2,740 research outputs found
Ion beam micromachining of integrated optics components
Thin film integrated optics components such as light guides, modulators, directional couplers, and polarizers demand high quality edge smoothness and high resolution pattern formation in dimensions down to submicrometer size. Fabrication techniques combining holographic and scanning electron beam lithography with ion beam micromachining have produced planar phase gratings with intervals as small as 2800 Å, guiding channel couplers in GaAs, and also wire- grid polarizers for 10.6-µm radiation
A linear CO chemistry parameterization in a chemistry-transport model: evaluation and application to data assimilation
This paper presents an evaluation of a new linear parameterization valid for the troposphere and the stratosphere, based on a first order approximation of the carbon monoxide (CO) continuity equation. This linear scheme (hereinafter noted LINCO) has been implemented in the 3-D Chemical Transport Model (CTM) MOCAGE (MOdèle de Chimie Atmospherique Grande Echelle). First, a one and a half years of LINCO simulation has been compared to output obtained from a detailed chemical scheme output. The mean differences between both schemes are about ±25 ppbv (part per billion by volume) or 15% in the troposphere and ±10 ppbv or 100% in the stratosphere. Second, LINCO has been compared to diverse observations from satellite instruments covering the troposphere (Measurements Of Pollution In The Troposphere: MOPITT) and the stratosphere (Microwave Limb Sounder: MLS) and also from aircraft (Measurements of ozone and water vapour by Airbus in-service aircraft: MOZAIC programme) mostly flying in the upper troposphere and lower stratosphere (UTLS). In the troposphere, the LINCO seasonal variations as well as the vertical and horizontal distributions are quite close to MOPITT CO observations. However, a bias of ~−40 ppbv is observed at 700 Pa between LINCO and MOPITT. In the stratosphere, MLS and LINCO present similar large-scale patterns, except over the poles where the CO concentration is underestimated by the model. In the UTLS, LINCO presents small biases less than 2% compared to independent MOZAIC profiles. Third, we assimilated MOPITT CO using a variational 3D-FGAT (First Guess at Appropriate Time) method in conjunction with MOCAGE for a long run of one and a half years. The data assimilation greatly improves the vertical CO distribution in the troposphere from 700 to 350 hPa compared to independent MOZAIC profiles. At 146 hPa, the assimilated CO distribution is also improved compared to MLS observations by reducing the bias up to a factor of 2 in the tropics. This study confirms that the linear scheme is able to simulate reasonably well the CO distribution in the troposphere and in the lower stratosphere. Therefore, the low computing cost of the linear scheme opens new perspectives to make free runs and CO data assimilation runs at high resolution and over periods of several years
Cm-Wavelength Total Flux and Linear Polarization Properties of Radio-Loud BL Lacertae Objects
Results from a long-term program to quantify the range of behavior of the
cm-wavelength total flux and linear polarization variability properties of a
sample of 41 radio-loud BL Lac objects using weekly to tri-monthly observations
with the University of Michigan 26-m telescope operating at 14.5, 8.0, and 4.8
GHz are presented; these observations are used to identify class-dependent
differences between these BL Lacs and QSOs in the Pearson-Readhead sample. The
BL Lacs are found to be more highly variable in total flux density than the
QSOs, exhibiting changes that are often nearly-simultaneous and of comparable
amplitude at 14.5 and 4.8 GHz in contrast to the behavior in the QSOs and
supporting the existence of class-dependent differences in opacity within the
parsec-scale jet flows. Structure function analyses of the flux observations
quantify that a characteristic timescale is identifiable in only 1/3 of the BL
Lacs. The time-averaged fractional linear polarizations are only on the order
of a few percent and are consistent with the presence of tangled magnetic
fields within the emitting regions. In many sources a preferred long-term
orientation of the EVPA is present; when compared with the VLBI structural
axis, no preferred position angle difference is identified. The polarized flux
typically exhibits variability with timescales of months to a few years and
shows the signature of a propagating shock during several resolved outbursts.
