1,850 research outputs found
On the state of low luminous accreting neutron stars
Observational appearance of a neutron star in the subsonic propeller state
which is a companion of a wind-fed mass-exchange close binary system is
discussed. During the subsonic propeller state the neutron star magnetosphere
is surrounded by a spherical quasi-static plasma envelope, which is extended
from the magnetospheric boundary up to the star accretion radius. The energy
input to the envelope due to the propeller action by the neutron star exceeds
the radiative losses and the plasma temperature in the envelope is of the order
of the free-fall temperature. Under this condition the magnetospheric boundary
is interchange stable. Nevertheless, I find that the rate of plasma penetration
from the envelope into the magnetic field of the neutron star due to diffusion
and magnetic field line reconnection processes is large enough for the
accretion power to dominate the spindown power. I show that the accretion
luminosity of the neutron star in the subsonic propeller state is 5*10**{30} -
10**{33} (dM/dt)_{15} erg/s, where dM/dt is the strength of the normal
companion stellar wind which is parametrized in terms of the maximum possible
mass accretion rate onto the neutron star magnetosphere. On this basis I
suggest that neutron stars in the subsonic propeller state are expected to be
observed as low luminous accretion-powered pulsars. The magnetospheric radius
of the neutron star in this state is determined by the strength of the stellar
wind, (dM/dt)_c, while the accretion luminosity is determined by the rate of
plasma penetration into the star magnetosphere, (dM/dt)_a, which is (dM/dt)_a
<< (dM/dt)_c. That is why the classification of the neutron star state in these
objects using the steady accretion model (i.e. setting (dM/dt)_a = (dM/dt)_c)
can lead to a mistaken conclusion.Comment: 6 pages, accepted for publication in A&
Large-scale circulation departures related to wet episodes in northeast Brazil
Large scale circulation features are presented as related to wet spells over northeast Brazil (Nordeste) during the rainy season (March and April) of 1979. The rainy season season is devided into dry and wet periods, the FGGE and geostationary satellite data was averaged and mean and departure fields of basic variables and cloudiness were studied. Analysis of seasonal mean circulation features show: lowest sea level easterlies beneath upper level westerlies; weak meridional winds; high relative humidity over the Amazon basin and relatively dry conditions over the South Atlantic Ocean. A fluctuation was found in the large scale circulation features on time scales of a few weeks or so over Nordeste and the South Atlantic sector. Even the subtropical High SLP's have large departures during wet episodes, implying a short period oscillation in the Southern Hemisphere Hadley circulation
On the duration of the subsonic propeller state of neutron stars in wind-fed mass-exchange close binary systems
The condition for the subsonic propeller - accretor state transition of
neutron stars in wind-fed mass-exchange binary systems is discussed. I show
that the value of the break period, at which the neutron star change its state
to accretor, presented by Davies & Pringle (1981) is underestimated by a factor
of 7.5. The correct value is P_{\rm br} = 450 \mu_{30}^{16/21}
\dot{M}_{15}^{-5/7} (M/M_{\sun})^{-4/21} s. This result forced us to reconsider
some basic conclusions on the efficiency of the propeller spindown mechanism.Comment: 3 pages, published in A&A 368, L
Stabilized lanthanum sulphur compounds
Lanthanum sulfide is maintained in the stable cubic phase form over a temperature range of from 500 C to 1500 C by adding to it small amounts of calcium, barium, or strontium. This compound is an excellent thermoelectric material
Accretion by Isolated Neutron Stars
Accretion of interstellar material by an isolated neutron star is discussed.
The point I address here is the interaction between the accretion flow and the
stellar magnetosphere. I show that the interchange instabilities of the
magnetospheric boundary under the conditions of interest are basically
suppressed. The entry of the material into the magnetosphere is governed by
diffusion. Due to this reason the persistent accretion luminosity of isolated
neutron stars is limited to < 4E+26 erg/s. These objects can also appear as
X-ray bursters with the burst durations of about 30 minutes and repetition time
of about 1E+5 yr. This indicates that the number of the accreting isolated
neutron stars which could be observed with recent and modern X-ray missions is
a few orders of magnitude smaller than that previously estimated.Comment: 3 pages, to appear in Astrophysics and Space Science, in the
proceedings of "Isolated Neutron Stars: from the Interior to the Surface",
edited by D. Page, R. Turolla and S. Zan
Subsonic propellers in a strong wind as anomalous X-ray pulsars
The appearance of subsonic propellers situated in a strong wind is discussed.
