2,139 research outputs found
Temporal evolution of magnetic molecular shocks II. Analytics of the steady state and semi-analytical construction of intermediate ages
In the first paper of this series (Paper I) we computed time dependent
simulations of multifluid shocks with chemistry and a transverse magnetic field
frozen in the ions, using an adaptive moving grid. In this paper, we present
new analytical results on steady-state molecular shocks. Relationships between
density and pressure in the neutral fluid are derived for the cold magnetic
precursor, hot magnetic precursor, adiabatic shock front, and the following
cooling layer. The compression ratio and temperature behind a fully
dissociative adiabatic shock is also derived. To prove that these results may
even hold for intermediate ages, we design a test to locally characterise the
validity of the steady state equations in a time-dependent shock simulation.
Applying this tool to the results of Paper I, we show that most of these shocks
(all the stable ones) are indeed in a quasi-steady state at all times, i.e. : a
given snapshot is composed of one or more truncated steady shock. Finally, we
use this property to produce a construction method of any intermediate time of
low velocity shocks (u < 20 km/s) with only a steady-state code. In particular,
this method allows one to predict the occurrence of steady CJ-type shocks more
accurately than previously proposed criteria.Comment: A&A in pres
Converts and Islamist Terrorism: An Introduction
Converts to Islam represent a small percentage of the Muslim community in Western countries. Yet when it comes to Islamist extremism and terrorism, research has suggested that converts are considerably overrepresented. This ICCT Policy Brief serves as an introduction to this topic by providing an overview of what is known about convertsâ involvement in homegrown jihadism and the foreign fighter phenomenon. Notwithstanding considerable reservations about the quantity and quality of the available data, this Policy Brief finds support for the notion of convert overrepresentation in these activities. This is especially so in the case of foreign fighters. What little data was found on convertsâ involvement in homegrown jihadism provided a more nuanced picture, emphasizing that overrepresentation may not be the norm in all Western countries and that it may be a relatively recent development. Numerous explanations for convertsâ involvement in Islamist extremism and terrorism have been provided, running the gamut from structural-level explanations to distinctly personal motives. At present, however, a comprehensive, theoretically sound and empirically grounded understanding of how and why converts become involved in Islamist militancy is absent. The Policy Brief concludes by stressing the need to develop our understanding of this important yet under-researched topic.Security and Global Affair
Detection of FeO towards SgrB2
We have observed the J=5-4 ground state transition of FeO at a frequency of
153 GHz towards a selection of galactic sources.
Towards the galactic center source SgrB2, we see weak absorption at
approximately the velocity of other features towards this source (62 km
s LSR).
Towards other sources, the results were negative as they were also for
MgOH(3-2) and FeC(6-5). We tentatively conclude that the absorption seen toward
SgrB2 is due to FeO in the hot ( 500 K) relatively low density absorbing
gas known to be present in this line of sight.
This is the first (albeit tentative) detection of FeO or any iron--containing
molecule in the interstellar gas. Assuming the observed absorption to be due to
FeO, we estimate [FeO]/[SiO] to be of order or less than 0.002 and
[FeO]/[H] of order . This is compatible with our negative
results in other sources.
Our results suggest that the iron liberated from grains in the shocks
associated with SgrB2 remains atomic and is not processed into molecular form.Comment: 1 postscrit figure,10 page
On the evolution of the molecular line profiles induced by the propagation of C-shock waves
We present the first results of the expected variations of the molecular line
emission arising from material recently affected by C-shocks (shock
precursors). Our parametric model of the structure of C-shocks has been coupled
with a radiative transfer code to calculate the molecular excitation and line
profiles of shock tracers such as SiO, and of ion and neutral molecules such as
H13CO+ and HN13C, as the shock propagates through the unperturbed medium. Our
results show that the SiO emission arising from the early stage of the magnetic
precursor typically has very narrow line profiles slightly shifted in velocity
with respect to the ambient cloud. This narrow emission is generated in the
region where the bulk of the ion fluid has already slipped to larger velocities
in the precursor as observed toward the young L1448-mm outflow. This strongly
suggests that the detection of narrow SiO emission and of an ion enhancement in
young shocks, is produced by the magnetic precursor of C-shocks. In addition,
our model shows that the different velocity components observed toward this
outflow can be explained by the coexistence of different shocks at different
evolutionary stages, within the same beam of the single-dish observations.Comment: 7 pages, 4 figures, accepted for publication in Ap
Water emission from the high-mass star-forming region IRAS 17233-3606. High water abundances at high velocities
We investigate the physical and chemical processes at work during the
formation of a massive protostar based on the observation of water in an
outflow from a very young object previously detected in H2 and SiO in the IRAS
17233-3606 region. We estimated the abundance of water to understand its
chemistry, and to constrain the mass of the emitting outflow. We present new
observations of shocked water obtained with the HIFI receiver onboard Herschel.
