5,250 research outputs found
A Monte Carlo study of surface critical phenomena: The special point
We study the special point in the phase diagram of a semi-infinite system,
where the bulk transition is in the three-dimensional Ising universality class.
To this end we perform a finite size scaling study of the improved Blume-Capel
model on the simple cubic lattice with two different types of surface
interactions. In order to check for the effect of leading bulk corrections we
have also simulated the spin-1/2 Ising model on the simple cubic lattice. We
have accurately estimated the surface enhancement coupling at the special point
of these models. We find and for the
surface renormalization group exponents of the special transitions. These
results are compared with previous ones obtained by using field theoretic
methods and Monte Carlo simulations of the spin-1/2 Ising model. Furthermore we
study the behaviour of the surface transition near the special point and
finally we discuss films with special boundary conditions at one surface and
fixed ones at the other.Comment: 21 pages, 2 figures. figure 1 replaced, various typos correcte
Galactic annihilation emission from nucleosynthesis positrons
The Galaxy hosts a widespread population of low-energy positrons revealed by
successive generations of gamma-ray telescopes through a bright annihilation
emission from the bulge region, with a fainter contribution from the inner
disk. The exact origin of these particles remains currently unknown. We
estimate the contribution to the annihilation signal of positrons generated in
the decay of radioactive 26Al, 56Ni and 44Ti. We adapted the GALPROP
propagation code to simulate the transport and annihilation of radioactivity
positrons in a model of our Galaxy. Using plausible source spatial
distributions, we explored several possible propagation scenarios to account
for the large uncertainties on the transport of ~1MeV positrons in the
interstellar medium. We then compared the predicted intensity distributions to
the INTEGRAL/SPI observations. We obtain similar intensity distributions with
small bulge-to-disk ratios, even for extreme large-scale transport
prescriptions. At least half of the positrons annihilate close to their
sources, even when they are allowed to travel far away. In the high-diffusion,
ballistic case, up to 40% of them escape the Galaxy. In proportion, this
affects bulge positrons more than disk positrons because they are injected
further off the plane in a tenuous medium, while disk positrons are mostly
injected in the dense molecular ring. The predicted intensity distributions are
fully consistent with the observed longitudinally-extended disk-like emission,
but the transport scenario cannot be strongly constrained by the current data.
Nucleosynthesis positrons alone cannot account for the observed annihilation
emission in the frame of our model. An additional component is needed to
explain the strong bulge contribution, and the latter is very likely
concentrated in the central regions if positrons have initial energies in the
100keV-1MeV range.Comment: 16 pages, 7 figures, accepted for publication in A&
Renormalized field theory and particle density profile in driven diffusive systems with open boundaries
We investigate the density profile in a driven diffusive system caused by a
plane particle source perpendicular to the driving force. Focussing on the case
of critical bulk density we use a field theoretic renormalization
group approach to calculate the density as a function of the distance
from the particle source at first order in (: spatial
dimension). For we find reasonable agreement with the exact solution
recently obtained for the asymmetric exclusion model. Logarithmic corrections
to the mean field profile are computed for with the result for .Comment: 32 pages, RevTex, 4 Postscript figures, to appear in Phys. Rev.
Kinematics of massive star ejecta in the Milky Way as traced by Al
Context. Massive stars form in groups and their winds and supernova explosions create superbubbles up to kpc in size. The fate of their ejecta is of vital importance for the dynamics of the interstellar medium, for chemical evolution models, and the chemical enrichment of galactic halos and the intergalactic medium. However, ejecta kinematics and the characteristic scales in space and time have not been explored in great detail beyond ~10 Ka. Aims: Through measurement of radioactive 26Al with its decay time constant at ~106 years, we aim to trace the kinematics of cumulative massive-star and supernova ejecta independent of the uncertain gas parameters over million-year time scales. Our goal is to identify the mixing time scale and the spatio-kinematics of such ejecta from the pc to kpc scale in our Milky Way. Methods: We use the SPI spectrometer on the INTEGRAL observatory and its observations along the Galactic ridge to trace the detailed line shape systematics of the 1808.63 keV gamma-ray line from 26Al decay. We determine line centroids and compare these to Doppler shift expectations from large-scale systematic rotation around the Galaxy centre, as observed in other Galactic objects. Results: We measure the radial velocities of gas traced by 26Al, averaged over the line of sight, as a function of Galactic