2,476 research outputs found
Radionuclide Ionization in Protoplanetary Disks: Calculations of Decay Product Radiative Transfer
We present simple analytic solutions for the ionization rate
arising from the decay of short-lived radionuclides (SLRs)
within protoplanetary disks. We solve the radiative transfer problem for the
decay products within the disk, and thereby allow for the loss of radiation at
low disk surface densities; energy loss becomes important outside
for typical disk masses M. Previous studies of
chemistry/physics in these disks have neglected the impact of ionization by
SLRs, and often consider only cosmic rays (CRs), because of the high CR-rate
present in the ISM. However, recent work suggests that the flux of CRs present
in the circumstellar environment could be substantially reduced by relatively
modest stellar winds, resulting in severely modulated CR ionization rates,
, equal to or substantially below that of SLRs
( s). We compute the net ionizing
particle fluxes and corresponding ionization rates as a function of position
within the disk for a variety of disk models. The resulting expressions are
especially simple for the case of vertically gaussian disks (frequently assumed
in the literature). Finally, we provide a power-law fit to the ionization rate
in the midplane as a function of gas disk surface density and time. Depending
on location in the disk, the ionization rates by SLRs are typically in the
range s.Comment: 7 pages, 4 figures, accepted to Ap
Exploiting flow dynamics for super-resolution in contrast-enhanced ultrasound
Ultrasound localization microscopy offers new radiation-free diagnostic tools
for vascular imaging deep within the tissue. Sequential localization of echoes
returned from inert microbubbles with low-concentration within the bloodstream
reveal the vasculature with capillary resolution. Despite its high spatial
resolution, low microbubble concentrations dictate the acquisition of tens of
thousands of images, over the course of several seconds to tens of seconds, to
produce a single super-resolved image. %since each echo is required to be well
separated from adjacent microbubbles. Such long acquisition times and stringent
constraints on microbubble concentration are undesirable in many clinical
scenarios. To address these restrictions, sparsity-based approaches have
recently been developed. These methods reduce the total acquisition time
dramatically, while maintaining good spatial resolution in settings with
considerable microbubble overlap. %Yet, non of the reported methods exploit the
fact that microbubbles actually flow within the bloodstream. % to improve
recovery. Here, we further improve sparsity-based super-resolution ultrasound
imaging by exploiting the inherent flow of microbubbles and utilize their
motion kinematics. While doing so, we also provide quantitative measurements of
microbubble velocities. Our method relies on simultaneous tracking and
super-localization of individual microbubbles in a frame-by-frame manner, and
as such, may be suitable for real-time implementation. We demonstrate the
effectiveness of the proposed approach on both simulations and {\it in-vivo}
contrast enhanced human prostate scans, acquired with a clinically approved
scanner.Comment: 11 pages, 9 figure
A Preliminary Investigation for Application of an Advanced X-Ray Diffraction Analyzer to In-Process Texture Assessment of Aluminum Alloy Sheet
The objective of the this preliminary investigation was to assess the possibility of the use of rapid x-ray diffraction technology to non-destructively distinguish various texture conditions of aluminum can stock
Outflow forces of low mass embedded objects in Ophiuchus: a quantitative comparison of analysis methods
The outflow force of molecular bipolar outflows is a key parameter in
theories of young stellar feedback on their surroundings. The focus of many
outflow studies is the correlation between the outflow force, bolometric
luminosity and envelope mass. However, it is difficult to combine the results
of different studies in large evolutionary plots over many orders of magnitude
due to the range of data quality, analysis methods and corrections for
observational effects such as opacity and inclination. We aim to determine the
outflow force for a sample of low luminosity embedded sources. We will quantify
the influence of the analysis method and the assumptions entering the
calculation of the outflow force. We use the James Clerk Maxwell Telescope to
map 12CO J=3-2 over 2'x2' regions around 16 Class I sources of a well-defined
sample in Ophiuchus at 15" resolution. The outflow force is then calculated
using seven different methods differing e.g. in the use of intensity-weighted
emission and correction factors for inclination. The results from the analysis
methods differ from each other by up to a factor of 6, whereas observational
properties and choices in the analysis procedure affect the outflow force by up
to a factor of 4. For the sample of Class I objects, bipolar outflows are
detected around 13 sources including 5 new detections, where the three
non-detections are confused by nearby outflows from other sources. When
combining outflow forces from different studies, a scatter by up to a factor of
5 can be expected. Although the true outflow force remains unknown, the
separation method (separate calculation of dynamical time and momentum) is
least affected by the uncertain observational parameters. The correlations
between outflow force, bolometric luminosity and envelope mass are further
confirmed down to low luminosity sources.Comment: 24 pages, 13 figures, Accepted by A&
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