194 research outputs found
Oscillatory and localized perturbations of periodic structures and the bifurcation of defect modes
Let denote a periodic function on the real line. The Schr\"odinger
operator, , has spectrum equal to
the union of closed real intervals separated by open spectral gaps. In this
article we study the bifurcation of discrete eigenvalues (point spectrum) into
the spectral gaps for the operator , where is
spatially localized and highly oscillatory in the sense that its Fourier
transform, is concentrated at high frequencies. Our
assumptions imply that may be pointwise large but is
small in an average sense. For the special case where
with smooth, real-valued, localized in
, and periodic or almost periodic in , the bifurcating eigenvalues are at
a distance of order from the lower edge of the spectral gap. We
obtain the leading order asymptotics of the bifurcating eigenvalues and
eigenfunctions. Underlying this bifurcation is an effective Hamiltonian
associated with the lower edge of the spectral band:
where is the Dirac distribution,
and effective-medium parameters are
explicit and independent of . The potentials we consider are a
natural model for wave propagation in a medium with localized, high-contrast
and rapid fluctuations in material parameters about a background periodic
medium.Comment: To appear in SIAM Journal on Mathematical Analysi
Homogenized description of defect modes in periodic structures with localized defects
A spatially localized initial condition for an energy-conserving wave
equation with periodic coefficients disperses (spatially spreads) and decays in
amplitude as time advances. This dispersion is associated with the continuous
spectrum of the underlying differential operator and the absence of discrete
eigenvalues. The introduction of spatially localized perturbations in a
periodic medium leads to defect modes, states in which energy remains trapped
and spatially localized. In this paper we study weak, localized perturbations
of one-dimensional periodic Schr\"odinger operators. Such perturbations give
rise to such defect modes, and are associated with the emergence of discrete
eigenvalues from the continuous spectrum. Since these isolated eigenvalues are
located near a spectral band edge, there is strong scale-separation between the
medium period and the localization length of the defect mode. Bound states
therefore have a multi-scale structure: a "carrier Bloch wave" times a "wave
envelope", which is governed by a homogenized Schr\"odinger operator with
associated effective mass, depending on the spectral band edge which is the
site of the bifurcation. Our analysis is based on a reformulation of the
eigenvalue problem in Bloch quasi-momentum space, using the Gelfand-Bloch
transform and a Lyapunov-Schmidt reduction to a closed equation for the
near-band-edge frequency components of the bound state. A rescaling of the
latter equation yields the homogenized effective equation for the wave
envelope, and approximations to bifurcating eigenvalues and eigenfunctions.Comment: The title differs from version 1. To appear in Communications in
Mathematical Science
Acanthamoeba castellanii : growth on human cell layers reactivates attenuated properties after prolonged axenic culture
The free-living, but potentially pathogenic, bacteriovorous amoebae of the genus Acanthamoeba can be easily grown axenically in a laboratory culture. This, however, often leads to considerable losses in virulence, and encystment capacity, and to changes in drug susceptibility. We evaluated potential options for a reactivation of a number of physiological properties, attenuated by prolonged axenic laboratory culture, including encystment potential, protease activity, heat resistance, growth rates and drug susceptibility against N-chlorotaurine (NCT). Toward this end, a strain that had been grown axenically for 10 years was repeatedly passaged on human HEp-2 cell monolayers or treated with 5âČ-azacytidine (AzaC), a methyltransferase inhibitor, and trichostatin A (TSA), a histone deacetylase inhibitor, in order to uplift epigenetic gene regulation. Culture on human cell monolayers resulted in significantly enhanced encystment potentials and protease activities, and higher susceptibility against NCT, whereas the resistance against heat shock was not altered. Treatment with AzaC/TSA resulted in increased encystment rates and protease activities, indicating the participation of epigenetic mechanisms. However, lowered resistances against heat shock indicate that possible stress responses to AzaC/TSA have to be taken into account. Repeated growth on human cell monolayers appears to be a potential method to reactivate attenuated characteristics in Acanthamoeba
Wave operator bounds for 1-dimensional Schr\"odinger operators with singular potentials and applications
Boundedness of wave operators for Schr\"odinger operators in one space
dimension for a class of singular potentials, admitting finitely many Dirac
delta distributions, is proved. Applications are presented to, for example,
dispersive estimates and commutator bounds.Comment: 16 pages, 0 figure
Wave operator bounds for one-dimensional Schrödinger operators with singular potentials and applications
