (PECVD) and carbon based materials with a focus on a-C:H and Si-DLC coatings is given. This survey includes a brief overview of the 60 years history of DLC coating.In the second part of the thesis, the experimental set up for the layer preparation by a magnetron based deposition method and a basic description of the process are presented. Using sputtering from a solid SiC target as a source of the Si for the Si-DLC instead of the commonly used PECVD process with Si-containing precursors Si-DLC/a-C:H nanolayers were deposited at high deposition rates.For the coatings, the influence of the acetylene gas flow, the bias voltage, and the hydrogen and silicon concentration are discussed. Undoped a-C:H coatings with high indentation hardness above 40 GPa and very low abrasive wear rates of 0.6 x 10[-15] m[3]/(Nm) and low hydrogen content of about 11 at.% were deposited. Si-DLC/a-C:H nanolayer coatings with high hardness of 20 to 30 GPa and a high temperature stability up to 500 °C were prepared. For these nanolayer films low friction coefficients of 0.06 to 0.11 and high abrasive wear rates of >2.5 x 10[-15] m3/(Nm) were achieved at high silicon contents above 15 at.%. On the other hand nanolayer coatings with low silicon contents of less than 10 at.% showed low wear rates below 1.7 x 10[-15] m3/(Nm) combined with higher friction coefficients of 0.12 to 0.15. In order to combine the low abrasive wear rate and the low friction coefficient at first a Si-DLC/a-C:H layer with low Si-concentration followed by a second layer with high Si-concentration was deposited. Due to the low surface free energy of Si-DLC coatings of 30 to 35 mN/m the wetting behavior for some lubricants may deteriorate. In this case a further optimization of the nanolayer system is required
Physiological responses of the calcifying rhodophyte, Corallina officinalis (L.), to future CO2 levels
Future atmospheric CO2 levels will most likely have complex consequences for marine organisms, particulary photosynthetic calcifying organisms. Corallina officinalis L. is an erect calcifying macroalga found in the inter- and subtidal regions of temperate rocky coastlines and provides important substrate and refugia for marine meiofauna. The main goal of the current study was to determine the physiological responses of C. officinalis to increased CO2 concentrations expected to occur within the next century and beyond. Our results show that growth and production of inorganic material decreased under high CO2 levels, while carbonic anhydrase activity was stimulated and negatively correlated to algal inorganic content. Photosynthetic efficiency based on oxygen evolution was also negatively affected by increased CO2. The results of this study indicate that C. officinalis may become less competitive under future CO2 levels, which could result in structural changes in future temperate intertidal communities
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