254 research outputs found
Statistical Assessment of Shapes and Magnetic Field Orientations in Molecular Clouds through Polarization Observations
We present a novel statistical analysis aimed at deriving the intrinsic
shapes and magnetic field orientations of molecular clouds using dust emission
and polarization observations by the Hertz polarimeter. Our observables are the
aspect ratio of the projected plane-of-the-sky cloud image, and the angle
between the mean direction of the plane-of-the-sky component of the magnetic
field and the short axis of the cloud image. To overcome projection effects due
to the unknown orientation of the line-of-sight, we combine observations from
24 clouds, assuming that line-of-sight orientations are random and all are
equally probable. Through a weighted least-squares analysis, we find that the
best-fit intrinsic cloud shape describing our sample is an oblate disk with
only small degrees of triaxiality. The best-fit intrinsic magnetic field
orientation is close to the direction of the shortest cloud axis, with small
(~24 deg) deviations toward the long/middle cloud axes. However, due to the
small number of observed clouds, the power of our analysis to reject
alternative configurations is limited.Comment: 14 pages, 8 figures, accepted for publication in MNRA
Magnetic Fields in Star-Forming Molecular Clouds II. The Depolarization Effect in the OMC-3 Filament of Orion A
Polarized 850 micron thermal emission data of the region OMC-3 in the Orion A
molecular cloud are presented. These data, taken in 1998 with the SCUBA
polarimeter mounted on the James Clerk Maxwell Telescope, have been re-reduced
using improved software. The polarization pattern is not suggestive of a
uniform field structure local to OMC-3, nor does the orientation of the vectors
align with existing polarimetry maps of the OMC-1 core 20' to the south. The
depolarization toward high intensity regions cannot be explained by uniform
field geometry except in the presence of changing grain structure, which is
most likely to occur in regions of high density or temperature (i.e. the
embedded cores). The depolarization in fact occurs along the length of the
filamentary structure of OMC-3 and is not limited to the vicinity of the bright
cores. Such a polarization pattern is predicted by helical field models for
filamentary clouds. We present three scenarios to explain the observed
polarization pattern of OMC-3 in terms of a helical field geometry. Qualitative
models incorporating a helical field geometry are presented for two cases.Comment: 57 pages, 12 figures, 3 tables; accepted for publication in Ap
Measurement of the magnetic field direction in the NGC2024-FIR5 protostellar outflow
Molecular outflows from young protostars are widely believed to be collimated
by magnetic fields, but there has been little observational evidence to support
this hypothesis. Using the new technique of millimetre-wavelength
spectro-polarimetry, we demonstrate the existence of a magnetic field in the
NGC2024-FIR5 outflow lobe. The 1.3mm J=2-1 transition of carbon monoxide (CO)
is polarized at a level of approximately 1%, in a direction within 10-15
degrees of the outflow axis. This agrees with theoretical models where the
magnetic field channels the outflowing gas, and shows that the process can be
effective as far as 0.1pc from the protostar.Comment: Accepted by ApJL. 10 pages, including 2 figure
Effects of Kynurenine Pathway Inhibition on NAD+ Metabolism and Cell Viability in Human Primary Astrocytes and Neurons
The kynurenine pathway (KP) is the principle route of L-Tryptophan (TRP) metabolism, producing several neurotoxic and neuroprotective metabolic precursors before complete oxidation to the essential pyridine nucleotide nicotinamide adenine dinucleotide (NAD+). KP inhibition may prove therapeutic in central nervous system (CNS) inflammation by reducing the production of excitotoxins such as quinolinic acid (QUIN). However, KP metabolism may also be cytoprotective through the de novo synthesis of intracellular NAD+. We tested the hypothesis that the KP is directly involved in the maintenance of intracellular NAD+ levels and SIRT1 function in primary astrocytes and neurons through regulation of NAD+ synthesis. Competitive inhibition of indoleamine 2,3 dioxygenase (IDO), and quinolinic acid phosphoribosyltransferase (QPRT) activities with 1-methyl-L-Tryptophan (1-MT), and phthalic acid (PA) respectively, resulted in a dose-dependent decrease in intracellular NAD+ levels and sirtuin deacetylase-1 (SIRT1) activity, and correlated directly with reduced cell viability. These results support the hypothesis that the primary role of KP activation during neuroinflammation is to maintain NAD+ levels through de novo synthesis from TRP. Inhibition of KP metabolism under these conditions can compromise cell viability, NAD-dependent SIRT1 activity and CNS function, unless alternative precursors for NAD+ synthesis are made available
A Genetic Algorithm-Based Exploration of Three Filament Models: A Case for the Magnetic Support of the G11.11-0.12 Infrared-Dark Cloud
The G11.11-0.12 infrared-dark cloud has a filamentary appearance, both in
extinction against the diffuse infrared emission of the Galactic plane and in
emission at 850 microns. We use a novel computational technique based on an
advanced genetic algorithm to explore thoroughly 3 different models of
self-gravitating, pressure truncated filaments and to constrain their
parameters. Specifically, the models tested are the non-magnetic Ostriker
(1964) model, a generalized version of the magnetic Stodolkiewicz (1963) model,
and the magnetic Fiege & Pudritz (2000) model. Previous results showed that
G11.11-0.12 has a much steeper r^{-4} radial density profile than other
filaments, where the density varies approximately as r^{-2}, and that this
steep density profile is consistent with the Ostriker (1964) model. We present
a more complete analysis that shows that the radial structure of G11.11-0.12 is
consistent with regimes of each of these models. All of the magnetic models
that agree with the data are threaded by a dominant poloidal magnetic field,
and most have dynamically significant fields. Thus, G11.11-0.12 is an excellent
candidate for radial support by a magnetic field that is predominantly
poloidal. We predict the polarization patterns expected for both magnetic
models and show that the two magnetic models produce different polarization
patterns that should be distingished by observations.Comment: To appear in Ap.J. Dec. 1 edition, volume 616. 40 pages and 42
figures. Figures are severely reduced to satisfy astro-ph size limits. A
version with higher quality figures is available by contacting the first
autho
Structure and Stability of Keplerian MHD Jets
MHD jet equilibria that depend on source properties are obtained using a
simplified model for stationary, axisymmetric and rotating magnetized outflows.
