1,367 research outputs found
Interstellar and Ejecta Dust in the Cas A Supernova Remnant
Infrared continuum observations provide a means of investigating the physical
composition of the dust in the ejecta and swept up medium of the Cas A
supernova remnant. Using low resolution Spitzer IRS spectra (5-35 m), and
broad-band Herschel PACS imaging (70, 100, and 160 m), we identify
characteristic dust spectra, associated with ejecta layers that underwent
distinct nuclear burning histories. The most luminous spectrum exhibits strong
emission features at and 21 m and is closely associated with
ejecta knots with strong Ar emission lines. The dust features can be reproduced
by magnesium silicate grains with relatively low Mg to Si ratios. Another dust
spectrum is associated with ejecta having strong Ne emission lines. It has no
indication of any silicate features, and is best fit by AlO dust. A
third characteristic dust spectrum shows features that are best matched by
magnesium silicates with a relatively high Mg to Si ratio. This dust is
primarily associated with the X-ray emitting shocked ejecta, but it is also
evident in regions where shocked interstellar or circumstellar material is
expected. However, the identification of dust composition is not unique, and
each spectrum includes an additional featureless dust component of unknown
composition. Colder dust of indeterminate composition is associated with
emission from the interior of the SNR, where the reverse shock has not yet
swept up and heated the ejecta. Most of the dust mass in Cas A is associated
with this unidentified cold component, which is . The
mass of warmer dust is only .Comment: 45 pages. 21 Figures. Accepted for publication in Ap
Eta Carinae across the 2003.5 Minimum: Analysis in the visible and near infrared spectral region
We present an analysis of the visible through near infrared spectrum of Eta
Carinae and its ejecta obtained during the "Eta Carinae Campaign with the UVES
at the ESO VLT". This is a part of larger effort to present a complete Eta
Carinae spectrum, and extends the previously presented analyses with the
HST/STIS in the UV (1240-3159 A) to 10,430 A. The spectrum in the mid and near
UV is characterized by the ejecta absorption. At longer wavelengths, stellar
wind features from the central source and narrow emission lines from the
Weigelt condensations dominate the spectrum. However, narrow absorption lines
from the circumstellar shells are present. This paper provides a description of
the spectrum between 3060 and 10,430 A, including line identifications of the
ejecta absorption spectrum, the emission spectrum from the Weigelt
condensations and the P-Cygni stellar wind features. The high spectral
resolving power of VLT/UVES enables equivalent width measurements of atomic and
molecular absorption lines for elements with no transitions at the shorter
wavelengths. However, the ground based seeing and contributions of nebular
scattered radiation prevent direct comparison of measured equivalent widths in
the VLT/UVES and HST/STIS spectra. Fortunately, HST/STIS and VLT/UVES have a
small overlap in wavelength coverage which allows us to compare and adjust for
the difference in scattered radiation entering the instruments' apertures. This
paper provides a complete online VLT/UVES spectrum with line identifications
and a spectral comparison between HST/STIS and VLT/UVES between 3060 and 3160
A.Comment: 13 pages, 11 figures + atlas. The paper accepted for the ApJS and is
accompanied with an atlas in the online edition pape
Texture and shape of two-dimensional domains of nematic liquid crystal
We present a generalized approach to compute the shape and internal structure
of two-dimensional nematic domains. By using conformal mappings, we are able to
compute the director field for a given domain shape that we choose from a rich
class, which includes drops with large and small aspect ratios, and sharp
domain tips as well as smooth ones. Results are assembled in a phase diagram
that for given domain size, surface tension, anchoring strength, and elastic
constant shows the transitions from a homogeneous to a bipolar director field,
from circular to elongated droplets, and from sharp to smooth domain tips. We
find a previously unaccounted regime, where the drop is nearly circular, the
director field bipolar and the tip rounded. We also find that bicircular
director fields, with foci that lie outside the domain, provide a remarkably
accurate description of the optimal director field for a large range of values
of the various shape parameters.Comment: 12 pages, 10 figure
DAC-Less amplifier-less generation and transmission of QAM signals using sub-volt silicon-organic hybrid modulators
We demonstrate generation and transmission of optical signals by directly interfacing highly efficient silicon-organic hybrid (SOH) modulators to binary output ports of a field-programmable gate array. Using an SOH Mach-Zehnder modulator (MZM) and an SOH IQ modulator we generate ON-OFF- keying and binary phase-shift keying signals as well as quadrature phase-shift keying and 16-state quadrature amplitude modulation (16QAM) formats. Peak-to-peak voltages amount to only 0.27 V-pp for driving the MZM and 0.41 V-pp for the IQ modulator. Neither digital-to-analog converters nor drive amplifiers are required, and the RF energy consumption in the modulator amounts to record-low 18 fJ/bit for 16QAM signaling
Discovery of CH and OH in the -513 km s-1 Ejecta of Eta Carinae
The very massive star, Eta Carinae, is enshrouded in an unusual complex of
stellar ejecta, which is highly depleted in C and O, and enriched in He and N.
This circumstellar gas gives rise to distinct absorption components
corresponding to at least 20 different velocities along the line-of-sight. The
velocity component at -513 kms-1 exhibits very low ionization with
predominantly neutral species of iron-peak elements. Our statistical
equilibrium/photoionization modeling indicates that the low temperature (T =
760 K) and high density (n_H=10^7 cm^-3) of the -513 kms-1 component is
conducive to molecule formation including those with the elements C and O.
Examination of echelle spectra obtained with the Space Telescope Imaging
Spectrograph (STIS) aboard the confirms the model's predictions. The molecules,
H_2, CH, and most likely OH, have been identified in the -513 kms-1 absorption
spectrum. This paper presents the analysis of the HST/STIS spectra with the
deduced column densities for CH, OH and C I, and upper limit for CO. It is
quite extraordinary to see molecular species in a cool environment at such a
high velocity. The sharp molecular and ionic absorptions in this extensively
CNO- processed material offers us a unique environment for studying the
chemistry, dust formation processes, and nucleosynthesis in the ejected layers
of a highly evolved massive star.Comment: tentatively scheduled for the ApJ 1 September 2005, v630, 1 issu
Geometry-dependent critical currents in superconducting nanocircuits
In this paper we calculate the critical currents in thin superconducting
strips with sharp right-angle turns, 180-degree turnarounds, and more
complicated geometries, where all the line widths are much smaller than the
Pearl length . We define the critical current as the
current that reduces the Gibbs free-energy barrier to zero. We show that
current crowding, which occurs whenever the current rounds a sharp turn, tends
to reduce the critical current, but we also show that when the radius of
curvature is less than the coherence length this effect is partially
compensated by a radius-of-curvature effect. We propose several patterns with
rounded corners to avoid critical-current reduction due to current crowding.
These results are relevant to superconducting nanowire single-photon detectors,
where they suggest a means of improving the bias conditions and reducing dark
counts. These results also have relevance to normal-metal nanocircuits, as
these patterns can reduce the electrical resistance, electromigration, and hot
spots caused by nonuniform heating.Comment: 29 pages, 24 figure
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