491 research outputs found
Adaptive and non-adaptive group sequential tests
Adaptive and non-adaptive group sequential test
Meta-analyses and adaptive group sequential designs in the clinical development process
Meta-analyses and adaptive group sequential designs in the clinical development proces
Displacement- and Timing-Noise Free Gravitational-Wave Detection
Motivated by a recently-invented scheme of displacement-noise-free
gravitational-wave detection, we demonstrate the existence of
gravitational-wave detection schemes insusceptible to both displacement and
timing (laser) noises, and are thus realizable by shot-noise-limited laser
interferometry. This is possible due to two reasons: first, gravitational waves
and displacement disturbances contribute to light propagation times in
different manners; second, for an N-detector system, the number of signal
channels is of the order O(N^2), while the total number of timing- and
displacement-noise channels is of the order O(N).Comment: 4 pages, 3 figures; mistake correcte
Boundary crossing Random Walks, clinical trials and multinomial sequential estimation
A sufficient condition for the uniqueness of multinomial sequential unbiased
estimators is provided generalizing a classical result for binomial samples.
Unbiased estimators are applied to infer the parameters of multidimensional or
multinomial Random Walks which are observed until they reach a boundary. An
application to clinical trials is presented
Optimal sequential fingerprinting: Wald vs. Tardos
We study sequential collusion-resistant fingerprinting, where the
fingerprinting code is generated in advance but accusations may be made between
rounds, and show that in this setting both the dynamic Tardos scheme and
schemes building upon Wald's sequential probability ratio test (SPRT) are
asymptotically optimal. We further compare these two approaches to sequential
fingerprinting, highlighting differences between the two schemes. Based on
these differences, we argue that Wald's scheme should in general be preferred
over the dynamic Tardos scheme, even though both schemes have their merits. As
a side result, we derive an optimal sequential group testing method for the
classical model, which can easily be generalized to different group testing
models.Comment: 12 pages, 10 figure
Direct Detection of the Brown Dwarf GJ 802B with Adaptive Optics Masking Interferometry
We have used the Palomar 200" Adaptive Optics (AO) system to directly detect
the astrometric brown dwarf GJ 802B reported by Pravdo et al. 2005. This
observation is achieved with a novel combination of aperture masking
interferometry and AO. The dynamical masses are 0.1750.021 M and
0.0640.032 M for the primary and secondary respectively. The
inferred absolute H band magnitude of GJ 802B is M=12.8 resulting in a
model-dependent T of 1850 50K and mass range of
0.057--0.074 M.Comment: 4 Pages, 5 figures, emulateapj format, submitted to ApJ
Recommended from our members
Auger spectroscopy and electronically-stimulated surface processes
Electronic excitations in adsorbate layers stimulate desorption and dissociation of adsorbed molecules as well as chemical reactions between adsorbates. The highest-probability stimulated processes produce neutral desorbates and determine how surface composition is altered by electron or photon radiation. A basic understanding has emerged, due largely to laser resonance-enhanced multi-photon ionization (REMPI) experiments, which provide quantum-state resolution of the gas-phase products. Auger phenomena enter this understanding in several ways. For example, CVV Auger spectroscopy determines the screened hole-hole interaction, U, in adsorbates, which in turn provides insight into the degree of charge-transfer screening from the substrate. In those systems where screening charge is used in excitation Auger decay, screening directly determines the lifetime, which in turn can exponentially affect the yield. Reductions in screening, e.g. induced by coadsorption of electro-negative species, thus can result in giant yield enhancements. As separate issues, a finite U may prevent the fast resonant decay and thus increase the yield from two-hole excitations, as has been suggested for NO{sub 2} dissociation on Pt (111), or may assist in the localization (self-trapping) of two-hole excitations in dense adsorbate layers, as apparently is the case for NO desorption from the same surface. The latter causes the yields from one- and two-hole excitations to differ in their coverage dependence. Finally, CVV Auger spectroscopy, of course, measures the energies of two-hole excitations, which can be correlated with observed stimulated thresholds. 27 refs., 15 figs
Metastable precursors during the oxidation of the Ru(0001) surface
Using density-functional theory, we predict that the oxidation of the
Ru(0001) surface proceeds via the accumulation of sub-surface oxygen in
two-dimensional islands between the first and second substrate layer. This
leads locally to a decoupling of an O-Ru-O trilayer from the underlying metal.