longitude. We find substantially higher velocities than expected from Galactic rotation, the average bulk velocity being ~200 km s-1 larger than predicted from Galactic rotation. The observed radial velocity spread implies a Doppler broadening of the gamma-ray line that is consistent with our measurements of the overall line width. We can reproduce the observed characteristics with 26Al sources located along the inner spiral arms, when we add a global blow-out preference into the forward direction away from arms into the inter-arm region, as is expected when massive stars are offset towards the spiral-arm leading edge. With the known connection of superbubbles to the gaseous halo, this implies angular-momentum transfer in the disk-halo system and consequently also radial gas flows. The structure of the interstellar gas above the disk affects how ionizing radiation may escape and ionize intergalactic gas.Peer reviewe
Regionalentwicklung und Armut: Theoretische Grundlagen einer regional-orientierten Politik der Armutsbekämpfung unter besonderer Berücksichtigung Chinas
Ungleichverteilungen von Wohlstand und Armut treten nicht nur im internationalen, sondern auch innerhalb einzelner Länder im interregionalen Vergleich auf. Innerhalb der einzelnen Länder zeigt sich zudem oftmals eine Konzentration der absolut armen Menschen auf einzelne Regionen. Dieses Phänomen ist nicht neu,1 aber von aktueller Bedeutung, da im Zuge des ordnungspolitischen Transformationsprozesses2 in vielen Ländern zumindest in den ersten Transformationsphasen meist ein spürbarer Anstieg der Zahl der absolut Armen erfolgt. Die gleichzeitige regionale Konzentration dieser Armutszunahme kann sich durchaus als Sprengsatz für die Fortsetzung der Reformpolitik erweisen. Insofern ist die Analyse der Ursachen dieser regional ungleichen Entwicklung sowie die Erarbeitung geeigneter Gegenmaßnahmen von hoher Bedeutung für die Transformations- und Entwicklungspolitik. Der vorliegende Aufsatz versucht, hierzu einen Beitrag zu leisten. Zur empirischen Untermauerung der einzelnen Aussagen wird dabei ein Akzent auf die VR China gelegt, in der derzeit erhebliche regionale Disparitäten vorliegen. --
Connection between the elastic GEp/GMp and P to Delta form factors
It is suggested that the falloff in Qsq of the P to Delta magnetic form
factor GM* is related to the recently observed falloff of the elastic electric
form factor GEp/GMp. Calculation is carried out in the framework of a GPD
mechanism
Surface critical behavior of driven diffusive systems with open boundaries
Using field theoretic renormalization group methods we study the critical
behavior of a driven diffusive system near a boundary perpendicular to the
driving force. The boundary acts as a particle reservoir which is necessary to
maintain the critical particle density in the bulk. The scaling behavior of
correlation and response functions is governed by a new exponent eta_1 which is
related to the anomalous scaling dimension of the chemical potential of the
boundary. The new exponent and a universal amplitude ratio for the density
profile are calculated at first order in epsilon = 5-d. Some of our results are
checked by computer simulations.Comment: 10 pages ReVTeX, 6 figures include
Generalized Parton Distributions -- theoretical review --
In this talk an introduction to generalized parton distributions is given.
Recent developments are shortly reviewed, including non-perturbative
calculations, phenomenological aspects and evaluation of higher order
perturbative and power corrections.Comment: 11 pages, 5 figure
Comparing simulated Al maps to gamma-ray measurements
© ESO 2019.Context. The diffuse gamma-ray emission of at 1.8 MeV reflects ongoing nucleosynthesis in the Milky Way, and traces massive-star feedback in the interstellar medium due to its 1 Myr radioactive lifetime. Interstellar-medium morphology and dynamics are investigated in astrophysics through 3D hydrodynamic simulations in fine detail, as only few suitable astronomical probes are available. Aims. We compare a galactic-scale hydrodynamic simulation of the Galaxy's interstellar medium, including feedback and nucleosynthesis, with gamma-ray data on emission in the Milky Way extracting constraints that are only weakly dependent on the particular realisation of the simulation or Galaxy structure. Methods. Due to constraints and biases in both the simulations and the gamma-ray observations, such comparisons are not straightforward. For a direct comparison, we perform maximum likelihood fits of simulated sky maps as well as observation-based maximum entropy maps to measurements with INTEGRAL/SPI. To study general morphological properties, we compare the scale heights of emission produced by the simulation to INTEGRAL/SPI measurements.} Results. The direct comparison shows that the simulation describes the observed inner Galaxy well, but differs significantly from the observed full-sky emission morphology. Comparing the scale height distribution, we see similarities for small scale height features and a mismatch at larger scale heights. We attribute this to the prominent foreground emission sites that are not captured by the simulation.Peer reviewedFinal Accepted Versio
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