6,788 research outputs found
The complex network of global cargo ship movements
Transportation networks play a crucial role in human mobility, the exchange
of goods, and the spread of invasive species. With 90% of world trade carried
by sea, the global network of merchant ships provides one of the most important
modes of transportation. Here we use information about the itineraries of
16,363 cargo ships during the year 2007 to construct a network of links between
ports. We show that the network has several features which set it apart from
other transportation networks. In particular, most ships can be classified in
three categories: bulk dry carriers, container ships and oil tankers. These
three categories do not only differ in the ships' physical characteristics, but
also in their mobility patterns and networks. Container ships follow regularly
repeating paths whereas bulk dry carriers and oil tankers move less predictably
between ports. The network of all ship movements possesses a heavy-tailed
distribution for the connectivity of ports and for the loads transported on the
links with systematic differences between ship types. The data analyzed in this
paper improve current assumptions based on gravity models of ship movements, an
important step towards understanding patterns of global trade and bioinvasion.Comment: 7 figures Accepted for publication by Journal of the Royal Society
Interface (2010) For supplementary information, see
http://www.icbm.de/~blasius/publications.htm
AGC 226067: A possible interacting low-mass system
We present Arecibo, GBT, VLA and WIYN/pODI observations of the ALFALFA source
AGC 226067. Originally identified as an ultra-compact high velocity cloud and
candidate Local Group galaxy, AGC 226067 is spatially and kinematically
coincident with the Virgo cluster, and the identification by multiple groups of
an optical counterpart with no resolved stars supports the interpretation that
this systems lies at the Virgo distance (D=17 Mpc). The combined observations
reveal that the system consists of multiple components: a central HI source
associated with the optical counterpart (AGC 226067), a smaller HI-only
component (AGC 229490), a second optical component (AGC 229491), and extended
low surface brightness HI. Only ~1/4 of the single-dish HI emission is
associated with AGC 226067; as a result, we find M_HI/L_g ~ 6 Msun/Lsun, which
is lower than previous work. At D=17 Mpc, AGC 226067 has an HI mass of 1.5 x
10^7 Msun and L_g = 2.4 x 10^6 Lsun, AGC 229490 (the HI-only component) has
M_HI = 3.6 x 10^6 Msun, and AGC 229491 (the second optical component) has L_g =
3.6 x 10^5 Lsun. The nature of this system of three sources is uncertain: AGC
226067 and AGC 229490 may be connected by an HI bridge, and AGC 229490 and AGC
229491 are separated by only 0.5'. The current data do not resolve the HI in
AGC 229490 and its origin is unclear. We discuss possible scenarios for this
system of objects: an interacting system of dwarf galaxies, accretion of
material onto AGC 226067, or stripping of material from AGC 226067.Comment: Accepted for publication in A&A. 6 pages, 4 figure
Two-Stream Instability of Counter-Rotating Galaxies
The present study of the two-stream instability in stellar disks with
counter-rotating components of stars and/or gas is stimulated by recently
discovered counter-rotating spiral and S0 galaxies. Strong linear two-stream
instability of tightly-wrapped spiral waves is found for one and two-armed
waves with the pattern angular speed of the unstable waves always intermediate
between the angular speed of the co-rotating matter () and that of the
counter-rotating matter (). The instability arises from the
interaction of positive and negative energy modes in the co- and
counter-rotating components. The unstable waves are in general convective -
they move in radius and radial wavenumber space - with the result that
amplification of the advected wave is more important than the local growth
rate. For a galaxy of co-rotating stars and counter-rotating stars of
mass-fraction , or of counter-rotating gas of mass-fraction
, the largest amplification is usually for the one-armed
leading waves (with respect to the co-rotating stars). For the case of both
counter-rotating stars and gas, the largest amplifications are for , also for one-armed leading waves. The two-armed trailing
waves usually have smaller amplifications. The growth rates and amplifications
all decrease as the velocity spreads of the stars and/or gas increase. It is
suggested that the spiral waves can provide an effective viscosity for the gas
causing its accretion.Comment: 14 pages, submitted to ApJ. One table and 17 figures can be obtained
by sending address to R. Lovelace at [email protected]
Polarized radio emission over the southern Galactic plane at 2.4 GHz
Polarimetric results from the Parkes 2.4-GHz survey of the southern Galactic plane are presented. These take the form of a series of images, detailing the polarized intensity and vector position angles over the survey area. The observations were made using the 64-m Parkes radio telescope, and cover the Galactic plane within the region 238°≤l≤5°, with a latitude range of at least |b|≤5°, with some coverage to b = +7° and b = -8°. The resolution of the images is 10.4 arcmin. The rms noise of the polarized intensity images is 11 mJy beam area-1 (5.3 mK), and the rms variation in the vector position angles is of the order of several dgrees. The images show many polarized structures, over a wide range of intensities and angular sizes. Bright, extended regions of polarized emission (of the order of 5° across) are detected, including the Vela supernova remnant and a large 'cap' structure appearing to the north of Sgr A. A quasi-uniform 'background' component of faint, patchy emission is seen over the length of the survey. This faint component appears to originate over a range of distances, out to greater than 5 kpc, and shows considerable structure in the orientations of the polarization vectors. Several bright H II complexes are seen to exhibit bipolar, depolarizing 'plumes', several degrees in length, which are interpreted as outflows of low-density thermal material (with densities in the range 1-10 cm-3)
Extraluminal Colonic Carcinoma Invading into Kidney: A Case Report and Review of the Literature
Renal metastasis from primary colon cancer is very rare, comprising less than 3% of secondary renal neoplasms. There are just 11 cases reported in the medical literature of colonic adenocarcinoma metastatic to the kidney. Of these cases, none occurred via direct invasion. We report a unique case of a 51-year-old female with extraluminal colonic adenocarcinoma which directly invaded into the kidney. Additionally, we investigate the causal relationship between the site of invasion and a previous stab injury by reviewing the role of the peritoneum and Gerota's fascia in preventing the spread of metastatic cancer into the perirenal space. Due to the rarity of this event, we present this case including a review of the existing literature relative to the diagnosis and treatment
