4,318 research outputs found
A representation formula for radially weighted biharmonic functions in the unit disc
Let w : D → (0, ∞) be a radial continuous weight function. We consider the weighted biharmonic equation ∆w −1 ∆u = 0 in D with Dirichlet boundary conditions u = f0 and ∂n u = f1 on T = ∂D. Under some extra conditions on the weight function w, we establish existence and uniqueness of a distributional solution u of this biharmonic Dirichlet problem. Furthermore, we give a representation formula for the solution u. The key to our analysis is a series representation of Almansi type
EMBRACE@Nancay: An Ultra Wide Field of View Prototype for the SKA
A revolution in radio receiving technology is underway with the development
of densely packed phased arrays for radio astronomy. This technology can
provide an exceptionally large field of view, while at the same time sampling
the sky with high angular resolution. Such an instrument, with a field of view
of over 100 square degrees, is ideal for performing fast, all-sky, surveys,
such as the "intensity mapping" experiment to measure the signature of Baryonic
Acoustic Oscillations in the HI mass distribution at cosmological redshifts.
The SKA, built with this technology, will be able to do a billion galaxy
survey. I will present a very brief introduction to radio interferometry, as
well as an overview of the Square Kilometre Array project. This will be
followed by a description of the EMBRACE prototype and a discussion of results
and future plans.Comment: to appear in proceedings of the INFIERI Summer School INtelligent
Signal Processing for FrontIEr Research and Industry, Paris 201
Gas signatures of Herbig Ae/Be disks probed with Herschel SPIRE spectroscopy
Herbig Ae/Be objects, like their lower mass counterparts T Tauri stars, are
seen to form a stable circumstellar disk which is initially gas-rich and could
ultimately form a planetary system. We present Herschel SPIRE 460-1540 GHz
spectra of five targets out of a sample of 13 young disk sources, showing line
detections mainly due to warm CO gas.Comment: to be published in proceedings of IAU symposium 299 (Victoria, BC,
Canada, June 2013
Molecular gas in high-mass filament WB673
We studied the distribution of dense gas in a filamentary molecular cloud
containing several dense clumps. The center of the filament is given by the
dense clump WB673. The clumps are high-mass and intermediate-mass star-forming
regions. We observed CS(2-1), 13CO(1-0), C18O(1-0) and methanol lines at 96GHz
toward WB673 with the Onsala Space Observatory 20-m telescope. We found CS(2-1)
emission in the inter-clump medium so the clumps are physically connected and
the whole cloud is indeed a filament. Its total mass is M and
mass-to-length ratio is 360 Mpc from 13CO(1-0) data.
Mass-to-length ratio for the dense gas is Mpc from
CS(2-1) data. The PV-diagram of the filament is V-shaped. We estimated physical
conditions in the molecular gas using methanol lines. Location of the filament
on the sky between extended shells suggests that it could be a good example to
test theoretical models of formation of the filaments via multiple compression
of interstellar gas by supersonic waves
Near-infrared observations of water-ice in OH/IR stars
A search for the near-infrared water-ice absorption band was made in a number
of very red OH/IR stars which are known to exhibit the 10um silicate
absorption. As a by-product, accurate positions of these highly reddened
objects are obtained. We derived a dust mass loss rate for each object by
modelling the spectral energy distribution and the gas mass loss rate by
solving the equation of motion for the dust drag wind. The derived mass loss
rates show a strong correlation with the silicate optical depth as well as that
of the water-ice. The stars have a high mass loss rate (> 1.0E-4 Msun/yr) with
an average gas-to-dust mass ratio of 110. In objects which show the 3.1um
water-ice absorption, the near-IR slope is much steeper than those with no
water-ice. Comparison between our calculated mass loss rates and those derived
from OH and CO observations indicates that these stars have recently increased
their mass loss rates.Comment: 10 pages, 6 figures : accepted for publication in A&
Characterization of a dense aperture array for radio astronomy
EMBRACE@Nancay is a prototype instrument consisting of an array of 4608
densely packed antenna elements creating a fully sampled, unblocked aperture.
