7,705 research outputs found
Three-State Feshbach Resonances Mediated By Second-Order Couplings
We present an analytical study of three-state Feshbach resonances induced by
second-order couplings. Such resonances arise when the scattering amplitude is
modified by the interaction with a bound state that is not directly coupled to
the scattering state containing incoming flux. Coupling occurs indirectly
through an intermediate state. We consider two problems: (i) the intermediate
state is a scattering state in a distinct open channel; (ii) the intermediate
state is an off-resonant bound state in a distinct closed channel. The first
problem is a model of electric-field-induced resonances in ultracold collisions
of alkali metal atoms [Phys. Rev. A 75, 032709 (2007)] and the second problem
is relevant for ultracold collisions of complex polyatomic molecules, chemical
reaction dynamics, photoassociation of ultracold atoms, and electron - molecule
scattering. Our analysis yields general expressions for the energy dependence
of the T-matrix elements modified by three-state resonances and the dependence
of the resonance positions and widths on coupling amplitudes for the
weak-coupling limit. We show that the second problem can be generalized to
describe resonances induced by indirect coupling through an arbitrary number of
sequentially coupled off-resonant bound states and analyze the dependence of
the resonance width on the number of the intermediate states.Comment: 27 pages, 4 figures; added a reference; journal reference/DOI refer
to final published version, which is a shortened and modified version of this
preprin
Measuring forces between protein fibers by microscopy
We propose a general scheme for measuring the attraction between mechanically frustrated semiflexible fibers by measuring their thermal fluctuations and shape. We apply this analysis to a system of sickle hemoglobin (HbS) fibers that laterally attract one another. These fibers appear to āzipā together before reaching mechanical equilibrium due to the existence of cross-links into a dilute fiber network. We are also able to estimate the rigidities of the fibers. These rigidities are found to be consistent with sickle hemoglobin āsingleā fibers 20 nm in diameter, despite recent experiments indicating that fiber bundling sometimes occurs. Our estimate of the magnitude of the interfiber attraction for HbS fibers is in the range 8 Ā± 7 kBT/Ī¼m, or 4 Ā± 3 kBT/Ī¼m if the fibers are assumed, a priori to be single fibers (such an assumption is fully consistent with the data). This value is sufficient to bind the fibers, overcoming entropic effects, although extremely chemically weak. Our results are compared to models for the interfiber attraction that include depletion and van der Waals forces. This technique should also facilitate a similar analysis of other filamentous protein assembles in the future, including Ī²-amyloid, actin, and tubulin
Millimeter and Submillimeter Survey of the R Corona Australis Region
Using a combination of data from the Antarctic Submillimeter Telescope and
Remote Observatory (AST/RO), the Arizona Radio Observatory Kitt Peak 12m
telescope and the Arizona Radio Observatory 10m Heinrich Hertz Telescope, we
have studied the most active part of the R CrA molecular cloud in multiple
transitions of Carbon Monoxide, HCO and 870\micron continuum emission.
Since R CrA is nearby (130 pc), we are able to obtain physical spatial
resolution as high as 0.01pc over an area of 0.16 pc, with velocity
resolution finer than 1 km/s. Mass estimates of the protostar driving the
mm-wave emission derived from HCO, dust continuum emission and kinematic
techniques point to a young, deeply embedded protostar of 0.5-0.75
M, with a gaseous envelope of similar mass. A molecular outflow is
driven by this source that also contains at least 0.8 M of molecular
gas with 0.5 L of mechanical luminosity. HCO lines show the
kinematic signature of infall motions as well as bulk rotation. The source is
most likely a Class 0 protostellar object not yet visible at near-IR
wavelengths. With the combination of spatial and spectral resolution in our
data set, we are able to disentangle the effects of infall, rotation and
outflow towards this young object.Comment: 29 pages, 9 figures. Accepted for publication in the Astrophysical
Journa
Strategies towards statistically robust interpretations of in situ UāPb zircon geochronology
