9,437 research outputs found
Experiments with calibrated digital sideband separating downconversion
This article reports on the first step in a focused program to re-optimize
radio astronomy receiver architecture to better take advantage of the latest
advancements in commercial digital technology. Specifically, an L-Band
sideband-separating downconverter has been built using a combination of careful
(but ultimately very simple) analog design and digital signal processing to
achieve wideband downconversion of an RFI-rich frequency spectrum to baseband
in a single mixing step, with a fixed-frequency Local Oscillator and stable
sideband isolation exceeding 50 dB over a 12 degree C temperature range.Comment: 10 pages, 12 figures, to be published in PAS
Accretion of planetary matter and the lithium problem in the 16 Cygni stellar system
The 16 Cyg system is composed of two solar analogs with similar masses and
ages. A red dwarf is in orbit around 16 Cyg A whereas 16 Cyg B hosts a giant
planet. The abundances of heavy elements are similar in the two stars but
lithium is much more depleted in 16 Cyg B that in 16 Cyg A, by a factor of at
least 4.7. The interest of studying the 16 Cyg system is that the two star have
the same age and the same initial composition. The presently observed
differences must be due to their different evolution, related to the fact that
one of them hosts a planet contrary to the other one. We computed models of the
two stars which precisely fit the observed seismic frequencies. We used the
Toulouse Geneva Evolution Code (TGEC) that includes complete atomic diffusion
(including radiative accelerations). We compared the predicted surface
abundances with the spectroscopic observations and confirmed that another
mixing process is needed. We then included the effect of accretion-induced
fingering convection. The accretion of planetary matter does not change the
metal abundances but leads to lithium destruction which depends on the accreted
mass. A fraction of earth mass is enough to explain the lithium surface
abundances of 16 Cyg B. We also checked the beryllium abundances. In the case
of accretion of heavy matter onto stellar surfaces, the accreted heavy elements
do not remain in the outer convective zones but they are mixed downwards by
fingering convection induced by the unstable -gradient. Depending on the
accreted mass, this mixing process may transport lithium down to its nuclear
destruction layers and lead to an extra lithium depletion at the surface. A
fraction of earth mass is enough to explain a lithium ratio of 4.7 in the 16
Cyg system. In this case beryllium is not destroyed. Such a process may be
frequent in planet host stars and should be studied in other cases in the
future.Comment: 8 pages, 10 figures, publication in A&
Applied linguistics and mathematics education: More than words and numbers
The preceding set of papers has explored various aspects of the role of language in mathematics education. The papers reflect the work of individual contributors. An important part of our collaboration, however, has been the conversation between us. This paper reflects aspects of that conversation, as we draw together some of the themes that have emerged during our work. In particular, we discuss some of the implications of our analyses for theory, policy, practice and inter-disciplinarity in mathematics education and applied linguistics
Black Holes and Galactic Density Cusps Spherically Symmetric Anisotropic Cusps
Aims: In this paper we study density cusps that may contain central black
holes. The actual co-eval self-similar growth would not distinguish between the
central object and the surroundings. Methods: To study the environment of a
growing black hole we seek descriptions of steady `cusps' that may contain a
black hole and that retain at least a memory of self-similarity. We refer to
the environment in brief as the `bulge' and on smaller scales, the `halo'.
Results: We find simple descriptions of the simulations of collisionless matter
by comparing predicted densities, velocity dispersions and distribution
functions with the simulations. In some cases central point masses may be
included by iteration. We emphasize that the co-eval self-similar growth allows
an explanation of the black hole bulge mass correlation between approximately
similar collisionless systems. Conclusions: We have derived our results from
first principles assuming adiabatic self-similarity and either self-similar
virialisation or normal steady virialisation. We conclude that distribution
functions that retain a memory of self-similar evolution provide an
understanding of collisionless systems. The implied energy relaxation of the
collisionless matter is due to the time dependence. Phase mixing relaxation may
be enhanced by clump-clump interactions.Comment: 9 pp, 3 figs, accepted by A\&
The Law at War: Counterinsurgency Operations and the Use of Indigenous Legal Institutions
Success in counterinsurgency campaigns requires the U.S. military to train, equip, and ultimately turn over responsibility for public safety to indigenous legal institutions. Doing so presents many challenges, as pragmatic concerns for operational security and use of intelligence as legal evidence must be reconciled with cultural differences and the weakness of indigenous legal institutions. This article argues, however, that such participation may be required under international law. Further, participation may help to legitimize counterinsurgency goals in the eyes of the local populace, and bring additional resources to military efforts. In order to realize such benefits, this article argues that military commanders must make a commitment to the use of indigenous criminal justice systems through all stages of a counterinsurgency campaign - during pre-deployment planning, active combat, and post-conflict rebuilding efforts
Component library retrieval using property models
The re-use of products such as code, specifications, design decisions and documentation has been proposed as a method for increasing software productivity and reliability. A major problem that has still to be adequately solved is the storage and retrieval of re-usable 'components'. Current methods, such as keyword retrieval and catalogues, rely on the use of names to describe components or categories. This is inadequate for all but a few well established components and categories; in the majority of cases names do not convey sufficient information on which to base a decision to retrieve. One approach to this problem is to describe components using a formal specification. However this is impractical for two reasons; firstly, the limitations of theorem proving would severely restrict the complexity of components that could be retrieved and secondly the retrieval mechanism would need to have a method of retrieving components with 'similar' specifications. This thesis proposes the use of formal 'property' models to represent the key functionality of components. Retrieval of components can then take place on the basis of a property model produced by the library's users. These models only describe the key properties of a component, thereby making the task of comparing properties feasible. Views are introduced as a method of relating similar, non identical property models, and the use of these views facilitates the re-use of components with similar properties. The language Miramod has been developed for the purpose of describing components, and a Miramod compiler and property prover which allow Miramod models to be compared for similarity, have been designed and implemented. These tools have indicated that model based component library retrieval is feasible at relatively low levels of the programming process, and future work is suggested to extend the method to encompass earlier stages in the development of large systems
Investigation into the variations of moisture content of two buildings constructed with light earth walls
This paper briefly describes the background to light earth buildings and details a series of moisture measurements undertaken upon the clay and straw, (light earth) constructed walls of two UK based buildings. The methodology of measurement that was based upon previous studies undertaken on walls made from straw bales is described. A novel âin-wallâ wet heating system used in one of the two buildings allows the investigation of the effects of direct wall heating upon the distribution of moisture in the walls. The influence of exterior and interior temperature and humidity are described as are the variations in moisture migration introduced by the in-wall heating system. It was concluded that both buildings have exterior wall moisture content readings that indicate little risk of degradation due to interior wall moisture levels (although the Studio walls do exhibit higher and if suffered over long time periods, dangerous moisture readings for part of the measurement period)
- âŠ