1,814 research outputs found
High-contrast imaging in the Hyades with snapshot LOCI
To image faint substellar companions obscured by the stellar halo and
speckles, scattered light from the bright primary star must be removed in
hardware or software. We apply the "locally-optimized combination of images"
(LOCI) algorithm to 1-minute Keck Observatory snapshots of GKM dwarfs in the
Hyades using source diversity to determine the most likely PSF. We obtain a
mean contrast of 10^{-2} at 0.01", 10^{-4} at <1", and 10^{-5} at 5". New brown
dwarf and low-mass stellar companions to Hyades primaries are found in a third
of the 84 targeted systems. This campaign shows the efficacy of LOCI on
snapshot imaging as well as on bright wide binaries with off-axis LOCI,
reaching contrasts sufficient for imaging 625-Myr late-L/early-T dwarfs purely
in post-processing.Comment: 12 pages, 12 figures, to appear in SPIE Astronomy 2012, paper
8447-16
Astrometric Monitoring of the HR 8799 Planets: Orbit Constraints from Self-Consistent Measurements
We present new astrometric measurements from our ongoing monitoring campaign
of the HR 8799 directly imaged planetary system. These new data points were
obtained with NIRC2 on the W.M. Keck II 10 meter telescope between 2009 and
2014. In addition, we present updated astrometry from previously published
observations in 2007 and 2008. All data were reduced using the SOSIE algorithm,
which accounts for systematic biases present in previously published
observations. This allows us to construct a self-consistent data set derived
entirely from NIRC2 data alone. From this dataset, we detect acceleration for
two of the planets (HR 8799b and e) at 3. We also assess possible
orbital parameters for each of the four planets independently. We find no
statistically significant difference in the allowed inclinations of the
planets. Fitting the astrometry while forcing coplanarity also returns
consistent to within 1 of the best fit values, suggesting that if
inclination offsets of 20 are present, they are not detectable
with current data. Our orbital fits also favor low eccentricities, consistent
with predictions from dynamical modeling. We also find period distributions
consistent to within 1 with a 1:2:4:8 resonance between all planets.
This analysis demonstrates the importance of minimizing astrometric systematics
when fitting for solutions to highly undersampled orbits.Comment: 18 pages, 11 figures. Accepted for publication in A
Simulation study on the impact of the transmission power on the performance of routing protocols under different mobility models
© 2014 IEEE. the dynamic topology of a mobile ad hoc network poses a real challenge in designing the routing protocol. This paper examine through simulation the fundamental factors, mobility models and transmission power which have a major impacts on the performance of position based routing protocols. We analyse the effect of the transmission power of on the performance of protocols under two different mobility models. Using OPNET simulation tool, results show the evaluation and performance of the proposed protocol under a unified simulation environment for different scenarios
Measuring the Mass of a Pre-Main Sequence Binary Star Through the Orbit of TWA 5A
We present the results of a five year monitoring campaign of the close binary TWA 5Aab in the TW Hydrae association, using speckle and adaptive optics on the W.M. Keck 10 m telescopes. These measurements were taken as part of our ongoing monitoring of pre-main sequence (PMS) binaries in an effort to increase the number of dynamically determined PMS masses and thereby calibrate the theoretical PMS evolutionary tracks. Our observations have allowed us to obtain the first determination of this system's astrometric orbit. We find an orbital period of 5.94 +- 0.09 years and a semi-major axis of 0.066" +- 0.005". Combining these results with a kinematic distance, we calculate a total mass of 0.71 +- 0.14 M_sun (D/44 pc)^3. for this system. This mass measurement, as well as the estimated age of this system, are consistent to within 2 of all theoretical models considered. In this analysis, we properly account for correlated uncertainties, and show that while these correlations are generally ignored, they increase the formal uncertainties by up to a factor of five and therefore are important to incorporate. With only a few more years of observation, this type of measurement will allow the theoretical models to be distinguished
A new 1.6-micron map of Titanâs surface
We present a new map of Titan's surface obtained in the spectral 'window' at âŒ1.6 ÎŒm between strong methane absorption. This pre-Cassini view of Titan's surface was created from images obtained using adaptive optics on the W.M. Keck II telescope and is the highest resolution map yet made of Titan's surface. Numerous surface features down to the limits of the spatial resolution (âŒ200â300 km) are apparent. No features are easily identifiable in terms of their geologic origin, although several are likely craters