Zircon UāPb geochronology has become a keystone tool across Earth science, arguably providing the gold standard in resolving deep geological time. The development of rapid in situ analysis of zircon (via laser ablation and secondary ionization mass spectrometry) has allowed for large amounts of data to be generated in a relatively short amount of time and such large volume datasets offer the ability to address a range of geological questions that would otherwise remain intractable (e.g. detrital zircons as a sediment fingerprinting method). The ease of acquisition, while bringing benefit to the Earth science community, has also led to diverse interpretations of geochronological data. In this work we seek to refocus UāPb zircon geochronology toward best practice by providing a robust statistically coherent workflow. We discuss a range of data filtering approaches and their inherent limitations (e.g. discordance and the reduced chi-squared; MSWD). We evaluate appropriate mechanisms to calculate the most geologically appropriate age from both 238U/206Pb and 207Pb/206Pb ratios and demonstrate the cross over position when chronometric power swaps between these ratios. As our in situ analytical techniques become progressively more precise, appropriate statistical handing of UāPb datasets will become increasingly pertinent
Long range scattering resonances in strong-field seeking states of polar molecules
We present first steps toward understanding the ultracold scattering
properties of polar molecules in strong electric field-seeking states. We have
found that the elastic cross section displays a quasi-regular set of potential
resonances as a function of the electric field, which potentially offers
intimate details about the inter-molecular interaction. We illustrate these
resonances in a ``toy'' model composed of pure dipoles, and in more physically
realistic systems. To analyze these resonances, we use a simple WKB
approximation to the eigenphase, which proves both reasonably accurate and
meaningful. A general treatment of the Stark effect and dipolar interactions is
also presented
High Spatial Resolution Observations of Two Young Protostars in the R Corona Australis Region
We present multi-wavelength, high spatial resolution imaging of the IRS 7
region in the R Corona Australis molecular cloud. Our observations include 1.1
mm continuum and HCO^+ J = images from the SMA, ^{12}CO J =
outflow maps from the DesertStar heterodyne array receiver on the HHT, 450
m and 850 m continuum images from SCUBA, and archival Spitzer IRAC
and MIPS 24 \micron images. The accurate astrometry of the IRAC images allow us
to identify IRS 7 with the cm source VLA 10W (IRS 7A) and the X-ray source X_W.
The SMA 1.1 mm image reveals two compact continuum sources which are also
distinguishable at 450 m. SMA 1 coincides with X-ray source CXOU
J190156.4-365728 and VLA cm source 10E (IRS 7B) and is seen in the IRAC and
MIPS images. SMA 2 has no infrared counterpart but coincides with cm source VLA
9. Spectral energy distributions constructed from SMA, SCUBA and Spitzer data
yield bolometric temperatures of 83 K for SMA 1 and 70 K for SMA 2. These
temperatures along with the submillimeter to total luminosity ratios indicate
that SMA 2 is a Class 0 protostar, while SMA 1 is a Class 0/Class I
transitional object (L= \Lsun). The ^{12}CO J = outflow map
shows one major and possibly several smaller outflows centered on the IRS 7
region, with masses and energetics consistent with previous work. We identify
the Class 0 source SMA 2/VLA 9 as the main driver of this outflow. The complex
and clumpy spatial and velocity distribution of the HCO^+ J =
emission is not consistent with either bulk rotation, or any known molecular
outflow activity.Comment: 31 pages, 8 figures, Accepted to Ap
Shear-induced breaking of internal gravity waves
Motivated by observations of turbulence in the strongly stratified ocean
thermocline, we use direct numerical simulations to investigate the interaction
of a sinusoidal shear flow and a large-amplitude internal gravity wave. Despite
strong nonlinearities in the flow and a lack of scale separation, we find that
linear ray tracing theory is qualitatively useful in describing the early
development of the flow as the wave is refracted by the shear. Consistent with
the linear theory, the energy of the wave accumulates in regions of negative
mean shear where we observe evidence of convective and shear instabilities.
Streamwise-aligned convective rolls emerge the fastest, but their contribution
to irreversible mixing is dwarfed by shear-driven billow structures that
develop later. Although the wave strongly distorts the buoyancy field on which
these billows develop, the mixing efficiency of the subsequent turbulence is
similar to that arising from Kelvin-Helmholtz instability in a stratified shear
layer. We run simulations at Reynolds numbers of 5000 and 8000, and vary the
initial amplitude of the internal gravity wave. For high values of initial wave
amplitude, the results are qualitatively independent of . Smaller initial
wave amplitudes delay the onset of the instabilities, and allow for significant
laminar diffusion of the internal wave, leading to reduced turbulent activity.
We discuss the complex interaction between the mean flow, internal gravity wave
and turbulence, and its implications for internal wave-driven mixing in the
ocean.Comment: 27 pages, 12 figures, accepted to J. Fluid. Mec
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