3,692 research outputs found
A universal scaling law for the evolution of granular gases
Dry, freely evolving granular materials in a dilute gaseous state coalesce
into dense clusters only due to dissipative interactions. This clustering
transition is important for a number of problems ranging from geophysics to
cosmology. Here we show that the evolution of a dilute, freely cooling granular
gas is determined in a universal way by the ratio of inertial flow and thermal
velocities, that is, the Mach number. Theoretical calculations and direct
numerical simulations of the granular Navier--Stokes equations show that
irrespective of the coefficient of restitution, density or initial velocity
distribution, the density fluctuations follow a universal quadratic dependence
on the system's Mach number. We find that the clustering exhibits a scale-free
dynamics but the clustered state becomes observable when the Mach number is
approximately of . Our results provide a method to determine
the age of a granular gas and predict the macroscopic appearance of clusters
High Latitude Radio Emission in a Sample of Edge-On Spiral Galaxies
We have mapped 16 edge-on galaxies at 20 cm using the VLA. For 5 galaxies, we
could form spectral index, energy and magnetic field maps. We find that all but
one galaxy show evidence for non-thermal high latitude radio continuum
emission, suggesting that cosmic ray halos are common in star forming galaxies.
The high latitude emission is seen over a variety of spatial scales and in
discrete and/or smooth features. In general, the discrete features emanate from
the disk, but estimates of CR diffusion lengths suggest that diffusion alone is
insufficient to transport the particles to the high latitudes seen (> 15 kpc in
one case). Thus CRs likely diffuse through low density regions and/or are
assisted by other mechanisms (e.g. winds). We searched for correlations between
the prevalence of high latitude radio emission and a number of other
properties, including the global SFR, supernova input rate per unit star
forming, and do not find clear correlations with any of these properties.Comment: 40 pages of text, 3 figures, 6 tables, and an appendix of 21 jpeg
figures (which is a radio continuum catalogue of 17 galaxies). to appear in
A. J. (around January 1999
Antigen-driven clonal proliferation of B cells within the target tissue of an autoimmune disease: the salivary glands of patients with Sjögren's syndrome
Structures resembling germinal centers are seen in the salivary glands of patients with Sjögren's syndrome, but it is not known whether the microenvironment of these cell clusters is sufficient for the induction of a germinal center response. Therefore, we cloned and sequenced rearranged Ig V genes expressed by B cells isolated from sections of labial salivary gland biopsies from two Sjögren's syndrome patients. Rearranged V genes from B cells within one cell cluster were polyclonal and most had few somatic mutations. Two adjacent clusters from another patient each contained one dominant B cell clone expressing hypermutated V genes. None of the rearranged V genes was found in both clusters, suggesting that cells are unable to migrate out into the surrounding tissue and seed new clusters. The ratios of replacement to silent mutations in the framework and complementarity determining regions suggest antigen selection of high-affinity mutants. These results show that an antigen-driven, germinal center-type B cell response is taking place within the salivary glands of Sjögren's syndrome patients. In view of the recent demonstration of a germinal center response within the rheumatoid synovial membrane and the existence of similar structures in the target tissues of other autoimmune. diseases, we propose that germinal center- type responses can be induced in the nonlymphoid target tissues of a variety of autoimmune diseases
VLT observations of the magnetar CXO J164710.2-455216 and the detection of a candidate infrared counterpart
We present deep observations of the field of the magnetar CXOJ164710.2-455216
in the star cluster Westerlund 1, obtained in the near-infrared with the
adaptive optics camera NACO@VLT. We detected a possible candidate counterpart
at the {\em Chandra} position of the magnetar, of magnitudes , , and . The K-band measurements available for two epochs (2006 and
2013) do not show significant signs of variability but only a marginal
indication that the flux varied (at the 2 level), consistent with the
fact that the observations were taken when CXOJ164710.2-455216 was in
quiescence. At the same time, we also present colour--magnitude and
colour--colour diagrams in the J, H, and K bands from the 2006 epoch
only, the only one with observations in all three bands, showing that the
candidate counterpart lies in the main bulk of objects describing a relatively
well--defined sequence. Therefore, based on its colours and lack of
variability, we cannot yet associate the candidate counterpart to
CXOJ164710.2-455216. Future near-infrared observations of the field,
following-up a source outburst, would be crucial to confirm the association
from the detection of near-infrared variability and colour evolution.Comment: 5 pages, 3 figures, accepted for publication in MNRA
Jet stability, dynamics and energy transport
Relativistic jets carry energy and particles from compact to very large
scales compared with their initial radius. This is possible due to their
remarkable collimation despite their intrinsic unstable nature. In this
contribution, I review the state-of-the-art of our knowledge on instabilities
growing in those jets and several stabilising mechanisms that may give an
answer to the question of the stability of jets. In particular, during the last
years we have learned that the limit imposed by the speed of light sets a
maximum amplitude to the instabilities, contrary to the case of classical jets.
