1,583 research outputs found
Optimal positron-beam excited plasma wakefields in Hollow and Ion-Wake channels
A positron-beam interacting with the plasma electrons drives radial suck-in,
in contrast to an electron-beam driven blow-out in the over-dense regime,
. In a homogeneous plasma, the electrons are radially sucked-in from
all the different radii. The electrons collapsing from different radii do not
simultaneously compress on-axis driving weak fields. A hollow-channel allows
electrons from its channel-radius to collapse simultaneously exciting coherent
fields. We analyze the optimal channel radius. Additionally, the low ion
density in the hollow allows a larger region with focusing phase which we show
is linearly focusing. We have shown the formation of an ion-wake channel behind
a blow-out electron bubble-wake. Here we explore positron acceleration in the
over-dense regime comparing an optimal hollow-plasma channel to the ion-wake
channel. The condition for optimal hollow-channel radius is also compared. We
also address the effects of a non-ideal ion-wake channel on positron-beam
excited fields.Comment: Proceedings of IPAC2015, Richmond, VA, USA 3: Alternative Particle
Sources and Acceleration Techniques A22 - Plasma Wake eld Acceleration
http://accelconf.web.cern.ch/AccelConf/IPAC2015/papers/wepje001.pdf, 2015
(ISBN 978-3-95450-168-7) pp 2674-267
The Coldest Place in the Universe: Probing the Ultra-Cold Outflow and Dusty Disk in the Boomerang Nebula
Our Cycle 0 ALMA observations confirmed that the Boomerang Nebula is the
coldest known object in the Universe, with a massive high-speed outflow that
has cooled significantly below the cosmic background temperature. Our new CO
1-0 data reveal heretofore unseen distant regions of this ultra-cold outflow,
out to AU. We find that in the ultra-cold outflow, the
mass-loss rate (dM/dt) increases with radius, similar to its expansion velocity
() - taking , we find . The mass in
the ultra-cold outflow is Msun, and the Boomerang's main-sequence
progenitor mass is Msun. Our high angular resolution (0".3) CO
J=3-2 map shows the inner bipolar nebula's precise, highly-collimated shape,
and a dense central waist of size (FWHM) 1740 AU AU. The
molecular gas and the dust as seen in scattered light via optical HST imaging
show a detailed correspondence. The waist shows a compact core in thermal dust
emission at 0.87-3.3 mm, which harbors Msun~of very large
(mm-to-cm sized), cold ( K) grains. The central waist
(assuming its outer regions to be expanding) and fast bipolar outflow have
expansion ages of yr and yr: the "jet-lag" (i.e.,
torus age minus the fast-outflow age) in the Boomerang supports models in which
the primary star interacts directly with a binary companion. We argue that this
interaction resulted in a common-envelope configuration while the Boomerang's
primary was an RGB or early-AGB star, with the companion finally merging into
the primary's core, and ejecting the primary's envelope that now forms the
ultra-cold outflow.Comment: accepted ApJ, 12 Apr, 201
A Fast bipolar H2 outflow from IRAS 16342-3814: an old star reliving its youth
Some evolved stars in the pre-planetary nebula phase produce
highly-collimated molecular outflows that resemble the accretion-driven jets
and outflows from pre-main sequence stars. We show that IRAS 16342-3814 (the
Water Fountain Nebula) is such an object and present K-band integral field
spectroscopy revealing a fast (> 150 km/s) bipolar H2 outflow. The H2 emission
is shock excited and may arise in fast-moving clumps, accelerated by the
previously observed precessing jet. The total luminosity in H2 is 0.37
L which is comparable with that of accretion-powered outflows from
Class 0 protostars. We also detect CO overtone bandhead emission in the
scattered continuum, indicating hot molecular gas close to the centre, a
feature also observed in a number of protostars with active jets. It seems
likely that the jet and outflow in IRAS 16342-3814 are powered by accretion
onto a binary companion.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Societ
Decompositions of some classes of regular graphs into cycles and paths of length eight
Let (resp. ) denote the cycle (resp. path) of length . In
this paper, we examine the necessary and sufficient conditions for the
existence of a -decomposition of tensor product and wreath product
of complete graphs
Spectral signatures of moisture-stressed wheat
One of the important parameters affecting crop yield is the availability of soil moisture to the crop. Lackof it may bring about moistur~ stress
in plants which manifests itself in terms of changes in the spectral reflectance and emittence properties of plants. An experiment involving