Boundedness of wave operators for Schrödinger operators in one space dimension for a class of singular potentials, admitting finitely many Dirac delta distributions, is proved. Applications are presented to, for example, dispersive estimates and commutator bounds
An Adaptive Optics Survey for Close Protostellar Binaries
In order to test the hypothesis that Class I protostellar binary stars are a
product of ejections during the dynamical decay of non-hierarchical multiple
systems, we combined the results of new adaptive optics (AO) observations of
Class I protostars with our previously published AO data to investigate whether
Class I protostars with a widely separated companion (r>200 AU) are more likely
to also have a close companion (r<200 AU). In total, we observed 47 embedded
young stellar objects (YSOs) with either the Subaru natural guide star AO
system or the Keck laser guide star AO system. We found that targets with a
widely separated companion within 5,000 AU are not more likely to have a close
companion. However, targets with another YSO within a projected separation of
25,000 AU are much more likely to have a close companion. Most importantly,
every target with a close companion has another YSO within a projected
separation of 25,000 AU. We came to the same conclusions after considering a
restricted sample of targets within 500 pc and close companions wider than 50
AU to minimize incompleteness effects. The Orion star forming region was found
to have an excess of both close binaries and YSOs within 25,000 AU compared to
other star forming regions. We interpret these observations as strong evidence
that many close Class I binary stars form via ejections and that many of the
ejected stars become unbound during the Class I phase.Comment: 22 pages, 3 figures, 2 table
The Evolution of the Multiplicity of Embedded Protostars II: Binary Separation Distribution & Analysis
We present the Class I protostellar binary separation distribution based on
the data tabulated in the companion paper. We verify the excess of Class I
binary stars over solar-type main-sequence stars, especially at separations
beyond 500 AU. Although our sources are in nearby star forming regions
distributed across the entire sky (including Orion), none of our objects are in
a high stellar density environment. The binary separation distribution changes
significantly during the Class I phase, and the binary frequency at separations
greater than 1000 AU declines steadily with respect to spectral index. Despite
these changes, the binary frequency remains constant until the end of the Class
I phase, when it drops sharply. We propose a scenario to account for the
changes in the Class I binary separation distribution. This scenario postulates
that companions with a separation greater than ~1000 AU were ejected during the
Class 0 phase, but remain gravitationally bound due to the mass of the
envelope. As the envelope dissipates, these companions become unbound and the
binary frequency at wide separations declines. This scenario predicts that a
large number of Class 0 objects should be non-hierarchical multiple systems,
and that many Class I YSOs with a widely separated companion should also have a
very close companion. We also find that Class I protostars are not dynamically
pristine, and have experienced dynamical evolution before they are visible as
Class I objects. For the first time, evidence is presented showing that the
Class I binary frequency and the binary separation distribution strongly depend
on the star forming environment. The reason for this dependence remains
unclear.Comment: 33 pages, 16 figures, accepted by the Astronomical Journa
Disk and Envelope Structure in Class 0 Protostars: I. The Resolved Massive Disk in Serpens FIRS 1
We present the first results of a program to characterize the disk and
envelope structure of typical Class 0 protostars in nearby low-mass star
forming regions. We use Spitzer IRS mid-infrared spectra, high resolution CARMA
230 GHz continuum imaging, and 2-D radiative transfer models to constrain the
envelope structure, as well as the size and mass of the circum-protostellar
disk in Serpens FIRS 1. The primary envelope parameters (centrifugal radius,
outer radius, outflow opening angle, and inclination) are well constrained by
the spectral energy distribution (SED), including Spitzer IRAC and MIPS
photometry, IRS spectra, and 1.1 mm Bolocam photometry. These together with the
excellent uv-coverage (4.5-500 klam) of multiple antenna configurations with
CARMA allow for a robust separation of the envelope and a resolved disk. The
SED of Serpens FIRS 1 is best fit by an envelope with the density profile of a
rotating, collapsing spheroid with an inner (centrifugal) radius of
approximately 600 AU, and the millimeter data by a large resolved disk with
Mdisk~1.0 Msun and Rdisk~300 AU. These results suggest that large, massive
disks can be present early in the main accretion phase. Results for the larger,
unbiased sample of Class~0 sources in the Perseus, Serpens, and Ophiuchus
molecular clouds are needed to determine if relatively massive disks are
typical in the Class 0 stage.Comment: Comments: 13 pages, 8 figures, 3 tables; accepted for publication in
the Ap
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