21,213 research outputs found
A Turbulent Model for the Interstellar Medium. II. Magnetic Fields and Rotation
We present results from two-dimensional numerical simulations of a supersonic
turbulent flow in the plane of the galactic disk, incorporating shear,
thresholded and discrete star formation (SF), self-gravity, rotation and
magnetic fields. A test of the model in the linear regime supports the results
of the linear theory of Elmegreen (1991). In the fully nonlinear turbulent
regime, while some results of the linear theory persist, new effects also
emerge. Some exclusively nonlinear effects are: a) Even though there is no
dynamo in 2D, the simulations are able to maintain or increase their net
magnetic energy in the presence of a seed uniform azimuthal component. b) A
well-defined power-law magnetic spectrum and an inverse magnetic cascade are
observed in the simulations, indicating full MHD turbulence. Thus, magnetic
field energy is generated in regions of SF and cascades up to the largest
scales. c) The field has a slight but noticeable tendency to be aligned with
density features. d) The magnetic field prevents HII regions from expanding
freely, as in the recent results of Slavin \& Cox (1993). e) A tendency to
exhibit {\it less} filamentary structures at stronger values of the uniform
component of the magnetic field is present in several magnetic runs. f) For
fiducial values of the parameters, the flow in general appears to be in rough
equipartition between magnetic and kinetic energy. There is no clear domination
of either the magnetic or the inertial forces. g) A median value of the
magnetic field strength within clouds is G, while for the
intercloud medium a value of G is found. Maximum contrasts of up to
a factor of are observed.Comment: Plain TeX file, 25 pages. Gzipped, tarred set of Tex file plus 17
figures and 3 tables (Postscript) available at
ftp://kepler.astroscu.unam.mx/incoming/enro/papers/mhdgturb.tar.g
Highly Compressible MHD Turbulence and Gravitational Collapse
We investigate the properties of highly compressible turbulence and its
ability to produce self-gravitating structures. The compressibility is
parameterized by an effective polytropic exponent gama-eff. In the limit of
small gama-eff, the density jump at shocks is shown to be of the order of
e^{M^2}, and the production of vorticity by the nonlinear terms appears to be
negligible. In the presence of self-gravity, we suggest that turbulence can
produce bound structures for gama-eff < 2(1-1/n), where 'n' is the typical
dimensionality of the turbulent compressions. We show, by means of numerical
simulations, that, for sufficiently small gama-eff, small-scale turbulent
density fluctuations eventually collapse even though the medium is globally
stable. This result is preserved in the presence of a magnetic field for
supercritical mass-to-flux ratios.Comment: 4 pages, 3 postscript figures. Latex, uses aipproc.sty Contribution
to the Conference Proc. of the 7th Annual Astrophysics Conference in
Maryland, STAR FORMATION, NEAR AND FAR, eds. Stephen S. Holt and Lee G. Mund
Activities of \gamma-ray emitting isotopes in rainwater from Greater Sudbury, Canada following the Fukushima incident
We report the activity measured in rainwater samples collected in the Greater
Sudbury area of eastern Canada on 3, 16, 20, and 26 April 2011. The samples
were gamma-ray counted in a germanium detector and the isotopes 131I and 137Cs,
produced by the fission of 235U, and 134Cs, produced by neutron capture on
133Cs, were observed at elevated levels compared to a reference sample of
ice-water. These elevated activities are ascribed to the accident at the
Fukushima Dai-ichi nuclear reactor complex in Japan that followed the 11 March
earthquake and tsunami. The activity levels observed at no time presented
health concerns.Comment: 4 pages, 8 figure
The Micro Slit Gas Detector
We describe the first tests with a new proportional gas detector. Its
geometry consists in slits opened in a copper metallized kapton foil with 30
micron anode strips suspended in these openings. In this way the multiplication
process is similar to a standard MSGC. The fundamental difference is the
absence of an insulating substrate around the anode. Also the material budget
is significantly reduced, and the problems related to charging-up or
polarization are removed. Ageing properties of this detector are under study.Comment: 13 pages tex file, 10 figures ep
Cluster analysis to define distinct clinical phenotypes among septic patients with bloodstream infections
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The Wonder of Delays
Letâs get to, and celebrate loud and now, the raptor-like intensity of not being distracted that is Licia Fiol-Mattaâs The Great Woman Singer. By creating the complex badass ensemble of Myrta Silva, Ruth Fernandez, La Calandria, and Lucecita, Fiol-Matta asks us: what if we start here? What if we begin with the presumption of womenâs musical activity rather than from that shorthanded place of negation? A place that doesnât begin with how women entered, but made possible, the larger musical scene. This place, the beautiful migrant non-place of this book, goes straight for the historical fact of women in music, but also suggests how criticism (as a whole) has never been able to make this presumption ordinary to talk about them in a sustained, rather than corrective way. What links these performers is their multi-decade careersâwhich means that we get to follow them from girlhood to golden age. The robust longevity of these musicians, of women who made and make things in colonial modernity, suggests that we have to refigure just about everything that we think we know about the culture industry. Of the wonder of Lucecita and her bendy artistry of the mid-1960s, Fiol-Matta writes: âShe had arrived at the scene, but she could not be interpreted yetâ (184). I love this delay that the author leaves for herself and for all us here to say: these musicians require time and new words and worlds
Agent Based Models of Language Competition: Macroscopic descriptions and Order-Disorder transitions
We investigate the dynamics of two agent based models of language
competition. In the first model, each individual can be in one of two possible
states, either using language or language , while the second model
incorporates a third state XY, representing individuals that use both languages
(bilinguals). We analyze the models on complex networks and two-dimensional
square lattices by analytical and numerical methods, and show that they exhibit
a transition from one-language dominance to language coexistence. We find that
the coexistence of languages is more difficult to maintain in the Bilinguals
model, where the presence of bilinguals in use facilitates the ultimate
dominance of one of the two languages. A stability analysis reveals that the
coexistence is more unlikely to happen in poorly-connected than in fully
connected networks, and that the dominance of only one language is enhanced as
the connectivity decreases. This dominance effect is even stronger in a
two-dimensional space, where domain coarsening tends to drive the system
towards language consensus.Comment: 30 pages, 11 figure
Baryon chemical potential and in-medium properties of BPS skyrmions
We continue the investigation of thermodynamical properties of the BPS Skyrme
model. In particular, we analytically compute the baryon chemical potential
both in the full field theory and in a mean-field approximation. In the full
field theory case, we find that the baryon chemical potential is always exactly
proportional to the baryon density, for arbitrary solutions. We further find
that, in the mean-field approximation, the BPS Skyrme model approaches the
Walecka model in the limit of high density - their thermodynamical functions as
well as the equation of state agree in this limit. This fact allows to read off
some properties of the -meson from the BPS Skyrme action, even though
the latter model is entirely based on the (pionic) Skyrme field. On the
other hand, at low densities, at the order of the usual nuclear matter density,
the equations of state of the two models are no longer universal, such that a
comparison depends on some model details. Still, also the BPS Skyrme model
gives rise to nuclear saturation in this regime, leading, in fact, to an exact
balance between repulsive and attractive forces. The perfect fluid aspects of
the BPS Skyrme model, which, together with its BPS properties, form the base of
our results, are shown to be in close formal analogy with the Eulerian
formulation of relativistic fluid dynamics. Within this analogy, the BPS Skyrme
model, in general, corresponds to a non-barotropic perfect fluid.Comment: Latex, 28 pages, 3 figure
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