1,466 research outputs found
Exact Baryon, Strangeness and Charge Conservation in Hadronic Gas Models
Relativistic heavy ion collisions are studied assuming that particles can be
described by a hadron gas in thermal and chemical equilibrium. The exact
conservation of baryon number, strangeness and charge are explicitly taken into
account. For heavy ions the effect arising from the neutron surplus becomes
important and leads to a substantial increase in e.g. the ratio.
A method is developed which is very well suited for the study of small systems.Comment: 5 pages, 5 Postscript figure
The Snowball Stratosphere
According to the Snowball Earth hypothesis, Earth has experienced periods of
low-latitude glaciation in its deep past. Prior studies have used general
circulation models (GCMs) to examine the effects such an extreme climate state
might have on the structure and dynamics of Earth's troposphere, but the
behavior of the stratosphere has not been studied in detail. Understanding the
snowball stratosphere is important for developing an accurate account of the
Earth's radiative and chemical properties during these episodes. Here we
conduct the first analysis of the stratospheric circulation of the Snowball
Earth using ECHAM6 general circulation model simulations. In order to
understand the factors contributing to the stratospheric circulation, we extend
the Statistical Transformed Eulerian Mean framework. We find that the
stratosphere during a snowball with prescribed modern ozone levels exhibits a
weaker meridional overturning circulation, reduced wave activity, stronger
zonal jets, and is extremely cold relative to modern conditions. Notably, the
snowball stratosphere displays no sudden stratospheric warmings. Without ozone,
the stratosphere displays slightly weaker circulation, a complete lack of polar
vortex, and even colder temperatures. We also explicitly quantify for the first
time the cross-tropopause mass exchange rate and stratospheric mixing
efficiency during the snowball and show that our values do not change the
constraints on CO inferred from geochemical proxies during the Marinoan
glaciation (635 Ma), unless the O concentration during the snowball
was orders of magnitude less than the CO concentration.Comment: 28 pages, 11 figures, submitted to JGR-Atmosphere
First upper limit analysis and results from LIGO science data: stochastic background
I describe analysis of correlations in the outputs of the three LIGO
interferometers from LIGO's first science run, held over 17 days in August and
September of 2002, and the resulting upper limit set on a stochastic background
of gravitational waves. By searching for cross-correlations between the LIGO
detectors in Livingston, LA and Hanford, WA, we are able to set a 90%
confidence level upper limit of h_{100}^2 Omega_0 < 23 +/- 4.6.Comment: 7 pages; 1 eps figures; proceeding from 2003 Edoardo Amaldi Meeting
on Gravitational Wave
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Initiation of a Marinoan Snowball Earth in a state-of-the-art atmosphere-ocean general circulation model
We study the initiation of a Marinoan Snowball Earth (~635 million years before present) with the state-of-the-art atmosphere-ocean general circulation model ECHAM5/MPI-OM. This is the most sophisticated model ever applied to Snowball initiation. A comparison with a pre-industrial control climate shows that the change of surface boundary conditions from present-day to Marinoan, including a shift of continents to low latitudes, induces a global-mean cooling of 4.6 K. Two thirds of this cooling can be attributed to increased planetary albedo, the remaining one third to a weaker greenhouse effect. The Marinoan Snowball Earth bifurcation point for pre-industrial atmospheric carbon dioxide is between 95.5 and 96% of the present-day total solar irradiance (TSI), whereas a previous study with the same model found that it was between 91 and 94% for present-day surface boundary conditions. A Snowball Earth for TSI set to its Marinoan value (94% of the present-day TSI) is prevented by doubling carbon dioxide with respect to its pre-industrial level. A zero-dimensional energy balance model is used to predict the Snowball Earth bifurcation point from only the equilibrium global-mean ocean potential temperature for present-day TSI. We do not find stable states with sea-ice cover above 55%, and land conditions are such that glaciers could not grow with sea-ice cover of 55%. Therefore, none of our simulations qualifies as a "slushball" solution. While uncertainties in important processes and parameters such as clouds and sea-ice albedo suggest that the Snowball Earth bifurcation point differs between climate models, our results contradict previous findings that Snowball Earth initiation would require much stronger forcings
Astrometric observations of Saturn's satellites from McDonald Observatory, 1972
