1,565 research outputs found
Testing Planet Formation Models with Gaia as Astrometry
In this paper, we first summarize the results of a large-scale double-blind
tests campaign carried out for the realistic estimation of the Gaia potential
in detecting and measuring planetary systems. Then, we put the identified
capabilities in context by highlighting the unique contribution that the Gaia
exoplanet discoveries will be able to bring to the science of extrasolar
planets during the next decade.Comment: 4 pages, 1 figure. To appear in the proceedings of "IAU Symposium 248
- A Giant Step: from Milli- to Micro-arcsecond Astrometry", held in Shanghai,
China, 15-19 Oct. 200
Chromaticity in all-reflective telescopes for astrometry
Chromatic effects are usually associated with refractive optics, so
reflective telescopes are assumed to be free from them. We show that
all-reflective optics still bears significant levels of such perturbations,
which is especially critical to modern micro-arcsecond astrometric experiments.
We analyze the image formation and measurement process to derive a precise
definition of the chromatic variation of the image position, and we evaluate
the key aspects of optical design with respect to chromaticity. The fundamental
requirement related to chromaticity is the symmetry of the optical design and
of the wavefront errors. Finally, we address some optical engineering issues,
such as manufacturing and alignment, providing recommendations to minimize the
degradation that chromaticity introduces into astrometry.Comment: 10 pages, 8 figure
Marker based Thermal-Inertial Localization for Aerial Robots in Obscurant Filled Environments
For robotic inspection tasks in known environments fiducial markers provide a
reliable and low-cost solution for robot localization. However, detection of
such markers relies on the quality of RGB camera data, which degrades
significantly in the presence of visual obscurants such as fog and smoke. The
ability to navigate known environments in the presence of obscurants can be
critical for inspection tasks especially, in the aftermath of a disaster.
Addressing such a scenario, this work proposes a method for the design of
fiducial markers to be used with thermal cameras for the pose estimation of
aerial robots. Our low cost markers are designed to work in the long wave
infrared spectrum, which is not affected by the presence of obscurants, and can
be affixed to any object that has measurable temperature difference with
respect to its surroundings. Furthermore, the estimated pose from the fiducial
markers is fused with inertial measurements in an extended Kalman filter to
remove high frequency noise and error present in the fiducial pose estimates.
The proposed markers and the pose estimation method are experimentally
evaluated in an obscurant filled environment using an aerial robot carrying a
thermal camera.Comment: 10 pages, 5 figures, Published in International Symposium on Visual
Computing 201
Growth hormone plus resistance exercise attenuate structural changes in rat myotendinous junctions resulting from chronic unloading.
Myotendinous junctions (MTJs) are specialized sites on the muscle surface where forces generated by myofibrils are transmitted across the sarcolemma to the extracellular matrix. At the ultrastructural level, the interface between the sarcolemma and extracellular matrix is highly folded and interdigitated at these junctions. In this study, the effect of exercise and growth hormone (GH) treatments on the changes in MTJ structure that occur during muscle unloading, has been analyzed. Twenty hypophysectomized rats were assigned randomly to one of five groups: ambulatory control, hindlimb unloaded, hindlimb unloaded plus exercise (3 daily bouts of 10 climbs up a ladder with 50% body wt attached to the tail), hindlimb unloaded plus GH (2 daily injections of 1 mg/kg body wt, i.p.), and hindlimb unloaded plus exercise plus GH. MTJs of the plantaris muscle were analyzed by electron microscopy and the contact between muscle and tendon was evaluated using an IL/B ratio, where B is the base and IL is the interface length of MTJ's digit-like processes. After 10 days of unloading, the mean IL/B ratio was significantly lower in unloaded (3.92), unloaded plus exercise (4.18), and unloaded plus GH (5.25) groups than in the ambulatory control (6.39) group. On the opposite, the mean IL/B ratio in the group treated with both exercise and GH (7.3) was similar to control. These findings indicate that the interaction between exercise and GH treatments attenuates the changes in MTJ structure that result from chronic unloading and thus can be used as a countermeasure to these adaptations
Signatures of clumpy dark matter in the global 21 cm background signal
We examine the extent to which the self-annihilation of supersymmetric
neutralino dark matter, as well as light dark matter, influences the rate of
heating, ionisation and Lyman-alpha pumping of interstellar hydrogen and helium