On top of this stabilising mechanism, the fast growth of unstable modes with
small wavelengths prevents the total disruption and entrainment of jets. I also
review several non-linear processes that can have an effect on the collimation
of extragalactic and microquasar jets. Within those, I remark possible causes
for the decollimation and decelleration of FRI jets, as opposed to the
collimated FRII's. Finally, I give a summary of the main reasons why jets can
propagate through such long distances.Comment: For the proceedings of High Energy Phenomena in Relativistic Outflows
III (HEPRO III, IJMPD, accepted). 12 page
Using the analytic hierarchy process to support teams in defining new product objectives
Defining new product objectives is a critical problem solving activity to new product success. The analytic hierarchy process appears to be an adequate technique for multi-criteria decision analysis to support the definition of new product objectives. To illustrate this support, we applied this technique to a project focused on the development of a liver perfusion system to preserve donor livers. It quantitatively supported discussions between technological developers and clinical practitioners focused on the product requirements and the pursued performance of the liver perfusion system relative to an envisaged competitor. The discussions significantly reduced disagreements about the new product objectives between the group members. They resulted in a quantitative overview of the importance of the product requirements and the relative performance of the alternatives with regard to these requirements. Furthermore, research activities necessary to fulfil these objectives were discerned. The group members were committed to these outcomes. This application shows the value of introducing the AHP as a means to steer new products to solve problems to a higher extent than competing products do
Dynamical mass of the O-type supergiant in Zeta Orionis A
A close companion of Zeta Orionis A was found in 2000 with the Navy Precision
Optical Interferometer (NPOI), and shown to be a physical companion. Because
the primary is a supergiant of type O, for which dynamical mass measurements
are very rare, the companion was observed with NPOI over the full 7-year orbit.
Our aim was to determine the dynamical mass of a supergiant that, due to the
physical separation of more than 10 AU between the components, cannot have
undergone mass exchange with the companion. The interferometric observations
allow measuring the relative positions of the binary components and their
relative brightness. The data collected over the full orbital period allows all
seven orbital elements to be determined. In addition to the interferometric
observations, we analyzed archival spectra obtained at the Calar Alto, Haute
Provence, Cerro Armazones, and La Silla observatories, as well as new spectra
obtained at the VLT on Cerro Paranal. In the high-resolution spectra we
identified a few lines that can be associated exclusively to one or the other
component for the measurement of the radial velocities of both. The combination
of astrometry and spectroscopy then yields the stellar masses and the distance
to the binary star. The resulting masses for components Aa of 14.0 solar masses
and Ab of 7.4 solar masses are low compared to theoretical expectations, with a
distance of 294 pc which is smaller than a photometric distance estimate of 387
pc based on the spectral type B0III of the B component. If the latter (because
it is also consistent with the distance to the Orion OB1 association) is
adopted, the mass of the secondary component Ab of 14 solar masses would agree
with classifying a star of type B0.5IV. It is fainter than the primary by about
2.2 magnitudes in the visual. The primary mass is then determined to be 33
solar masses
Small Groups, Big Weapons: The Nexus of Emerging Technologies and Weapons of Mass Destruction Terrorism
Historically, only nation-states have had the capacity and resources to develop weapons of mass destruction (WMD). This was due to the significant capital, infrastructure, and intellectual capacity required to develop and maintain a WMD program. This paradigm, however, is shifting. To be clear, non-state actors have been interested in WMD for decades. In fact, over a 26-year period, there were 525 incidents by non-state actors involving nuclear, biological, and chemical agents. But the scale of these incidents was relatively low level when compared to the impact of terrorist attacks using conventional weapons. However, this reality must be reexamined given the commercialization of emerging technologies that is reducing the financial, intellectual, and material barriers required for WMD development and employment. This report serves as a primer that surveys the key challenges facing non-state actors pursuing WMD capabilities, and the potential for certain emerging technologies to help overcome them. While there are numerous examples of such technologies, this report focuses on synthetic biology, additive manufacturing (AM) (commonly known as 3D printing), and unmanned aerial systems (UAS). There is a wide range of expert opinions regarding the dual-use nature of the technologies discussed in this report, the ease of their possible misuse, and the potential threats they pose. The varied opinions of scientists and government officials highlight the challenges these technologies pose to developing a cohesive strategy to prevent their proliferation for nefarious use by non-state actors. Much of the risk and threat associated with these dual-use technologies resides in the intent of the user
- …