8,652 research outputs found
GRAVITY: The AO-Assisted, Two-Object Beam-Combiner Instrument
We present the proposal for the infrared adaptive optics (AO) assisted,
two-object, high-throughput, multiple-beam-combiner GRAVITY for the VLTI. This
instrument will be optimized for phase-referenced interferometric imaging and
narrow-angle astrometry of faint, red objects. Following the scientific
drivers, we analyze the VLTI infrastructure, and subsequently derive the
requirements and concept for the optimum instrument. The analysis can be
summarized with the need for highest sensitivity, phase referenced imaging and
astrometry of two objects in the VLTI beam, and infrared wavefront-sensing.
Consequently our proposed instrument allows the observations of faint, red
objects with its internal infrared wavefront sensor, pushes the optical
throughput by restricting observations to K-band at low and medium spectral
resolution, and is fully enclosed in a cryostat for optimum background
suppression and stability. Our instrument will thus increase the sensitivity of
the VLTI significantly beyond the present capabilities. With its two fibers per
telescope beam, GRAVITY will not only allow the simultaneous observations of
two objects, but will also push the astrometric accuracy for UTs to 10
micro-arcsec, and provide simultaneous astrometry for up to six baselines.Comment: 12 pages, to be published in the Proceedings of the ESO Workshop on
"The Power of Optical/IR Interferometry: Recent Scientific Results and 2nd
Generation VLTI Instrumentation", eds. F. Paresce, A. Richichi, A. Chelli and
F. Delplancke, held in Garching, Germany, 4-8 April 200
Highly efficient energy excitation transfer in light-harvesting complexes: The fundamental role of noise-assisted transport
Excitation transfer through interacting systems plays an important role in
many areas of physics, chemistry, and biology. The uncontrollable interaction
of the transmission network with a noisy environment is usually assumed to
deteriorate its transport capacity, especially so when the system is
fundamentally quantum mechanical. Here we identify key mechanisms through which
noise such as dephasing, perhaps counter intuitively, may actually aid
transport through a dissipative network by opening up additional pathways for
excitation transfer. We show that these are processes that lead to the
inhibition of destructive interference and exploitation of line broadening
effects. We illustrate how these mechanisms operate on a fully connected
network by developing a powerful analytical technique that identifies the
invariant (excitation trapping) subspaces of a given Hamiltonian. Finally, we
show how these principles can explain the remarkable efficiency and robustness
of excitation energy transfer from the light-harvesting chlorosomes to the
bacterial reaction center in photosynthetic complexes and present a numerical
analysis of excitation transport across the Fenna-Matthew-Olson (FMO) complex
together with a brief analysis of its entanglement properties. Our results show
that, in general, it is the careful interplay of quantum mechanical features
and the unavoidable environmental noise that will lead to an optimal system
performance.Comment: 16 pages, 9 figures; See Video Abstract at
http://www.quantiki.org/video_abstracts/09014454 . New revised version;
discussion of entanglement properties enhance
Optical Spectroscopy of Galactic Cirrus Clouds: Extended Red Emission in the Diffuse Interstellar Medium
We present initial results from the first optical spectroscopic survey of
high latitude Galactic cirrus clouds. The observed shape of the cirrus spectrum
does not agree with that of scattered ambient Galactic starlight. This mismatch
can be explained by the presence of Extended Red Emission (ERE) in the diffuse
interstellar medium, as found in many other astronomical objects, probably
caused by photoluminescence of hydrocarbons. The integrated ERE intensity,
I_ERE \approx 1.2 x 10^{-5} erg s^{-1} cm^{-2} sr^{-1}, is roughly a third of
the scattered light intensity, consistent with recent color measurements of
diffuse Galactic light. The peak of the cirrus ERE (lambda_{0} \sim 6000 AA) is
shifted towards short (bluer) wavelengths compared to the ERE in sources
excited by intense ultraviolet radiation, such as HII regions (lambda_{0} sim
8000 AA); such a trend is seen in laboratory experiments on hydrogenated
amorphous carbon films.Comment: 7 pages, 2 figures. Accepted for publication in ApJ Letter
The Excitation of Extended Red Emission: New Constraints on its Carrier From HST Observations of NGC 7023
The carrier of the dust-associated photoluminescence process causing the
extended red emission (ERE) in many dusty interstellar environments remains
unidentified. Several competing models are more or less able to match the
observed broad, unstructured ERE band. We now constrain the character of the
ERE carrier further by determining the wavelengths of the radiation that
initiates the ERE. Using the imaging capabilities of the Hubble Space
Telescope, we have resolved the width of narrow ERE filaments appearing on the
surfaces of externally illuminated molecular clouds in the bright reflection
nebula NGC 7023 and compared them with the depth of penetration of radiation of
known wavelengths into the same cloud surfaces. We identify photons with
wavelengths shortward of 118 nm as the source of ERE initiation, not to be
confused with ERE excitation, however. There are strong indications from the
well-studied ERE in the Red Rectangle nebula and in the high-|b| Galactic
cirrus that the photon flux with wavelengths shortward of 118 nm is too small
to actually excite the observed ERE, even with 100% quantum efficiency. We
conclude, therefore, that ERE excitation results from a two-step process. While
none of the previously proposed ERE models can match these new constraints, we
note that under interstellar conditions most polycyclic aromatic hydrocarbon
(PAH) molecules are ionized to the di-cation stage by photons with E > 10.5 eV
and that the electronic energy level structure of PAH di-cations is consistent
with fluorescence in the wavelength band of the ERE. Therefore, PAH di-cations
deserve further study as potential carriers of the ERE. (abridged)Comment: Accepted for Publication in the Ap
Longevity of immediate rehabilitation with direct metal-wire reinforced composite fixed partial dentures.
