7,771 research outputs found
Stability control of nonlinear micromechanical resonators under simultaneous primary and superharmonic resonances
Fast effects of a slow excitation on the main resonance of a nonlinear micromechanical resonator are analytically and experimentally investigated. We show, in particular, how the bifurcation topology of an undesirable unstable behavior is modified when the resonator is simultaneously actuated at its primary and superharmonic resonances. A stabilization mechanism is proposed and demonstrated by increasing the superharmonic excitation
The carbon abundance in two h 2 regions of the small Megallanic Cloud
Observations of the ultraviolet spectra of two locations in the H II region NGC 346 and of the entire H II region IC 1644 in the Small Magellanic Cloud (SMC) were made using the International Ultraviolet Explorer (IUE) satellite. The abundance of carbon in the nebulae was derived using theoretical model analysis combined with ground-based spectrophotometry of other emission lines. The abundance of C relative to H in the SMC was found to be lower by -0.9 dex compared with the Sun and lower by -0.8 dex compared with the Orion Nebula. This C deficiency is similar to that of O, Ne, S, and Ar in the SMC, but not as great as found for N. The sites and history of C nucleosynthesis in galaxies is similar to that of O, Ne, S, and Ar, in contrast to that of N, which appears to be more complex, perhaps because of a mixture of secondary primary sources or a significant contribution from intermediate-mass long-lived stars
Casimir-Polder shifts on quantum levitation states
An ultracold atom above a horizontal mirror experiences quantum reflection
from the attractive Casimir-Polder interaction, which holds it against gravity
and leads to quantum levitation states. We analyze this system by using a
Liouville transformation of the Schr\"odinger equation and a Langer coordinate
adapted to problems with a classical turning point. Reflection on the
Casimir-Polder attractive well is replaced by reflection on a repulsive wall
and the problem is then viewed as an ultracold atom trapped inside a cavity
with gravity and Casimir-Polder potentials acting respectively as top and
bottom mirrors. We calculate numerically Casimir-Polder shifts of the energies
of the cavity resonances and propose a new approximate treatment which is
precise enough to discuss spectroscopy experiments aiming at tests of the weak
equivalence principle on antihydrogen. We also discuss the lifetimes by
calculating complex energies associated with cavity resonances.Comment: Accepted in PR
Land Use Policy in Local Historic Districts and Incentives for Compliance in the Vieux Carré
The Vieux Carré in New Orleans is the second oldest locally designated historic district and serves as a prominent example of local historic preservation efforts; however, the Vieux Carré has a high vacancy rate. This thesis examines the effects of land use policy, including the Comprehensive Zoning Ordinance and the design review process in the Vieux Carré, on attempts to return vacant buildings to commerce. The author examines three cases of redevelopment attempts of vacant properties in the Vieux Carré. In two of the three cases, constrictions from the Zoning Ordinance not the design review process held up the redevelopment process. The other case identified owner negligence as the cause for failure in the redevelopment attempt
Land Use Policy in Local Historic Districts and Incentives for Compliance in the Vieux Carré
The Vieux Carré in New Orleans is the second oldest locally designated historic district and serves as a prominent example of local historic preservation efforts; however, the Vieux Carré has a high vacancy rate. This thesis examines the effects of land use policy, including the Comprehensive Zoning Ordinance and the design review process in the Vieux Carré, on attempts to return vacant buildings to commerce. The author examines three cases of redevelopment attempts of vacant properties in the Vieux Carré. In two of the three cases, constrictions from the Zoning Ordinance not the design review process held up the redevelopment process. The other case identified owner negligence as the cause for failure in the redevelopment attempt
Accretion and activity on the post-common-envelope binary RR~Cae
Current scenarios for the evolution of interacting close binaries - such as
cataclysmic variables (CVs) - rely mainly on our understanding of low-mass star
angular momentum loss (AML) mechanisms. The coupling of stellar wind with its
magnetic field, i.e., magnetic braking, is the most promising mechanism to
drive AML in these stars. There are basically two properties driving magnetic
braking: the stellar magnetic field and the stellar wind. Understanding the
mechanisms that drive AML therefore requires a comprehensive understanding of
these two properties. RRCae is a well-known nearby (d=20pc) eclipsing DA+M
binary with an orbital period of P=7.29h. The system harbors a metal-rich cool
white dwarf (WD) and a highly active M-dwarf locked in synchronous rotation.
The metallicity of the WD suggests that wind accretion is taking place, which
provides a good opportunity to obtain the mass-loss rate of the M-dwarf
component. We analyzed multi-epoch time-resolved high-resolution spectra of
RRCae in search for traces of magnetic activity and accretion. We selected a
number of well-known activity indicators and studied their short and long-term
behavior. Indirect-imaging tomographic techniques were also applied to provide
the surface brightness distribution of the magnetically active M-dwarf, and
reveals a polar feature similar to those observed in fast-rotating solar-type
stars. The blue part of the spectrum was modeled using a atmosphere model to
constrain the WD properties and its metal enrichment. The latter was used to
improve the determination of the mass-accretion rate from the M-dwarf wind. The
presence of metals in the WD spectrum suggests that this component arises from
accretion of the M-dwarf wind. A model fit to the WD gives Teff=(7260+/-250)K
and logg=(7.8+/-0.1) dex with a metallicity of =(-2.8+/-0.1)dex,
and a mass-accretion rate of dotMacc=(7+/-2)x1e-16Msun/yr.Comment: 14 pages, 7 Figures, 6 Table
Precise Atmospheric Parameters for the Shortest Period Binary White Dwarfs: Gravitational Waves, Metals, and Pulsations
We present a detailed spectroscopic analysis of 61 low mass white dwarfs and
provide precise atmospheric parameters, masses, and updated binary system
parameters based on our new model atmosphere grids and the most recent
evolutionary model calculations. For the first time, we measure systematic
abundances of He, Ca and Mg for metal-rich extremely low mass white dwarfs and
examine the distribution of these abundances as a function of effective
temperature and mass. Based on our preliminary results, we discuss the
possibility that shell flashes may be responsible for the presence of the
observed He and metals. We compare stellar radii derived from our spectroscopic
analysis to model-independent measurements and find good agreement except for
those white dwarfs with Teff < 10,000 K. We also calculate the expected
gravitational wave strain for each system and discuss their significance to the
eLISA space-borne gravitational wave observatory. Finally, we provide an update
on the instability strip of extremely low mass white dwarf pulsators.Comment: 18 pages, 13 figures, 3 tables, accepted for publication in Ap
Quantum reflection of antihydrogen from nanoporous media
We study quantum reflection of antihydrogen atoms from nanoporous media due
to the Casimir-Polder (CP) potential. Using a simple effective medium model, we
show a dramatic increase of the probability of quantum reflection of
antihydrogen atoms if the porosity of the medium increases. We discuss the
limiting case of reflections at small energies, which have interesting
applications for trapping and guiding antihydrogen using material walls
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