398,076 research outputs found
The Hawaii Infrared Parallax Program. I. Ultracool Binaries and the L/T Transition
We present the first results from our high-precision infrared (IR) astrometry
program at the Canada-France-Hawaii Telescope. We measure parallaxes for 83
ultracool dwarfs (spectral types M6--T9) in 49 systems, with a median
uncertainty of 1.1 mas (2.3%) and as good as 0.7 mas (0.8%). We provide the
first parallaxes for 48 objects in 29 systems, and for another 27 objects in 17
systems, we significantly improve upon published results, with a median (best)
improvement of 1.7x (5x). Three systems show astrometric perturbations
indicative of orbital motion; two are known binaries (2MASSJ0518-2828AB and
2MASSJ1404-3159AB) and one is spectrally peculiar (SDSSJ0805+4812). In
addition, we present here a large set of Keck adaptive optics imaging that more
than triples the number of binaries with L6--T5 components that have both
multi-band photometry and distances. Our data enable an unprecedented look at
the photometric properties of brown dwarfs as they cool through the L/T
transition. Going from \approxL8 to \approxT4.5, flux in the Y and J bands
increases by \approx0.7 mag and \approx0.5 mag, respectively (the Y- and J-band
"bumps"), while flux in the H, K, and L' bands declines monotonically. This
wavelength dependence is consistent with cloud clearing over a narrow range of
temperature, since condensate opacity is expected to dominate at 1.0--1.3
micron. Interestingly, despite more than doubling the near-IR census of L/T
transition objects, we find a conspicuous paucity of objects on the
color--magnitude diagram just blueward of the late-L/early-T sequence. This
"L/T gap" occurs at MKO(J-H) = 0.1--0.3 mag, MKO(J-K) = 0.0--0.4 mag, and
implies that the last phases of cloud evolution occur rapidly. Finally, we
provide a comprehensive update to the absolute magnitudes of ultracool dwarfs
as a function of spectral type using a combined sample of 314 objects.Comment: Accepted to ApJ. New arXiv posting includes 4 new parallaxes and an
overall improvement in precision of 1.3x thanks to additional CFHT astrometry
for many targets. All data compiled in this paper (and more) are available
online: http://www.cfa.harvard.edu/~tdupuy/pl
Quantifying Confidence in DFT Predicted Surface Pourbaix Diagrams of Transition Metal Electrode-Electrolyte Interfaces
Density Functional Theory (DFT) calculations have been widely used to predict
the activity of catalysts based on the free energies of reaction intermediates.
The incorporation of the state of the catalyst surface under the
electrochemical operating conditions while constructing the free energy diagram
is crucial, without which even trends in activity predictions could be
imprecisely captured. Surface Pourbaix diagrams indicate the surface state as a
function of the pH and the potential. In this work, we utilize error-estimation
capabilities within the BEEF-vdW exchange correlation functional as an ensemble
approach to propagate the uncertainty associated with the adsorption energetics
in the construction of Pourbaix diagrams. Within this approach,
surface-transition phase boundaries are no longer sharp and are therefore
associated with a finite width. We determine the surface phase diagram for
several transition metals under reaction conditions and electrode potentials
relevant for the Oxygen Reduction Reaction (ORR). We observe that our surface
phase predictions for most predominant species are in good agreement with
cyclic voltammetry experiments and prior DFT studies. We use the OH
intermediate for comparing adsorption characteristics on Pt(111), Pt(100),
Pd(111), Ir(111), Rh(111), and Ru(0001) since it has been shown to have a
higher prediction efficiency relative to O, and find the trend
Ru>Rh>Ir>Pt>Pd for (111) metal facets, where Ru binds OH the strongest. We
robustly predict the likely surface phase as a function of reaction conditions
by associating c-values to quantifying the confidence in predictions within the
Pourbaix diagram. We define a confidence quantifying metric using which certain
experimentally observed surface phases and peak assignments can be better
rationalized.Comment: 21 pages, 8 figures and Supporting Informatio
Information hiding and retrieval in Rydberg wave packets using half-cycle pulses
We demonstrate an information hiding and retrieval scheme with the relative
phases between states in a Rydberg wave packet acting as the bits of a data
register. We use a terahertz half-cycle pulse (HCP) to transfer phase-encoded
information from an optically accessible angular momentum manifold to another
manifold which is not directly accessed by our laser pulses, effectively hiding
the information from our optical interferometric measurement techniques. A
subsequent HCP acting on these wave packets reintroduces the information back
into the optically accessible data register manifold which can then be `read'
out.Comment: 4 pages, 4 figure
A low cost scheme for high precision dual-wavelength laser metrology
A novel method capable of delivering relative optical path length metrology
with nanometer precision is demonstrated. Unlike conventional dual-wavelength
metrology which employs heterodyne detection, the method developed in this work
utilizes direct detection of interference fringes of two He-Ne lasers as well
as a less precise stepper motor open-loop position control system to perform
its measurement. Although the method may be applicable to a variety of
circumstances, the specific application where this metrology is essential is in
an astrometric optical long baseline stellar interferometer dedicated to
precise measurement of stellar positions. In our example application of this
metrology to a narrow-angle astrometric interferometer, measurement of
nanometer precision could be achieved without frequency-stabilized lasers
although the use of such lasers would extend the range of optical path length
the metrology can accurately measure. Implementation of the method requires
very little additional optics or electronics, thus minimizing cost and effort
of implementation. Furthermore, the optical path traversed by the metrology
lasers is identical with that of the starlight or science beams, even down to
using the same photodetectors, thereby minimizing the non-common-path between
metrology and science channels.Comment: 17 pages, 4 figures, accepted for publication in Applied Optic
Optimized Double-well quantum interferometry with Gaussian squeezed-states
A Mach-Zender interferometer with a gaussian number-difference squeezed input
state can exhibit sub-shot-noise phase resolution over a large phase-interval.
We obtain the optimal level of squeezing for a given phase-interval
and particle number , with the resulting phase-estimation
uncertainty smoothly approaching as approaches 10/N,
achieved with highly squeezed states near the Fock regime. We then analyze an
adaptive measurement scheme which allows any phase on to be
measured with a precision of requiring only a few measurements, even
for very large . We obtain an asymptotic scaling law of , resulting in a final
precision of . This scheme can be readily implemented in a
double-well Bose-Einstein condensate system, as the optimal input states can be
obtained by adiabatic manipulation of the double-well ground state.Comment: updated versio
Model independent sum rules for B-> pi K decays
We provide a set of sum rules relating CP-averaged branching ratios and
CP-asymmetries of the modes. They prove to be useful as a
mechanism to `test' experimental data given our expectations of the size of
isospin breaking. A set of observables emerges providing a simpler
interpretation of data in terms of isospin breaking. Moreover, the derivation
is done in a completely model independent way, i.e., they can accommodate also
New Physics contributions.Comment: 17 pages, 9 figure
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