We show that it is similar to the appearance of anomalous X-ray pulsars (AXPs)
provided the mass and the magnetic moment of neutron stars are 1.4 solar
masses, and 2E+30 G cm^3, respectively, and the strength of the wind is M_c =
3E+17 g/s. Under these conditions, the spin periods of subsonic propellers are
limited within the range of 5-15 s, and the expected spin-down rates are close
to 7E-11 s/s. The mass accretion rate onto the stellar surface is limited to
the rate of plasma penetration into its magnetosphere at the boundary. As this
process is governed by the reconnection of the field lines, the accretion rate
onto the stellar surface constitutes 1-2% of M_c. In this case the X-ray
luminosity of the objects under consideration can be evaluated as 4E+35 erg/s.
The model predicts the existence of at least two spatially separated sources of
the X-ray emission: hot spots at the stellar surface, and the hot atmosphere
surrounding the magnetosphere of the star. The ages of the subsonic propellers
under the conditions of interest are limited to 10^5 yr.Comment: 5 pages, 1 figure, accepted for publication in A&
Entoptic image quality of the retinal vasculature
AbstractSpatial details of entoptically visible retinal vessels were investigated using transcleral and Maxwellian-view stimulators. Nine normal subjects provided detailed drawings of the entoptic images which were digitized and superimposed onto digitized fundus photographs and fluorescein angiograms from the same eyes. Subjects also used a tracing method to locate visible entoptic features. The trans-scleral method provided images similar in detail to standard fundus photography (lacking capillary detail, but capturing larger arteries, veins, arterioles and venules) in the macula and around the disk. The Maxwellian-view method illuminated the fovea (7.7 degree field) and provided foveola capillary detail (capillaries traversing the foveola, the capillary arcade forming the FAZ) as well as the larger foveal vessels supplying the foveola, and often contained more foveal detail that available with fluorescein angiography
Testing of Milliwatt Power Source Components
A milliwatt power source (MPS) has been developed to satisfy the requirements of several potential solar system exploration missions. The MPS is a small power source consisting of three major components: a space qualified heat source (RHU), a thermopile (thermoelectric converter or TEC) and a container to direct the RHU heat to the TEC. Thermopiles from Hi-Z Technology, Inc. of San Diego and the Institute of Thermoelectricity of Chernivtsi Ukraine suitable for the MPS were tested and shown to perform as expected, producing 40 mW of power with a temperature difference of about 170°C. Such thermopiles were successfully life tested for up to a year. A MPS container designed and built by Swales Aerospace was tested with both a TEC simulator and actual TEC. The Swales unit, tested under dynamic vacuum, provided less temperature difference than anticipated, such that the TEC produced 20 mW of power with heat input equivalent to a RHU
Superconductivity in Organic Compounds with Pseudo-Triangular Lattice
We study spin fluctuation (SF) mediated superconductivity (SC) in a
half-filled square lattice Hubbard model with the transfer matrices -t between
nearest neighbor sites and -t' between a half of next nearest neighbor sites
neighboring along only one of the directions, considering application of
this model to organic kappa-(BEDT-TTF)2X compounds. Varying the t'/t value from
0 to 1, one can interpolate between a square and an equilateral triangular
lattice, the latter giving frustration to antiferromagnetically (AF) coupled
spin systems. Within the fluctuation exchange (FLEX) approximation, we
calculate chi(q,omega), Tc and the SC order parameter for various model
parameter values and find that both AF and SC are suppressed as one approaches
the frustration geometry or |(t'/t)-1| \to 0. The SC phase, however, extends
beyond the AF phase boundary fairly close to t'/t=1 for realistic U/t values.
The order parameter is of x2-y2-type for t'/t1.Comment: 4 pages, 5 eps figures, to appear in J. Phys. Soc. Jp
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