1,748 research outputs found
Frontier Masculinity, Femininity, and the Ideological Cleansing of Borderlands Teachers, 1924–1935
New stellar encounters discovered in the second Gaia data release
Passing stars may play an important role in the evolution of our solar
system. We search for close stellar encounters to the Sun among all 7.2 million
stars in Gaia-DR2 that have six-dimensional phase space data. We characterize
encounters by integrating their orbits through a Galactic potential and
propagating the correlated uncertainties via a Monte Carlo resampling. After
filtering to remove spurious data, we find 694 stars that have median (over
uncertainties) closest encounter distances within 5 pc, all occurring within 15
Myr from now. 26 of these have at least a 50% chance of coming closer than 1 pc
(and 7 within 0.5 pc), all but one of which are newly discovered here. We
further confirm some and refute several other previously-identified encounters,
confirming suspicions about their data. The closest encounter in the sample is
Gl 710, which has a 95% probability of coming closer than 0.08 pc (17 000 AU).
Taking mass estimates from Gaia astrometry and multiband photometry for
essentially all encounters, we find that Gl 710 also has the largest impulse on
the Oort cloud. Using a Galaxy model, we compute the completeness of the
Gaia-DR2 encountering sample as a function of perihelion time and distance.
Only 15% of encounters within 5 pc occurring within +/- 5 Myr of now have been
identified, mostly due to the lack of radial velocities for faint and/or cool
stars. Accounting for the incompleteness, we infer the present rate of
encounters within 1 pc to be 19.7 +/- 2.2 per Myr, a quantity expected to scale
quadratically with the encounter distance out to at least several pc.
Spuriously large parallaxes in our sample from imperfect filtering would tend
to inflate both the number of encounters found and this inferred rate. The
magnitude of this effect is hard to quantify.Comment: 12 pages. Accepted to A&A. Added to this version: section 3.2 and
Fig. 8 (CMD) with discussion of astrometric quality metrics; full versions of
tables 2 and 3 as ancillary dat
Comparative scanning near field optical microscopy studies of plasmonic nanoparticle concepts
We use scanning near field optical microscopy SNOM to characterize different plasmonic nanoparticle situations with high spatial and spectral resolution in this comparative study. The near field enhancement is measured with an aperture probe Al coated glass fiber and two CCD spectrometers for simultaneous detection of reflection and transmission. The images of transmission and reflection show a correlation to the topography. We present a new way to access the relative absorption and discuss the results with consideration of artifact influences. Near field enhancements are deeper understood by imaging isolated particles. This near field will be compared to measurements of random particle distributions. Therefore, we will show normalized reflection and transmission images of random structures that lay the foundation for an absolute interpretation of near field images. The normalization considers both the far field UV VIS results and a reference image of the substrate. The near field reflection of nanoparticle arrays shows an enhancement of 25 . In view of specific applications, particle distributions implemented in two ways as far field scatters and as near field enhancing object
Deconvolution with Shapelets
We seek to find a shapelet-based scheme for deconvolving galaxy images from
the PSF which leads to unbiased shear measurements. Based on the analytic
formulation of convolution in shapelet space, we construct a procedure to
recover the unconvolved shapelet coefficients under the assumption that the PSF
is perfectly known. Using specific simulations, we test this approach and
compare it to other published approaches. We show that convolution in shapelet
space leads to a shapelet model of order
with and being the maximum orders of the intrinsic
galaxy and the PSF models, respectively. Deconvolution is hence a
transformation which maps a certain number of convolved coefficients onto a
generally smaller number of deconvolved coefficients. By inferring the latter
number from data, we construct the maximum-likelihood solution for this
transformation and obtain unbiased shear estimates with a remarkable amount of
noise reduction compared to established approaches. This finding is
particularly valid for complicated PSF models and low images, which
renders our approach suitable for typical weak-lensing conditions.Comment: 9 pages, 9 figures, submitted to A&
Galaxy And Mass Assembly (GAMA): end of survey report and data release 2
The Galaxy And Mass Assembly (GAMA) survey is one of the largest contemporary spectroscopic surveys of low redshift galaxies. Covering an area of ˜286 deg2 (split among five survey regions) down to a limiting magnitude of r < 19.8 mag, we have collected spectra and reliable redshifts for 238 000 objects using the AAOmega spectrograph on the Anglo-Australian Telescope. In addition, we have assembled imaging data from a number of independent surveys in order to generate photometry spanning the wavelength range 1 nm-1 m. Here, we report on the recently completed spectroscopic survey and present a series of diagnostics to assess its final state and the quality of the redshift data. We also describe a number of survey aspects and procedures, or updates thereof, including changes to the input catalogue, redshifting and re-redshifting, and the derivation of ultraviolet, optical and near-infrared photometry. Finally, we present the second public release of GAMA data. In this release, we provide input catalogue and targeting information, spectra, redshifts, ultraviolet, optical and near-infrared photometry, single-component Sérsic fits, stellar masses, Hα-derived star formation rates, environment information, and group properties for all galaxies with r < 19.0 mag in two of our survey regions, and for all galaxies with r < 19.4 mag in a third region (72 225 objects in total). The data base serving these data is available at http://www.gama-survey.org/
Hindered Coulomb explosion of embedded Na clusters -- stopping, shape dynamics and energy transport
We investigate the dynamical evolution of a Na cluster embedded in Ar
matrices of various sizes from N=30 to 1048. The system is excited by an
intense short laser pulse leading to high ionization stages. We analyze the
subsequent highly non-linear motion of cluster and Ar environment in terms of
trajectories, shapes, and energy flow. The most prominent effects are:
temporary stabilization of high charge states for several ps, sudden stopping
of the Coulomb explosion of the embedded Na clusters associated with an
extremely fast energy transfer to the Ar matrix, fast distribution of energy
throughout the Ar layers by a sound wave. Other ionic-atomic transfer and
relaxation processes proceed at slower scale of few ps. The electron cloud is
almost thermally decoupled from ions and thermalizes far beyond the ps scale.Comment: 12 pages, 10 figures, accepted in Euro. Phys. J.
Time resolved fission in metal clusters
We explore from a theoretical point of view pump and probe (P&P) analysis for
fission of metal clusters where probe pulses are generalized to allow for
scanning various frequencies. We show that it is possible to measure the time
the system needs to develop to scission. This is achieved by a proper choice of
both delay and frequency of the probe pulse. A more detailed analysis even
allows to access the various intermediate stages of the fission process.Comment: 4 pages, 4 figure
Dynamics of metal clusters in rare gas clusters
We investigate the dynamics of Na clusters embedded in Ar matrices. We use a
hierarchical approach, accounting microscopically for the cluster's degrees of
freedom and more coarsely for the matrix. The dynamical polarizability of the
Ar atoms and the strong Pauli-repulsion exerted by the Ar-electrons are taken
into account. We discuss the impact of the matrix on the cluster gross
properties and on its optical response. We then consider a realistic case of
irradiation by a moderately intense laser and discuss the impact of the matrix
on the hindrance of the explosion, as well as a possible pump probe scenario
for analyzing dynamical responses.Comment: Proceedings of the 30th International Workshop on Condensed Matter
Theories, Dresden, June 05 - 10, 2006, World Scientific. 3 figure
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