10,203 research outputs found
Guidance, flight mechanics and trajectory optimization. Volume 1 - Coordinate systems and time measure
Coordinate measuring system for flight control, and trajectory optimizatio
HERD SIZE AND EFFICIENCY ON MIXED CROP AND LIVESTOCK FARMS: CASE STUDIES OF CHIWESHE AND GOKWE, ZIMBABWE
This study is based on two 1991 sample surveys, each of ninety farms, in the predominantly arable region of Chiweshe and in the low rainfall area of Gokwe, where animals are more important. The two samples are reasonably representative of the range of conditions found in the communal areas in Zimbabwe. Programming techniques are used to determine the efficiency levels of the farms in each region. The results show that efficiency is positively related to the numbers of both cows and oxen, with only a few farms in Gokwe possibly having too many animals. Farms in Gokwe are on average about two thirds as efficient as those in Chiweshe, which is a measure of the effects of the poorer climate and soils. Non-farm income is also lower, due to lesser opportunities in the more remote region. In both regions, the majority of farms are too small and the estimates suggest that increasing farm size could almost double productivity.Farm Management, Livestock Production/Industries,
Cosmology as Geodesic Motion
For gravity coupled to N scalar fields with arbitrary potential V, it is
shown that all flat (homogeneous and isotropic) cosmologies correspond to
geodesics in an (N+1)-dimensional `augmented' target space of Lorentzian
signature (1,N), timelike if V>0, null if V=0 and spacelike if V<0.
Accelerating cosmologies correspond to timelike geodesics that lie within an
`acceleration subcone' of the `lightcone'. Non-flat (k=-1,+1) cosmologies are
shown to evolve as projections of geodesic motion in a space of dimension
(N+2), of signature (1,N+1) for k=-1 and signature (2,N) for k=+1. This
formalism is illustrated by cosmological solutions of models with an
exponential potential, which are comprehensively analysed; the late-time
behviour for other potentials of current interest is deduced by comparison.Comment: 26 pages, 2 figures, journal version with additional reference
Self-gravitating Yang Monopoles in all Dimensions
The (2k+2)-dimensional Einstein-Yang-Mills equations for gauge group SO(2k)
(or SU(2) for k=2 and SU(3) for k=3) are shown to admit a family of
spherically-symmetric magnetic monopole solutions, for both zero and non-zero
cosmological constant Lambda, characterized by a mass m and a magnetic-type
charge. The k=1 case is the Reissner-Nordstrom black hole. The k=2 case yields
a family of self-gravitating Yang monopoles. The asymptotic spacetime is
Minkowski for Lambda=0 and anti-de Sitter for Lambda<0, but the total energy is
infinite for k>1. In all cases, there is an event horizon when m>m_c, for some
critical mass , which is negative for k>1. The horizon is degenerate when
m=m_c, and the near-horizon solution is then an adS_2 x S^{2k} vacuum.Comment: 16 pp. Extensive revision to include case of non-zero cosmological
constant and implications for adS/CFT. Numerous additional reference
Coexpression of rat P2X2 and P2X6 subunits in Xenopus oocytes.
Transcripts for P2X(2) and P2X(6) subunits are present in rat CNS and frequently colocalize in the same brainstem nuclei. When rat P2X(2) (rP2X(2)) and rat P2X(6) (rP2X(6)) receptors were expressed individually in Xenopus oocytes and studied under voltage-clamp conditions, only homomeric rP2X(2) receptors were fully functional and gave rise to large inward currents (2-3 microA) to extracellular ATP. Coexpression of rP2X(2) and rP2X(6) subunits in Xenopus oocytes resulted in a heteromeric rP2X(2/6) receptor, which showed a significantly different phenotype from the wild-type rP2X(2) receptor. Differences included reduction in agonist potencies and, in some cases (e.g., Ap(4)A), significant loss of agonist activity. ATP-evoked inward currents were biphasic at the heteromeric rP2X(2/6) receptor, particularly when Zn(2+) ions were present or extracellular pH was lowered. The pH range was narrower for H(+) enhancement of ATP responses at the heteromeric rP2X(2/6) receptor. Also, H(+) ions inhibited ATP responses at low pH levels (<pH 6.3). The pH-dependent blocking activity of suramin was changed at this heteromeric receptor, although the potentiating effect of Zn(2+) on ATP responses was unchanged. Thus, the rP2X(2/6) receptor is a functionally modified P2X(2)-like receptor with a distinct pattern of pH modulation of ATP activation and suramin blockade. Although homomeric P2X(6) receptors function poorly, the P2X(6) subunit can contribute to functional heteromeric P2X channels and may influence the phenotype of native P2X receptors in those cells in which it is expressed
Spin period change and the magnetic fields of neutron stars in Be X-ray binaries in the Small Magellanic Cloud
We report on the long-term average spin period, rate of change of spin period
and X-ray luminosity during outbursts for 42 Be X-ray binary systems in the
Small Magellanic Cloud. We also collect and calculate parameters of each system
and use these data to determine that all systems contain a neutron star which
is accreting via a disc, rather than a wind, and that if these neutron stars
are near spin equilibrium, then over half of them, including all with spin
periods over about 100 s, have magnetic fields over the quantum critical level
of 4.4x10^13 G. If these neutron stars are not close to spin equilibrium, then
their magnetic fields are inferred to be much lower, of the order of 10^6-10^10
G, comparable to the fields of neutron stars in low-mass X-ray binaries. Both
results are unexpected and have implications for the rate of magnetic field
decay and the isolated neutron star population.Comment: 22 pages, 50 figures; to appear in MNRA
Discovery of a strong magnetic field in the rapidly rotating B2Vn star HR 7355
We report the detection of a strong, organized magnetic field in the
helium-variable early B-type star HR 7355 using spectropolarimetric data
obtained with ESPaDOnS on the 3.6-m Canada-France-Hawaii Telescope within the
context of the Magnetism in Massive Stars (MiMeS) Large Program. HR 7355 is
both the most rapidly rotating known main-sequence magnetic star and the most
rapidly rotating helium-strong star, with = 300 15 km s
and a rotational period of 0.5214404 0.0000006 days. We have modeled our
eight longitudinal magnetic field measurements assuming an oblique dipole
magnetic field. Constraining the inclination of the rotation axis to be between
and , we find the magnetic obliquity angle to be
between and , and the polar strength of the magnetic
field at the stellar surface to be between 13-17 kG. The photometric light
curve constructed from HIPPARCOS archival data and new CTIO measurements shows
two minima separated by 0.5 in rotational phase and occurring 0.25 cycles
before/after the magnetic extrema. This photometric behavior coupled with
previously-reported variable emission of the H line (which we confirm)
strongly supports the proposal that HR 7355 harbors a structured magnetosphere
similar to that in the prototypical helium-strong star, Ori E.Comment: 6 pages, 3 figures. Accepted for publication in MNRAS Letter
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