A Radio and Optical Polarization Study of the Magnetic Field in the Small Magellanic Cloud
We present a study of the magnetic field of the Small Magellanic Cloud (SMC),
carried out using radio Faraday rotation and optical starlight polarization
data. Consistent negative rotation measures (RMs) across the SMC indicate that
the line-of-sight magnetic field is directed uniformly away from us with a
strength 0.19 +/- 0.06 microGauss. Applying the Chandrasekhar-Fermi method to
starlight polarization data yields an ordered magnetic field in the plane of
the sky of strength 1.6 +/- 0.4 microGauss oriented at a position angle 4 +/-
12 degs, measured counter-clockwise from the great circle on the sky joining
the SMC to the Large Magellanic Cloud (LMC). We construct a three-dimensional
magnetic field model of the SMC, under the assumption that the RMs and
starlight polarization probe the same underlying large-scale field. The vector
defining the overall orientation of the SMC magnetic field shows a potential
alignment with the vector joining the center of the SMC to the center of the
LMC, suggesting the possibility of a "pan-Magellanic'' magnetic field. A
cosmic-ray driven dynamo is the most viable explanation of the observed field
geometry, but has difficulties accounting for the observed uni-directional
field lines. A study of Faraday rotation through the Magellanic Bridge is
needed to further test the pan-Magellanic field hypothesis.Comment: 28 pages, 6 figures, accepted for publication in Ap
Spectral Evolution of Circinus X-1 Along its Orbit
We report on the spectral analysis of Circinus X-1 observed by the ASCA
satellite in March 1998 along one orbital period. The luminosity of the source
(in the 0.1-100 keV band) ranges from erg s at the
periastron (orbital phase 0.01) to erg s at orbital
phase 0.3. From the spectral analysis and the lightcurve we argue that Cir X-1
shows three states along the orbital evolution. The first state is at the
orbital phase interval 0.97-0.3: the luminosity becames super-Eddington and a
strong flaring activity is present. In this state a shock could form in the
inner region of the system due to the super-Eddington accretion rate, producing
an outflow of ionized matter whose observational signature could be the
prominent absorption edge at keV observed in the energy spectrum at
these phases. In the second state, corresponding to the orbital phase interval
between 0.3 and 0.7, the accretion rate is sub-Eddington and we observe a
weaker outflow, with smaller hydrogen column: the absorption edge is now at
keV with an optical depth a factor of 2.5 to 6 smaller. The third
state corresponds to the orbital phase interval 0.78-0.97. In this state the
best fit model to the spectrum requires the presence of a partial covering
component, indicating that the emission from the compact object is partially
absorbed by neutral matter, probably the atmosphere of the companion star
and/or the accreting matter from the companion.Comment: 18 pages, 3 figures. Accepted by Ap
Corrosion Resistant Coatings for High Temperature Applications
Efforts to increase efficiency of energy conversion devices have required their operation at ever higher temperatures. This will force the substitution of higher-temperature structural ceramics for lower temperature materials, largely metals. Yet, many of these ceramics will require protection from high temperature corrosion caused by combustion gases, atmospheric contaminants, or the operating medium. This paper discusses examples of the initial development of such coatings and materials for potential application in combustion, aluminum smelting, and other harsh environments
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Sponge layer feedbacks in middle-atmosphere models
Middle-atmosphere models commonly employ a sponge layer in the upper portion of their domain. It is shown that the relaxational nature of the sponge allows it to couple to the dynamics at lower levels in an artificial manner. In particular, the long-term zonally symmetric response to an imposed extratropical local force or diabatic heating is shown to induce a drag force in the sponge that modifies the response expected from the “downward control” arguments of Haynes et al. [1991]. In the case of an imposed local force the sponge acts to divert a fraction of the mean meridional mass flux upward, which for realistic parameter values is approximately equal to exp(−Δz/H), where Δz is the distance between the forcing region and the sponge layer and H is the density scale height. This sponge-induced upper cell causes temperature changes that, just below the sponge layer, are of comparable magnitude to those just below the forcing region. In the case of an imposed local diabatic heating, the sponge induces a meridional circulation extending through the entire depth of the atmosphere. This circulation causes temperature changes that, just below the sponge layer, are of opposite sign and comparable in magnitude to those at the heating region. In both cases, the sponge-induced temperature changes are essentially independent of the height of the imposed force or diabatic heating, provided the latter is located outside the sponge, but decrease exponentially as one moves down from the sponge. Thus the effect of the sponge can be made arbitrarily small at a given altitude by placing the sponge sufficiently high; e.g., its effect on temperatures two scale heights below is roughly at the 10% level, provided the imposed force or diabatic heating is located outside the sponge. When, however, an imposed force is applied within the sponge layer (a highly plausible situation for parameterized mesospheric gravity-wave drag), its effect is almost entirely nullified by the sponge-layer feedback and its expected impact on temperatures below largely fails to materialize. Simulations using a middle-atmosphere general circulation model are described, which demonstrate that this sponge-layer feedback can be a significant effect in parameter regimes of physical interest. Zonally symmetric (two dimensional) middle-atmosphere models commonly employ a Rayleigh drag throughout the model domain. It is shown that the long-term zonally symmetric response to an imposed extratropical local force or diabatic heating, in this case, is noticeably modified from that expected from downward control, even for a very weak drag coefficien
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