The Structure of High Strehl Ratio Point-Spread Functions
We describe the symmetries present in the point-spread function (PSF) of an
optical system either located in space or corrected by an adaptive o to Strehl
ratios of about 70% and higher. We present a formalism for expanding the PSF to
arbitrary order in terms of powers of the Fourier transform of the residual
phase error, over an arbitrarily shaped and apodized entrance aperture. For
traditional unapodized apertures at high Strehl ratios, bright speckles pinned
to the bright Airy rings are part of an antisymmetric perturbation of the
perfect PSF, arising from the term that is first order in the residual phase
error. There are two symmetric second degree terms. One is negative at the
center, and, like the first order term, is modulated by the perfect image's
field strength -- it reduces to the Marechal approximation at the center of the
PSF. The other is non-negative everywhere, zero at the image center, and can be
responsible for an extended halo -- which limits the dynamic range of faint
companion detection in the darkest portions of the image. In regimes where one
or the other term dominates the speckles in an image, the symmetry of the
dominant term can be exploited to reduce the effect of those speckles,
potentially by an order of magnitude or more. We demonstrate the effects of
both secondary obscuration and pupil apodization on the structure of residual
speckles, and discuss how these symmetries can be exploited by appropriate
telescope and instrument design, observing strategies, and filter bandwidths to
improve the dynamic range of high dynamic range AO and space-based
observations. Finally, we show that our analysis is relevant to high dynamic
range coronagraphy.Comment: Accepted for publication in ApJ; 20 pages, 4 figure
Detection of Carbon Monoxide and Water Absorption Lines in an Exoplanet Atmosphere
Determining the atmospheric structure and chemical composition of an
exoplanet remains a formidable goal. Fortunately, advancements in the study of
exoplanets and their atmospheres have come in the form of direct imaging -
spatially resolving the planet from its parent star - which enables
high-resolution spectroscopy of self-luminous planets in Jovian-like orbits.
Here, we present a spectrum with numerous, well-resolved, molecular lines from
both water and carbon monoxide from a massive planet orbiting less than 40 AU
from the star HR 8799. These data reveal the planet's chemical composition,
atmospheric structure, and surface gravity, confirming that it is indeed a
young planet. The spectral lines suggest an atmospheric carbon-to-oxygn ratio
greater than the host star's, providing hints about the planet's formation.Comment: Accepted for publication in Science. Published online on March 14,
2013. 24 pages (main text and supplementary materials), 8 figures.
Attachments to the supplementary material are available on Science websit
Identity dynamics as a barrier to organizational change
This article seeks to explore the construction of group and professional identities in situations of organizational change. It considers empirical material drawn from a health demonstration project funded by the Scottish Executive Health Department, and uses insights from this project to discuss issues that arise from identity construction(s) and organizational change. In the course of the project studied here, a new organizational form was developed which involved a network arrangement with a voluntary sector organization and the employment of âlay-workersâ in what had traditionally been a professional setting. Our analysis of the way actors made sense of their identities reveals that characterizations of both self and other became barriers to the change process. These identity dynamics were significant in determining the way people interpreted and responded to change within this project and which may relate to other change-oriented situations
Speckle Control with a remapped-pupil PIAA-coronagraph
The PIAA is a now well demonstrated high contrast technique that uses an
intermediate remapping of the pupil for high contrast coronagraphy
(apodization), before restoring it to recover classical imaging capabilities.
This paper presents the first demonstration of complete speckle control loop
with one such PIAA coronagraph. We show the presence of a complete set of
remapping optics (the so-called PIAA and matching inverse PIAA) is transparent
to the wavefront control algorithm. Simple focal plane based wavefront control
algorithms can thus be employed, without the need to model remapping effects.
Using the Subaru Coronagraphic Extreme AO (SCExAO) instrument built for the
Subaru Telescope, we show that a complete PIAA-coronagraph is compatible with a
simple implementation of a speckle nulling technique, and demonstrate the
benefit of the PIAA for high contrast imaging at small angular separation.Comment: 6 figures, submitted to PAS
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