radiometric measurements over six wheat plots subjected to different irrigation schedules was conducted to test this hypothesis, Vegetation
index defined in terms of cropreflectances in 0.6 to 0.7 and 0.8 to 1,1 micrometer bands was found to ben sensitive parameter to distinguish normal plants from moisture-stressed plants. The optimum period for the discrimination of such plants through remote sensing techniques has been indicated to be 45-80 days after sowing. The experiment also demonstrates that yield per unit area is linearly related to the maximum leaf-area index of the crop thus providing a possible method of crop yield prediction
Galactic bulge giants: probing stellar and galactic evolution I. Catalogue of Spitzer IRAC and MIPS sources
Aims: We aim at measuring mass-loss rates and the luminosities of a
statistically large sample of Galactic bulge stars at several galactocentric
radii. The sensitivity of previous infrared surveys of the bulge has been
rather limited, thus fundamental questions for late stellar evolution, such as
the stage at which substantial mass-loss begins on the red giant branch and its
dependence on fundamental stellar properties, remain unanswered. We aim at
providing evidence and answers to these questions. Methods: To this end, we
observed seven 15 times 15 arcmin^2 fields in the nuclear bulge and its
vicinity with unprecedented sensitivity using the IRAC and MIPS imaging
instruments on-board the Spitzer Space Telescope. In each of the fields, tens
of thousands of point sources were detected. Results: In the first paper based
on this data set, we present the observations, data reduction, the final
catalogue of sources, and a detailed comparison to previous mid-IR surveys of
the Galactic bulge, as well as to theoretical isochrones. We find in general
good agreement with other surveys and the isochrones, supporting the high
quality of our catalogue.Comment: 21 pages, accepted for publication in A&A. A version with
high-resolution figures, as well as the data catalogues (including cross-id
with GLIMPSE and GALCEN) and image mosaics are available at the anonymous
ftp://ftp.ster.kuleuven.be/dist/stefan/Spitzer
Nanostructure Accelerators: Novel concept and path to its realization
TeV/m acceleration gradients using crystals as originally envisioned by R.
Hofstadter, an early pioneer of HEP, have remained unrealizable. Fundamental
obstacles that have hampered efforts on particle acceleration using
bulk-crystals arise from collisional energy loss and emittance degradation in
addition to severe beam disruption despite the favorable effect of particle
channeling along interatomic planes in bulk. We aspire for the union of
nanoscience with accelerator science to not only overcome these problems using
nanostructured tubes to avoid direct impact of the beam on bulk ion-lattice but
also to utilize the highly tunable characteristics of nanomaterials. We pioneer
a novel surface wave mechanism in nanostructured materials with a strong
electrostatic component which not only attains tens of TeV/m gradients but also
has focusing fields. Under our initiative, the proof-of-principle demonstration
of tens of TeV/m gradients and beam nanomodulation is underway. Realizable
nanostructure accelerators naturally promise new horizons in HEP as well as in
a wide range of areas of research that utilize beams of high-energy particles
or photons.Comment: submission to Snowmass'21 Accelerator Frontie
Mid-Infrared Imaging of the Bipolar Planetary Nebula M2-9 from SOFIA
We have imaged the bipolar planetary nebula M2-9 using SOFIA's FORCAST
instrument in six wavelength bands between 6.6 and 37.1 . A bright
central point source, unresolved with SOFIA's 4-to-5 beam,
is seen at each wavelength, and the extended bipolar lobes are clearly seen at
19.7 and beyond. The photometry between 10 and 25 is well fit
by the emission predicted from a stratified disk seen at large inclination, as
has been proposed for this source by Lykou et al and by Smith and Gehrz. The
principal new results in this paper relate to the distribution and properties
of the dust that emits the infrared radiation. In particular, a considerable
fraction of this material is spread uniformly through the lobes, although the
dust density does increase at the sharp outer edge seen in higher resolution
optical images of M2-9. The dust grain population in the lobes shows that small
( 1 ) particles appear to be present in
roughly equal amounts by mass. We suggest that collisional processing within
the bipolar outflow plays an important role in establishing the particle size
distribution.Comment: 40 pages, 9 figures, 2 table
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