The present rotation laws are more complex than previously considered and
include a Keplerian disc. The ensuing jets have a dense, current-carrying
central core surrounded by an outer collar with a return current. The
intermediate part of the jet is almost current-free and is magnetically
dominated. Most of the momentum is located around the axis in the dense core
and this region is likely to dominate the dynamics of the jet. We address the
linear stability and the non-linear development of instabilities for our models
using both analytical and 2.5-D numerical simulation's. The instabilities seen
in the simulations develop with a wavelength and growth time that are well
matched by the stability analysis. The modes explored in this work may provide
a natural explanation for knots observed in astrophysical jets.Comment: 35 pages, accepted by the Ap
The G11.11-0.12 Infrared-Dark Cloud: Anomalous Dust and a Non-Magnetic Isothermal Model
The G11.11-0.12 Infrared-Dark Cloud has a filamentary appearance, both in
absorption against the diffuse 8micron Galactic background, and in emission
from cold dust at 850micron. Detailed comparison of the dust properties at
these two wavelengths reveals that standard models for the diffuse interstellar
dust in the Galaxy are not consistent with the observations. The ratio of
absorption coefficients within the cloud is kappa_8/kappa_850 <= 1010, which is
well below that expected for the diffuse ISM where kappa_8/kappa_850 ~ 1700.
This may be due to the formation of ice mantles on the dust and grain
coagulation, both of which are expected within dense regions of molecular
clouds. The 850micron emission probes the underlying radial structure of the
filament. The profile is well represented by a marginally resolved central
region and a steeply falling envelope, with Sigma(r) proportional to r^(-a),
where a <= 3, indicating that G11.11-0.12 is the first observed filament with a
profile similar to that of a non-magnetic isothermal cylinder.Comment: 13 pages, 4 figures, accepted for publication by ApJ Letter
The JCMT Gould Belt Survey: properties of star-forming filaments in Orion A North
We develop and apply a Hessian-based filament detection algorithm to submillimetre continuum observations of Orion A North. The resultant filament radial density profiles are fitted with beam-convolved line-of-sight Plummer-profiles using Markov chain Monte Carlo techniques. The posterior distribution of the radial decay parameter demonstrates that the majority of filaments exhibit p = 1.5–3, with a mode at p = 2.2, suggesting deviation from the Ostriker p = 4 isothermal, equilibrium, self-gravitating cylinder. The spatial distribution of young stellar objects relative to the high column density filaments is investigated, yielding a lower limit on the star-forming age of the integral-shaped filament ∼1.4 Myr. Additionally, inferred lifetimes of filaments are examined which suggest long-term filament accretion, varying rates of star formation, or both. Theoretical filament stability measures are determined with the aid of HARP C18O J = 3–2 observations and indicate that the majority of filaments are gravitationally subcritical, despite the presence of young protostars. The results from this investigation are consistent with the one-dimensional accretion flow filament model recently observed in numerical simulations
Efficient Passive ICS Device Discovery and Identification by MAC Address Correlation
Owing to a growing number of attacks, the assessment of Industrial Control
Systems (ICSs) has gained in importance. An integral part of an assessment is
the creation of a detailed inventory of all connected devices, enabling
vulnerability evaluations. For this purpose, scans of networks are crucial.
Active scanning, which generates irregular traffic, is a method to get an
overview of connected and active devices. Since such additional traffic may
lead to an unexpected behavior of devices, active scanning methods should be
avoided in critical infrastructure networks. In such cases, passive network
monitoring offers an alternative, which is often used in conjunction with
complex deep-packet inspection techniques. There are very few publications on
lightweight passive scanning methodologies for industrial networks. In this
paper, we propose a lightweight passive network monitoring technique using an
efficient Media Access Control (MAC) address-based identification of industrial
devices. Based on an incomplete set of known MAC address to device
associations, the presented method can guess correct device and vendor
information. Proving the feasibility of the method, an implementation is also
introduced and evaluated regarding its efficiency. The feasibility of
predicting a specific device/vendor combination is demonstrated by having
similar devices in the database. In our ICS testbed, we reached a host
discovery rate of 100% at an identification rate of more than 66%,
outperforming the results of existing tools.Comment: http://dx.doi.org/10.14236/ewic/ICS2018.
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