Continued oxidation results in the formation and stacking of more of these
trilayers, which unfold into the RuO_2(110) rutile structure once a critical
film thickness is exceeded. Along this oxidation pathway, we identify various
metastable configurations. These are found to be rather close in energy,
indicating a likely lively dynamics between them at elevated temperatures,
which will affect the surface chemical and mechanical properties of the
material.Comment: 11 pages including 9 figures. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
The last gasps of VY CMa: Aperture synthesis and adaptive optics imagery
We present new observations of the red supergiant VY CMa at 1.25 micron, 1.65
micron, 2.26 micron, 3.08 micron and 4.8 micron. Two complementary
observational techniques were utilized: non-redundant aperture masking on the
10-m Keck-I telescope yielding images of the innermost regions at unprecedented
resolution, and adaptive optics imaging on the ESO 3.6-m telescope at La Silla
attaining extremely high (~10^5) peak-to-noise dynamic range over a wide field.
For the first time the inner dust shell has been resolved in the near-infrared
to reveal a one-sided extension of circumstellar emission within 0.1" (~15
R_star) of the star. The line-of-sight optical depths of the circumstellar dust
shell at 1.65 micron, 2.26 micron, and 3.08 micron have been estimated to be
1.86 +/- 0.42, 0.85 +/- 0.20, and 0.44 +/- 0.11. These new results allow the
bolometric luminosity of VY~CMa to be estimated independent of the dust shell
geometry, yielding L_star ~ 2x10^5 L_sun. A variety of dust condensations,
including a large scattering plume and a bow-shaped dust feature, were observed
in the faint, extended nebula up to 4" from the central source. While the
origin of the nebulous plume remains uncertain, a geometrical model is
developed assuming the plume is produced by radially-driven dust grains forming
at a rotating flow insertion point with a rotational period between 1200-4200
years, which is perhaps the stellar rotational period or the orbital period of
an unseen companion.Comment: 25 pages total with 1 table and 5 figures. Accepted by Astrophysical
Journal (to appear in February 1999
Opacity in compact extragalactic radio sources and its effect on astrophysical and astrometric studies
The apparent position of the "core" in a parsec-scale radio jet (a compact,
bright emitting region at the narrow end of the jet) depends on the observing
frequency, owing to synchrotron self-absorption and external absorption. While
providing a tool probing physical conditions in the vicinity of the core, this
dependency poses problems for astrometric studies using compact radio sources.
We investigated the frequency-dependent shift in the positions of the cores
(core shift) observed with very long baseline interferometry (VLBI) in
parsec-scale jets. We discuss related physics, as well as its effect on radio
astrometry and the connection between radio and optical positions of
astrometric reference objects. We searched for the core shift in a sample of
277 radio sources imaged at 2.3 GHz (13 cm) and 8.6 GHz (4 cm) frequency bands
using VLBI observations made in 2002 and 2003. The core shift was measured by
referencing the core position to optically thin jet features whose positions
are not expected to change with frequency. We present here results for 29
selected active galactic nuclei (AGN) with bright distinct VLBI jet features
that can be used in differential measurements and that allow robust
measurements of the shift to be made. In these AGN, the magnitude of the
measured core shift between 2.3 and 8.6 GHz reaches 1.4 mas, with a median
value for the sample of 0.44 mas. Nuclear flares result in temporal variability
of the shift. An average shift between the radio (4 cm) and optical (6000
Angstrom) bands is estimated to be approximately 0.1 mas, and it should be
taken into account in order to provide the required accuracy of the
radio-optical reference frame connection. This can be accomplished with
multi-frequency VLBI measurements... (abridged)Comment: 11 pages, 8 figures, 2 tables, accepted for publication in the
Astronomy and Astrophysics; minor corrections to the manuscript are mad
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