and the extent to which this is manifested in the 21cm global background
signal. We fully consider the enhancements to the annihilation rate from DM
halos and substructures within them. We find that the influence of such
structures can result in significant changes in the differential brightness
temperature. The changes at redsfhits z<25 are likely to be undetectable due to
the presence of the astrophysical signal; however, in the most favourable
cases, deviations in the differential brightness temperature, relative to its
value in the absence of self-annihilating DM, of up to ~20 mK at z=30 can
occur. Thus we conclude that, in order to exclude these models, experiments
measuring the global 21cm signal, such as EDGES and CORE, will need to reduce
the systematics at 50 MHz to below 20 mK.Comment: V3: 32 pages, 14 figures, 4 tables. Replaced to match version
accepted for publication in PRD. Major revisions to address referee's
comment
Conformations of confined biopolymers
Nanoscale and microscale confinement of biopolymers naturally occurs in cells
and has been recently achieved in artificial structures designed for
nanotechnological applications. Here, we present an extensive theoretical
investigation of the conformations and shape of a biopolymer with varying
stiffness confined to a narrow channel. Combining scaling arguments, analytical
calculations, and Monte Carlo simulations, we identify various scaling regimes
where master curves quantify the functional dependence of the polymer
conformations on the chain stiffness and strength of confinement.Comment: 5 pages, 4 figures, minor correction
Fluctuation-Dissipation Theorem for the Microcanonical Ensemble
A derivation of the Fluctuation-Dissipation Theorem for the microcanonical
ensemble is presented using linear response theory. The theorem is stated as a
relation between the frequency spectra of the symmetric correlation and
response functions. When the system is not in the thermodinamic limit, this
result can be viewed as an extension of the fluctuation-dissipation relations
to a situation where dynamical fluctuations determine the response. Therefore,
the relation presented here between equilibrium fluctuations and response can
have a very different physical nature from the usual one in the canonical
ensemble. These considerations imply that the Fluctuation-Dissipation Theorem
is not restricted to the context of thermal equilibrium, where it is usually
derived. Dispersion relations and sum rules are also obtained and discussed in
the present case. Although analogous to the Kramers-Kronig relations, they are
not related to the frequency spectrum but to the energy dependence of the
response function.Comment: 15 pages, v3: final version, new text added, new reference
Controlling chaos in diluted networks with continuous neurons
Diluted neural networks with continuous neurons and nonmonotonic transfer
function are studied, with both fixed and dynamic synapses. A noisy stimulus
with periodic variance results in a mechanism for controlling chaos in neural
systems with fixed synapses: a proper amount of external perturbation forces
the system to behave periodically with the same period as the stimulus.Comment: 11 pages, 8 figure
Narrow-Angle Astrometry with the Space Interferometry Mission: The Search for Extra-Solar Planets. II. Detection and Characterization of Planetary Systems
(Abridged) The probability of detecting additional companions is essentially
unchanged with respect to the single-planet configurations, but after fitting
and subtraction of orbits with astrometric signal-to-noise ratio
the false detection rates can be enhanced by up to a
factor 2; the periodogram approach results in robust multiple-planet detection
for systems with periods shorter than the SIM mission length, even at low
values of , while the least squares technique combined with
Fourier series expansions is arguably preferable in the long-period regime. The
accuracy on multiple-planet orbit reconstruction and mass determination suffers
a typical degradation of 30-40% with respect to single-planet solutions; mass
and orbital inclination can be measured to better than 10% for periods as short
as 0.1 yr, and for as low as , while
is required in order to measure with similar
accuracy systems harboring objects with periods as long as three times the
mission duration. For systems with all components producing
or greater, quasi-coplanarity can be reliably
established with uncertainties of a few degrees, for periods in the range
yr; in systems where at least one component has
, coplanarity measurements are compromised, with typical
uncertainties on the mutual inclinations of order of . Our
findings are illustrative of the importance of the contribution SIM will make
to the fields of formation and evolution of planetary systems.Comment: 61 pages, 14 figures, 5 tables, to appear in the September 2003 Issue
of the Publications of the Astronomical Society of the Pacifi
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