2,382 research outputs found
Observation of sub-Bragg diffraction of waves in crystals
We investigate the diffraction conditions and associated formation of
stopgaps for waves in crystals with different Bravais lattices. We identify a
prominent stopgap in high-symmetry directions that occurs at a frequency below
the ubiquitous first-order Bragg condition. This sub-Bragg diffraction
condition is demonstrated by reflectance spectroscopy on two-dimensional
photonic crystals with a centred rectangular lattice, revealing prominent
diffraction peaks for both the sub-Bragg and first-order Bragg condition. These
results have implications for wave propagation in 2 of the 5 two-dimensional
Bravais lattices and 7 out of 14 three-dimensional Bravais lattices, such as
centred rectangular, triangular, hexagonal and body-centred cubic
Regularity of higher codimension area minimizing integral currents
This lecture notes are an expanded version of the course given at the
ERC-School on Geometric Measure Theory and Real Analysis, held in Pisa,
September 30th - October 30th 2013. The lectures aim to explain the main steps
of a new proof of the partial regularity of area minimizing integer rectifiable
currents in higher codimension, due originally to F. Almgren, which is
contained in a series of papers in collaboration with C. De Lellis (University
of Zurich).Comment: This text will appear in "Geometric Measure Theory and Real
Analysis", pp. 131--192, Proceedings of the ERC school in Pisa (2013), L.
Ambrosio Ed., Edizioni SNS (CRM Series
Far-UV Emissions of the Sun in Time: Probing Solar Magnetic Activity and Effects on Evolution of Paleo-Planetary Atmospheres
We present and analyze FUSE observations of six solar analogs. These are
single, main-sequence G0-5 strs selected as proxies for the Sun at several
stages of its main-sequence lifetime. The emission features in the FUSE
920-1180 A wavelength range allow for a critical probe of the hot plasma over
three decades in temperature. Using the flux ratio CIII 1176/977 as
diagnostics, we investigate the dependence of the electron pressure of the
transition region as a function of the rotation period, age and magnetic
activity. The results from these solar proxies indicate that the electron
pressure of the stellar ~10^5-K plasma decreases by a factor of about 70
between the young, fast-rotating magnetically active star and the old,
slow-rotating inactive star. Also, the observations indicate that the average
surface fluxes of emission features strongly decrease with increasing stellar
age and longer rotation period. The emission flux evolution with age or
rotation period is well fitted by power laws, which become steeper from cooler
chromospheric (10^4 K) to hotter coronal (10^7 K) plasma. The relationship for
the integrated (920-1180 A) FUSE flux indicates that the solar far-ultraviolet
emissions were about twice the present value 2.5 Gyr ago and about 4 times the
present value 3.5 Gyr ago. Note also that the FUSE/FUV flux of the Zero-Age
Main Sequence Sun could have been higher by as much as 50 times. Our analysis
suggests that the strong FUV emissions of the young Sun may have played a
crucial role in the developing planetary system, in particular through the
photoionization, photochemical evolution and possible erosion of the planetary
atmospheres. (abridged)Comment: 15 pages, 8 figures, accepted for publication in Ap
The Initial Mass Function of the Orion Nebula Cluster across the H-burning limit
We present a new census of the Orion Nebula Cluster (ONC) over a large field
of view (>30'x30'), significantly increasing the known population of stellar
and substellar cluster members with precisely determined properties. We develop
and exploit a technique to determine stellar effective temperatures from
optical colors, nearly doubling the previously available number of objects with
effective temperature determinations in this benchmark cluster. Our technique
utilizes colors from deep photometry in the I-band and in two medium-band
filters at lambda~753 and 770nm, which accurately measure the depth of a
molecular feature present in the spectra of cool stars. From these colors we
can derive effective temperatures with a precision corresponding to better than
one-half spectral subtype, and importantly this precision is independent of the
extinction to the individual stars. Also, because this technique utilizes only
photometry redward of 750nm, the results are only mildly sensitive to optical
veiling produced by accretion. Completing our census with previously available
data, we place some 1750 sources in the Hertzsprung-Russel diagram and assign
masses and ages down to 0.02 solar masses. At faint luminosities, we detect a
large population of background sources which is easily separated in our
photometry from the bona fide cluster members. The resulting initial mass
function of the cluster has good completeness well into the substellar mass
range, and we find that it declines steeply with decreasing mass. This suggests
a deficiency of newly formed brown dwarfs in the cluster compared to the
Galactic disk population.Comment: 16 pages, 18 figures. Accepted for publication in The Astrophysical
Journa
Light scattering from three-level systems: The T-matrix of a point-dipole with gain
We present an extension of the T-matrix approach to scattering of light by a
three-level system, using a description based on a Master equation. More
particularly, we apply our formalism to calculate the T-matrix of a pumped
three-level atom, providing an exact and analytical expression describing the
influence of a pump on the light scattering properties of an atomic three-level
system
Multiplicity of Nearby Free-floating Ultra-cool Dwarfs: a HST-WFPC2 search for companions
We present HST/WFPC2 observations of a sample of 134 ultra-cool objects
(spectral types later than M7) coming from the DENIS, 2MASS and SDSS surveys,
with distances estimated to range from 7 pc to 105 pc. Fifteen new ultra-cool
binary candidates are reported here. Eleven known binaries are confirmed and
orbital motion is detected in some of them. We estimate that the closest binary
systems in this sample have periods between 5 and 20 years, and thus dynamical
masses will be derived in the near future. For the calculation of binary
frequency we restrict ourselves to systems with distances less than 20 pc.