OBJECTIVES
This study aimed to analyze the longevity of direct metal-wire reinforced composite fixed partial dentures (MRC-FPD) and factors influencing their survival and success.
METHODS
Within one private practice 513 MRC-FPD were directly applied. The preparation of a proximal cavity in abutment teeth was not limited. MRC-FPD were reinforced by one to three metal-wires. At the last follow-up MRC-FPD were considered successful, if they were still in function without any need of therapy. MRC-FPD were considered as survived, if they were repaired or replaced. Multi-level Cox proportional hazard models were used to evaluate the association between clinical factors and time.
RESULTS
Mean follow-up period (range) was 59(2-249) months. Seventy-three bridges did not survive (cumulative survival rate(CSR):86%) and further 129 bridges had received a restorative follow-up treatment (CSR:61%). AFR was 2.2% for survival and 8.6% for success. In multivariate analysis MRC-FPD with> 1 wire showed a up to 2.3x higher failure rate than MRC-FPD with one wire(p â€Â 0.023). Dentist's experience in designing MRC-FDP (p â€Â 0.017), patient's caries risk (p â€Â 0.040) and bruxism (p = 0.033) significantly influenced the failure rate: the more experience, the lower caries risk and bruxism, the lower the failure rate.
SIGNIFICANCE
For directly prepared metal-wire reinforced composite bridges high survival and moderate success rates were observed. MRC-FPD might, thus, be an immediate, short- and medium-term solution for replacing missing teeth. However, several factors on the levels of practice (dentist's experience in designing MRC-FDP), patient (bruxism, caries risk) and restoration (number of wires) were identified as significant predictors for the failure rate. The study was registered in the German Clinical Trials Register (DRKS-ID: DRKS00021576)
Interferometer-Type Structures for Guided Atoms
We experimentally demonstrate interferometer-type guiding structures for
neutral atoms based on dipole potentials created by micro-fabricated optical
systems. As a central element we use an array of atom waveguides being formed
by focusing a red-detuned laser beam with an array of cylindrical microlenses.
Combining two of these arrays, we realize X-shaped beam splitters and more
complex systems like the geometries for Mach-Zehnder and Michelson-type
interferometers for atoms.Comment: 4 pages, 6 figure
The interferometric baselines and GRAVITY astrometric error budget
GRAVITY is a new generation beam combination instrument for the VLTI. Its
goal is to achieve microarsecond astrometric accuracy between objects separated
by a few arcsec. This accuracy on astrometric measurements is the most
important challenge of the instrument, and careful error budget have been
paramount during the technical design of the instrument. In this poster, we
will focus on baselines induced errors, which is part of a larger error budget.Comment: SPIE Meeting 2014 -- Montrea
An optical study of interdiffusion in ZnSe/ZnCdSe
Copyright 1996 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. This article appeared in Applied Physics Letters 69, 1579 (1996) and may be found at
An Upper Limit on the Reflected Light from the Planet Orbiting the Star tau Bootis
The planet orbiting tau Boo at a separation of 0.046 AU could produce a
reflected light flux as bright as 1e-4 relative to that of the star. A spectrum
of the system will contain a reflected light component which varies in
amplitude and Doppler-shift as the planet orbits the star. Assuming the
secondary spectrum is primarily the reflected stellar spectrum, we can limit
the relative reflected light flux to be less than 5e-5. This implies an upper
limit of 0.3 for the planetary geometric albedo near 480 nm, assuming a
planetary radius of 1.2 R_Jup. This albedo is significantly less than that of
any of the giant planets of the solar system, and is not consistent with
certain published theoretical predictions.Comment: 5 pages, 1 figure, accepted by ApJ Letter
Imaging the spotty surface of Betelgeuse in the H band
This paper reports on H-band interferometric observations of Betelgeuse made
at the three-telescope interferometer IOTA. We image Betelgeuse and its
asymmetries to understand the spatial variation of the photosphere, including
its diameter, limb darkening, effective temperature, surrounding brightness,
and bright (or dark) star spots. We used different theoretical simulations of
the photosphere and dusty environment to model the visibility data. We made
images with parametric modeling and two image reconstruction algorithms: MIRA
and WISARD. We measure an average limb-darkened diameter of 44.28 +/- 0.15 mas
with linear and quadratic models and a Rosseland diameter of 45.03 +/- 0.12 mas
with a MARCS model. These measurements lead us to derive an updated effective
temperature of 3600 +/- 66 K. We detect a fully-resolved environment to which
the silicate dust shell is likely to contribute. By using two imaging
reconstruction algorithms, we unveiled two bright spots on the surface of
Betelgeuse. One spot has a diameter of about 11 mas and accounts for about 8.5%
of the total flux. The second one is unresolved (diameter < 9 mas) with 4.5% of
the total flux. Resolved images of Betelgeuse in the H band are asymmetric at
the level of a few percent. The MOLsphere is not detected in this wavelength
range. The amount of measured limb-darkening is in good agreement with model
predictions. The two spots imaged at the surface of the star are potential
signatures of convective cells.Comment: 10 pages, 10 figures, accepted for publication in A&A, references
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