14,244 research outputs found
A search for solar-like oscillations in the Am star HD 209625
The goal is to test the structure of hot metallic stars, and in particular
the structure of a near-surface convection zone using asteroseismic
measurements. Indeed, stellar models including a detailed treatement of the
radiative diffusion predict the existence of a near-surface convection zone in
order to correctly reproduce the anomalies in surface abundances that are
observed in Am stars. The Am star HD 209625 was observed with the Harps
spectrograph mounted on the 3.6-m telescope at the ESO La Silla Observatory
(Chile) during 9 nights in August 2005. This observing run allowed us to
collect 1243 radial velocity (RV) measurements, with a standard deviation of
1.35 m/s. The power spectrum associated with these RV measurements does not
present any excess. Therefore, either the structure of the external layers of
this star does not allow excitation of solar-like oscillations, or the
amplitudes of the oscillations remain below 20-30 cm/s (depending on their
frequency range).Comment: 5 pages, 4 figures, A&A accepte
Graphene for Antenna Applications: Opportunities and Challenges from Microwaves to THz
The use of graphene for antennas and other electromagnetic passives could
bring significant benefit such as extreme miniaturization, monolithic
integration with graphene RF nanoelectronics, efficient dynamic tuning, and
even transparency and mechanical flexibility. Though recently different related
theoretical works have been presented, relatively few applications have been
proposed and realistically assessed. In this invited talk we will briefly
review the main properties of graphene and the state of the art in its
theoretical and experimental characterization. Then, we will discuss a number
of potential antenna applications from microwave to THz, providing in each case
a critical assessment of the benefits, limitations, and remaining issues
towards actual real-life implementations. Here we provide a brief overview of
different devices and associated developments in our group discussed in the
talk, including graphene antennas and reflectarrays at microwave and THz,
plasmonic switches, isotropic and anisotropic meta-surfaces, or graphene
RF-NEMS
Review of: Amer EI-Ahraf et al., The Impact of Public Policy on Environmental Quality and Health: The Case of Land Use Management and Planning
Review of the book The Impact of Public Policy on Environmental Quality and Health: The Case of Land Use Management and Planning by Amer El-Ahraf et al., (Quorum Books 1999). Acknowledgments, introduction, appendixes, selected bibliography, index. ISBN 1-56720-065-6 [181 pp. $59.95. Hardbound, 88 Post Road West, Westport, CT 06881]
Asteroseismology of the visual binary 70 Ophiuchi
Convection in stars excites resonant acoustic waves. The frequencies of these
oscillations depend on the sound speed inside the star, which in turn depends
on density, temperature, gas motion, and other properties of the stellar
interior. Therefore, analysis of the oscillations provides an unrivaled method
to probe the internal structure of a star. Solar-like oscillations in the
primary of the visual binary 70 Ophiuchi are investigated. 70 Ophiuchi A was
observed with the Harps spectrograph mounted on the 3.6-m telescope at the ESO
La Silla Observatory (Chile) during 6 nights in July 2004 allowing us to
collect 1758 radial velocity measurements with a standard deviation of about
1.39 m s-1. The power spectrum of the high precision velocity time series
clearly presents several identifiable peaks between 3 and 6 mHz showing
regularity with a large spacing of Delta_nu = 161.7 +- 0.3 uHz. Fourteen
individual modes were identified with amplitudes in the range 11 to 14 cm s-1.Comment: 5 pages, A&A in pres
Americans' Access to Prescription Drugs Stabilizes, 2007-2010
Examines trends in the percentage of Americans reporting difficulty affording prescription drugs by insurance status, age group, income, and health status. Considers contributing factors such as changes in the uninsured population and fewer doctor visits
MEMS-reconfigurable metamaterials and antenna applications
This paper reviews some of our contributions to reconfigurable metamaterials,
where dynamic control is enabled by micro-electro-mechanical systems (MEMS)
technology. First, we show reconfigurable composite right/left handed
transmission lines (CRLH-TLs) having state of the art phase velocity variation
and loss, thereby enabling efficient reconfigurable phase shifters and
leaky-wave antennas (LWA). Second, we present very low loss metasurface designs
with reconfigurable reflection properties, applicable in reflectarrays and
partially reflective surface (PRS) antennas. All the presented devices have
been fabricated and experimentally validated. They operate in X- and Ku-bands.Comment: 8 pages; 8 figures; International Journal of Antennas and Propagatio
Soil mechanics on the Moon, Mars, and Mulberry
From a soil mechanics point of view, the Moon is a relatively simple place. Without any water, organics, or clay minerals, the geotechnical properties of the lunar soil are confined to a fairly limited range. Furthermore, the major soil-forming agent is meteorite impact, which breaks the big particles into little particles; and simultaneously, cements the little particles back together again with molten glass. After about a hundred million years of exposure to meteorite impact, the distribution of particle sizes in the soil achieves a sort of steady state. The majority of the returned lunar soil samples have been found to be well-graded silty-sand to sandy-silt (SM in the Unified Soil Classification System). Each of the particle size distributions plots within a relatively narrow band, which appears to be uniform over the entire lunar surface. This further restricts the range of physical properties of the lunar surface. In contrast, Martian soils should exhibit an extremely wide range of properties. We already know that there is a small amount of water in the soil, greater than in the Martian atmosphere. Furthermore, the soil is suspected to be smectitic clay. That makes two out of the three factors that greatly affect the properties of terrestrial soils
Reconfigurable Reflectarrays and Array Lenses for Dynamic Antenna Beam Control: A Review
Advances in reflectarrays and array lenses with electronic beam-forming
capabilities are enabling a host of new possibilities for these
high-performance, low-cost antenna architectures. This paper reviews enabling
technologies and topologies of reconfigurable reflectarray and array lens
designs, and surveys a range of experimental implementations and achievements
that have been made in this area in recent years. The paper describes the
fundamental design approaches employed in realizing reconfigurable designs, and
explores advanced capabilities of these nascent architectures, such as
multi-band operation, polarization manipulation, frequency agility, and
amplification. Finally, the paper concludes by discussing future challenges and
possibilities for these antennas.Comment: 16 pages, 12 figure
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