We detected water at high velocities in a range similar to SiO. We
self-consistently fitted these observations along with previous SiO data
through a state-of-the-art, one-dimensional, stationary C-shock model. We found
that a single model can explain the SiO and H2O emission in the red and blue
wings of the spectra. Remarkably, one common area, similar to that found for H2
emission, fits both the SiO and H2O emission regions. This shock model
subsequently allowed us to assess the shocked water column density,
N(H2O)=1.2x10^{18} cm^{-2}, mass, M(H2O)=12.5 M_earth, and its maximum
fractional abundance with respect to the total density, x(H2O)=1.4x10^{-4}. The
corresponding water abundance in fractional column density units ranges between
2.5x10^{-5} and 1.2x10^{-5}, in agreement with recent results obtained in
outflows from low- and high-mass young stellar objects.Comment: accepted for publication as a Letter in Astronomy and Astrophysic
Ten years of different crop rotations in a no-tillage system â what happened to plant diseases and nematode pests?
Aim
To study the long-term effects of crop rotation and residue level on diseases and nematodes in Western Australian no-tillage systems
Rovibrationally resolved photodissociation of HeH+
Accurate photodissociation cross sections have been obtained for the A-X
electronic transition of HeH+ using ab initio potential curves and dipole
transition moments. Partial cross sections have been evaluated for all
rotational transitions from the vibrational levels v"=0-11 and over the entire
accessible wavelength range 100-1129 Angstrom. Assuming a Boltzmann
distribution of the rovibrational levels of the X state, photodissociation
cross sections are presented for temperatures between 500 and 12,000 K. A
similar set of calculations was performed for the pure rovibrational
photodissociation in the X-X electronic ground state, but covering photon
wavelengths into the far infrared. Applications of the cross sections to the
destruction of HeH+in the early Universe and in UV-irradiated environments such
as primordial halos and protoplanetary disks are briefly discussed
Converts and Islamist Terrorism: An Introduction
Converts to Islam represent a small percentage of the Muslim community in Western countries. Yet when it comes to Islamist extremism and terrorism, research has suggested that converts are considerably overrepresented. This ICCT Policy Brief serves as an introduction to this topic by providing an overview of what is known about convertsâ involvement in homegrown jihadism and the foreign fighter phenomenon. Notwithstanding considerable reservations about the quantity and quality of the available data, this Policy Brief finds support for the notion of convert overrepresentation in these activities. This is especially so in the case of foreign fighters. What little data was found on convertsâ involvement in homegrown jihadism provided a more nuanced picture, emphasizing that overrepresentation may not be the norm in all Western countries and that it may be a relatively recent development. Numerous explanations for convertsâ involvement in Islamist extremism and terrorism have been provided, running the gamut from structural-level explanations to distinctly personal motives. At present, however, a comprehensive, theoretically sound and empirically grounded understanding of how and why converts become involved in Islamist militancy is absent. The Policy Brief concludes by stressing the need to develop our understanding of this important yet under-researched topic.Security and Global Affair
Impossibility of spontaneously breaking local symmetries and the sign problem
Elitzur's theorem stating the impossibility of spontaneous breaking of local
symmetries in a gauge theory is reexamined. The existing proofs of this theorem
rely on gauge invariance as well as positivity of the weight in the Euclidean
partition function. We examine the validity of Elitzur's theorem in gauge
theories for which the Euclidean measure of the partition function is not
positive definite. We find that Elitzur's theorem does not follow from gauge
invariance alone. We formulate a general criterion under which spontaneous
breaking of local symmetries in a gauge theory is excluded. Finally we
illustrate the results in an exactly solvable two dimensional abelian gauge
theory.Comment: Latex 6 page
Massive expanding torus and fast outflow in planetary nebula NGC 6302
We present interferometric observations of CO and CO =21
emission from the butterfly-shaped, young planetary nebula NGC 6302. The high
angular resolution and high sensitivity achieved in our observations allow us
to resolve the nebula into two distinct kinematic components: (1) a massive
expanding torus seen almost edge-on and oriented in the North-South direction,
roughly perpendicular to the optical nebula axis. The torus exhibits very
complex and fragmentated structure; (2) high velocity molecular knots moving at
high velocity, higher than 20 \kms, and located in the optical bipolar lobes.
These knots show a linear position-velocity gradient (Hubble-like flow), which
is characteristic of fast molecular outflow in young planetary nebulae. From
the low but variable CO/CO =21 line intensity ratio we
conclude that the CO =21 emission is optically thick over much of
the nebula. Using the optically thinner line CO =21 we estimate a
total molecular gas mass of 0.1 M, comparable to the ionized gas
mass; the total gas mass of the NGC 6302 nebula, including the massive ionized
gas from photon dominated region, is found to be 0.5 M. From
radiative transfer modelling we infer that the torus is seen at inclination
angle of 75 with respect to the plane of the sky and expanding at
velocity of 15 \kms. Comparison with recent observations of molecular gas in
NGC 6302 is also discussed.Comment: 24 pages, 7 figures, accepted for publication in Astrophysical
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