The observations indicate that the source emission is predominately due to
evolving source components and support the occurrence of more frequent shock
formation in BL Lac parsec-scale flows than in QSO jets. The differences in
variability behavior and polarization between BL Lacs and QSOs can be explained
by differences in jet stability.Comment: 1 LaTex (aastex) file, 21 postscript figure files, 2 external LaTex
table files. To appear in the Astrophysical Journa
Multiwaveband Observations of Quasars with Flat Radio Spectra and Strong Millimeter Emission
We present multiwaveband observations of a well selected sample of 28 quasars
and two radio galaxies with flat radio spectra and strong millimeter wave
emission (referred to here as FSRQ's). The observations include multifrequency
VLBI measurements, X-ray observations with ROSAT and submillimeter observations
with the JCMT. Particularly interesting among many findings is a correlation
between the X-ray to millimeter spectral index and fraction of flux density
contained in the VLBI core. This tendency toward higher X-ray fluxes from
sources with stronger jet emission implies that the knots in the jet are the
prominent source of X-rays.Comment: 38 pages, 17 figures, 12 tables, accepted for publication in Ap J
Suppl, May 199
Fallout nuclides in Atlantic and Pacific water columns : GEOSECS data
This report contains results of measurements of the fallout radionuclides
90Sr, 137Cs , 239,240Pu , and 241Am in large volume seawater samples collected
between 1972 and 1974 in the Atlantic and Pacific as part of Geochemical Ocean
Sections (GEOSECS) program. The stations for which data are reported include
both the North and South Atlantic oceans and latitudes north of 20° S in the
Pacific Ocean. The 90Sr and 137Cs data set has been corrected by a procedure
which estimates independently the analytical blank for the laboratory
which made the analysis. When the data quality and spacing permit, water column
inventory estimates were made for each nuclide over depth intervals appropriate
to the nuclide's distribution.Funding was provided by the United States Department of Energy
under Contract DE-AC02-EV03563
Trigonometric Parallaxes of Central Stars of Planetary Nebulae
Trigonometric parallaxes of 16 nearby planetary nebulae are presented,
including reduced errors for seven objects with previous initial results and
results for six new objects. The median error in the parallax is 0.42 mas, and
twelve nebulae have parallax errors less than 20 percent. The parallax for
PHL932 is found here to be smaller than was measured by Hipparcos, and this
peculiar object is discussed. Comparisons are made with other distance
estimates. The distances determined from these parallaxes tend to be
intermediate between some short distance estimates and other long estimates;
they are somewhat smaller than estimated from spectra of the central stars.
Proper motions and tangential velocities are presented. No astrometric
perturbations from unresolved close companions are detected.Comment: 24 pages, includes 4 figures. Accepted for A
Granulocyte Macrophage Colony-Stimulating Factor: A New Putative Therapeutic Target in Multiple Sclerosis
Experimental autoimmune encephalomyelitis (EAE), a model for multiple sclerosis, can be induced by immunization with a number of myelin antigens. In particular, myelin oligodendrocyte glycoprotein, a central nervous system (CNS)-specific antigen expressed on the myelin surface, is able to induce a paralytic MS-like disease with extensive CNS inflammation and demyelination in several strains of animals. Although not well understood, the egress of immune cells into the CNS in EAE is governed by a complex interplay between pro and antiinflammatory cytokines and chemokines. The hematopoietic growth factor, granulocyte macrophage colony-stimulating factor (GM-CSF), is considered to play a central role in maintaining chronic inflammation. The present study was designed to investigate the previously unexplored role of GM-CSF in autoimmune-mediated demyelination. GM-CSF−/− mice are resistant to EAE, display decreased antigen-specific proliferation of splenocytes, and fail to sustain immune cell infiltrates in the CNS, thus revealing key activities for GM-CSF in the development of inflammatory demyelinating lesions and control of migration and/or proliferation of leukocytes within the CNS. These results hold implications for the pathogenesis of inflammatory and demyelinating diseases and may provide the basis for more effective therapies for inflammatory diseases, and more specifically for multiple sclerosis
Sympatric ecological speciation meets pyrosequencing: sampling the transcriptome of the apple maggot Rhagoletis pomonella
Background
The full power of modern genetics has been applied to the study of speciation in only a small handful of genetic model species - all of which speciated allopatrically. Here we report the first large expressed sequence tag (EST) study of a candidate for ecological sympatric speciation, the apple maggot Rhagoletis pomonella, using massively parallel pyrosequencing on the Roche 454-FLX platform. To maximize transcript diversity we created and sequenced separate libraries from larvae, pupae, adult heads, and headless adult bodies. Results
We obtained 239,531 sequences which assembled into 24,373 contigs. A total of 6810 unique protein coding genes were identified among the contigs and long singletons, corresponding to 48% of all known Drosophila melanogaster protein-coding genes. Their distribution across GO classes suggests that we have obtained a representative sample of the transcriptome. Among these sequences are many candidates for potential R. pomonella speciation genes (or barrier genes ) such as those controlling chemosensory and life-history timing processes. Furthermore, we identified important marker loci including more than 40,000 single nucleotide polymorphisms (SNPs) and over 100 microsatellites. An initial search for SNPs at which the apple and hawthorn host races differ suggested at least 75 loci warranting further work. We also determined that developmental expression differences remained even after normalization; transcripts expected to show different expression levels between larvae and pupae in D. melanogaster also did so in R. pomonella. Preliminary comparative analysis of transcript presences and absences revealed evidence of gene loss in Drosophila and gain in the higher dipteran clade Schizophora. Conclusions
These data provide a much needed resource for exploring mechanisms of divergence in this important model for sympatric ecological speciation. Our description of ESTs from a substantial portion of the R. pomonella transcriptome will facilitate future functional studies of candidate genes for olfaction and diapause-related life history timing, and will enable large scale expression studies. Similarly, the identification of new SNP and microsatellite markers will facilitate future population and quantitative genetic studies of divergence between the apple and hawthorn-infesting host races
Applying Decision-Theory Framework to Landscape Planning for Biodiversity: Follow-up to Watson et al
Because socioeconomic factors drive conservation planning, we believe that to be relevant to on-the-ground projects, conservation science should be focused more on formulating problems explicitly and showing how the broad variety of decision-making tools can be used to deliver solutions. Conservation biology cannot operate outside the reality of financial limitations
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