This technology is proposed for the Square Kilometre Array and has the
potential of providing an extremely large field of view making it the ideal
survey instrument. We describe the system,calibration procedures, and results
from the prototype.Comment: 17 pages, accepted for publication in A&
Modeling Growth and Telomere Dynamics in Saccharomyces Cerevisiae
A general branching process is proposed to model a population of cells of the yeast Saccharomyces cerevisiae following loss of telomerase. Previously published experimental data indicate that a population of telomerase-deficient cells regain exponential growth after a period of slowing due to critical telomere shortening. The explanation for this phenomenon is that some cells engage telomerase-independent pathways to maintain telomeres that allow them to become “survivors.” Our model takes into account random variation in individual cell cycle times, telomere length, finite lifespan of mother cells, and survivorship. We identify and estimate crucial parameters such as the probability of an individual cell becoming a survivor, and compare our model predictions to experimental data
Where is the warm H2 ? A search for H2 emission from disks around Herbig Ae/Be stars
Mid-IR emission lines of H2 are useful probes to determine the mass of warm
gas present in the surface layers of disks. Numerous observations of Herbig
Ae/Be stars (HAeBes) have been performed, but only 2 detections of mid-IR H2
toward HD97048 and AB Aur have been reported. We aim at tracing the warm gas in
the disks of 5 HAeBes with gas-rich environments and physical characteristics
close to those of AB Aur and HD97048, to discuss whether the detections toward
these 2 objects are suggestive of peculiar conditions for the gas. We search
for the H2 S(1) emission line at 17.035 \mu\m with VISIR, and complemented by
CH molecule observations with UVES. We gather the H2 measurements from the
literature to put the new results in context and search for a correlation with
some disk properties. None of the 5 VISIR targets shows evidence for H2
emission. From the 3sigma upper limits on the integrated line fluxes we
constrain the amount of optically thin warm gas to be less than 1.4 M_Jup in
the disk surface layers. There are now 20 HAeBes observed with VISIR and TEXES
instruments to search for warm H2, but only two detections (HD97048 and AB Aur)
were made so far. We find that the two stars with detected warm H2 show at the
same time high 30/13 \mu\m flux ratios and large PAH line fluxes at 8.6 and
11.3 \mu\m compared to the bulk of observed HAeBes and have emission CO lines
detected at 4.7 \mu\m. We detect the CH 4300.3A absorption line toward both
HD97048 and AB Aur with UVES. The CH to H2 abundance ratios that this would
imply if it were to arise from the same component as well as the radial
velocity of the CH lines both suggest that CH arises from a surrounding
envelope, while the detected H2 would reside in the disk. The two detections of
the S(1) line in the disks of HD97048 and AB Aur suggest either peculiar
physical conditions or a particular stage of evolution.Comment: accepted for publication in A&A : 10 pages, 6 figure
On the ubiquity of molecular anions in the dense interstellar medium
Results are presented from a survey for molecular anions in seven nearby
Galactic star-forming cores and molecular clouds. The hydrocarbon anion C6H- is
detected in all seven target sources, including four sources where no anions
have been previously detected: L1172, L1389, L1495B and TMC-1C. The C6H-/C6H
column density ratio is greater than about 1.0% in every source, with a mean
value of 3.0% (and standard deviation 0.92%). Combined with previous
detections, our results show that anions are ubiquitous in dense clouds
wherever C6H is present. The C6H-/C6H ratio is found to show a positive
correlation with molecular hydrogen number density, and with the apparent age
of the cloud. We also report the first detection of C4H- in TMC-1 (at 4.8-sigma
confidence), and derive an anion-to-neutral ratio C4H-/C4H = (1.2 +- 0.4) x
10^-5 (= 0.0012 +- 0.0004%). Such a low value compared with C6H- highlights the
need for a revised radiative electron attachment rate for C4H. Chemical model
calculations show that the observed C4H- could be produced as a result of
reactions of oxygen atoms with C5H- and C6H-
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