After correction of the binaries bias, we find a ratio of visual binaries (at
the HST limit of detection) of around 10%, and that ~15% of the 26 objects
within 20 parsecs are binary systems with separations between 1 and 8 A.U. The
observed frequency of ultra-cool binaries is similar than that of binaries with
G-type primaries in the separation range from 2.1 A.U. to 140 A.U. There is
also a clear deficit of ultra-cool binaries with separations greater than 15
A.U., and a possible tendency for the binaries to have mass ratios near unity.
Most systems have indeed visual and near-infrared brightness ratios between 1
and 0.3. We discuss our results in the framework of current scenarios for the
formation and evolution of free-floating brown dwarfs.Comment: 67 pages, 14 figures, Accepted for publication in AJ, September 2003.
First submission to AJ: august 2002, 5 submission
Mars Relay Lander and Orbiter Overflight Profile Estimation
This software allows science and mission operations to view graphs of geometric overflights of satellites and landers within the Mars (or other planetary) networks. It improves on the MaROS Web interface within any modern Web browser, in that it adds new capabilities to the MaROS suite. The profile for an overflight is an important element for selecting communication/ overflight opportunities between the landers and orbiters within the Mars network. Unfortunately, determining these estimates is very computationally expensive and difficult to compute by hand. This software allows the user to select different overflights (via the existing MaROS Web interface) and specify the smoothness of the estimation. Estimates for the geometric relationship between a lander and an orbiter are determined based upon the orbital conditions of the orbiter at the moment the orbiter rises above the horizon from the perspective of the lander. It utilizes 2-body orbital equations to propagate the trajectory through the duration of the view period, and returns profiles that represent the range between the two vehicles, and the elevation and azimuth angles of the orbiter as measured from the lander s position. The algorithms assume a 2-body relationship with an ideal, spherical planetary body, so therefore can see errors less than 2% at polar landing sites on Mars. These algorithms are being implemented to provide rough estimates rapidly for the geometry of a geometric view period where more complete data is unavailable, such as for planning purposes. While other software for this task exists, each at the time of this reporting has been contained within a much more complicated package. This tool allows science and mission operations to view the estimates with a few clicks of the mouse
Troubling identities: teacher education students` constructions of class and ethnicity
Working with diverse student populations productively depends on teachers and teacher educators recognizing and valuing difference. Too often, in teacher education programs, when markers of identity such as gender, ethnicity, \u27race\u27, or social class are examined, the focus is on developing student teachers\u27 understandings of how these discourses shape learner identities and rarely on how these also shape teachers\u27 identities. This article reports on a research project that explored how student teachers understand ethnicity and socio-economic status. In a preliminary stage of the research, we asked eight Year 3 teacher education students who had attended mainly Anglo-Australian, middle class schools as students and as student teachers, to explore their own ethnic and classed identities. The complexities of identity are foregrounded in both the assumptions we made in selecting particular students for the project and in the ways they constructed their own identities around ethnicity and social class. In this article we draw on these findings to interrogate how categories of identity are fluid, shifting and ongoing processes of negotiation, troubling and complex. We also consider the implications for teacher education.<br /
Temperature constraints on the coldest brown dwarf known WISE 0855-0714
Context. Nearby isolated planetary mass objects are beginning to be
discovered, but their individual properties are poorly constrained because
their low surface temperatures and strong molecular self-absorption make them
extremely faint.
Aims. We aimed to detect the near infrared emission of the coldest brown
dwarf (BD) found so far, WISE08550714, located 2.2 pc away, and to
improve its temperature estimate (T= 225-260 K) from a comparison
with state-of-the-art models of BD atmospheres.
Methods. We observed the field containing WISE0855-0714 with HAWK-I at the
VLT in the band. For BDs with T500\,K theoretical models
predict strong signal (or rather less molecular absorption) in this band.
Results. WISE0855-0714 was not detected in our Y-band images, thus placing an
upper limit on its brightness to Y>24.4 mag at 3- level, leading to
Y-[4.5]>10.5. Combining this limit with previous detections and upper limits at
other wavelengths, WISE08550714 is confirmed as the reddest BD detected,
further supporting its status as the coldest known brown dwarf. We applied
spectral energy distribution fitting with collections of models from two
independent groups for extremely cool BD atmospheres leading to an effective
temperature of T250\,K,.Comment: 4 pages, 4 figures. A&A letter Accepte
A Hubble Space Telescope ACS Search for Brown Dwarf Binaries in the Pleiades Open Cluster
We present the results of a high-resolution imaging survey for brown dwarf
binaries in the Pleiades open cluster. The observations were carried out with
the Advance Camera for Surveys onboard the Hubble Space Telescope. Our sample
consists of 15 bona-fide brown dwarfs. We confirm 2 binaries and detect their
orbital motion, but we did not resolve any new binary candidates in the
separation range between 5.4AU and 1700AU and masses in the range
0.035--0.065~Msun. Together with the results of our previous study (Martin et
al., 2003), we can derive a visual binary frequency of 13.3\%
for separations greater than 7~AU masses between 0.055--0.065~M_{\sun} and
mass ratios between 0.45--0.91.0. The other observed properties of
Pleiades brown dwarf binaries (distributions of separation and mass ratio)
appear to be similar to their older counterparts in the field.Comment: 29 pages, 7 figures, 6 tables, accepted for publication in Ap
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