Observations of Saturn's satellites were reduced by means of secondary reference stars obtained by reduction of Palomar Sky Survey (PSS) plates. This involved the use of 39 SAO stars and plate overlap technique to determine the coordinates of 59 fainter stars in the satellite field. Fourteen plate constants were determined for each of the two PSS plates. Comparison of two plate measurement and reduction techniques on the satellite measurements demonstrate the existence of a serious background gradient effect and the utility of microdensitometry to eliminate this error source in positional determinations of close satellites
Erythrocyte enrichment in hematopoietic progenitor cell cultures based on magnetic susceptibility of the hemoglobin
Using novel media formulations, it has been demonstrated that human placenta and umbilical cord blood-derived CD34+ cells can be expanded and differentiated into erythroid cells with high efficiency. However, obtaining mature and functional erythrocytes from the immature cell cultures with high purity and in an efficient manner remains a significant challenge. A distinguishing feature of a reticulocyte and maturing erythrocyte is the increasing concentration of hemoglobin and decreasing cell volume that results in increased cell magnetophoretic mobility (MM) when exposed to high magnetic fields and gradients, under anoxic conditions. Taking advantage of these initial observations, we studied a noninvasive (label-free) magnetic separation and analysis process to enrich and identify cultured functional erythrocytes. In addition to the magnetic cell separation and cell motion analysis in the magnetic field, the cell cultures were characterized for cell sedimentation rate, cell volume distributions using differential interference microscopy, immunophenotyping (glycophorin A), hemoglobin concentration and shear-induced deformability (elongation index, EI, by ektacytometry) to test for mature erythrocyte attributes. A commercial, packed column high-gradient magnetic separator (HGMS) was used for magnetic separation. The magnetically enriched fraction comprised 80% of the maturing cells (predominantly reticulocytes) that showed near 70% overlap of EI with the reference cord blood-derived RBC and over 50% overlap with the adult donor RBCs. The results demonstrate feasibility of label-free magnetic enrichment of erythrocyte fraction of CD34+ progenitor-derived cultures based on the presence of paramagnetic hemoglobin in the maturing erythrocytes. © 2012 Jin et al
The Primordial Gravitational Wave Background in String Cosmology
We find the spectrum P(w)dw of the gravitational wave background produced in
the early universe in string theory. We work in the framework of String Driven
Cosmology, whose scale factors are computed with the low-energy effective
string equations as well as selfconsistent solutions of General Relativity with
a gas of strings as source. The scale factor evolution is described by an early
string driven inflationary stage with an instantaneous transition to a
radiation dominated stage and successive matter dominated stage. This is an
expanding string cosmology always running on positive proper cosmic time. A
careful treatment of the scale factor evolution and involved transitions is
made. A full prediction on the power spectrum of gravitational waves without
any free-parameters is given. We study and show explicitly the effect of the
dilaton field, characteristic to this kind of cosmologies. We compute the
spectrum for the same evolution description with three differents approachs.
Some features of gravitational wave spectra, as peaks and asymptotic
behaviours, are found direct consequences of the dilaton involved and not only
of the scale factor evolution. A comparative analysis of different treatments,
solutions and compatibility with observational bounds or detection perspectives
is made.Comment: LaTeX, 50 pages with 2 figures. Uses epsfig and psfra
Reconstruction of source location in a network of gravitational wave interferometric detectors
This paper deals with the reconstruction of the direction of a gravitational
wave source using the detection made by a network of interferometric detectors,
mainly the LIGO and Virgo detectors. We suppose that an event has been seen in
coincidence using a filter applied on the three detector data streams. Using
the arrival time (and its associated error) of the gravitational signal in each
detector, the direction of the source in the sky is computed using a chi^2
minimization technique. For reasonably large signals (SNR>4.5 in all
detectors), the mean angular error between the real location and the
reconstructed one is about 1 degree. We also investigate the effect of the
network geometry assuming the same angular response for all interferometric
detectors. It appears that the reconstruction quality is not uniform over the
sky and is degraded when the source approaches the plane defined by the three
detectors. Adding at least one other detector to the LIGO-Virgo network reduces
the blind regions and in the case of 6 detectors, a precision less than 1
degree on the source direction can be reached for 99% of the sky.Comment: Accepted in Phys. Rev.
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