7,629 research outputs found
A ROSAT Survey of Contact Binary Stars
Contact binary stars are common variable stars which are all believed to emit
relatively large fluxes of x-rays. In this work we combine a large new sample
of contact binary stars derived from the ROTSE-I telescope with x-ray data from
the ROSAT All-Sky Survey (RASS) to estimate the x-ray volume emissivity of
contact binary stars in the galaxy. We obtained x-ray fluxes for 140 contact
binaries from the RASS, as well as 2 additional stars observed by the
XMM-Newton observatory. From these data we confirm the emission of x-rays from
all contact binary systems, with typical luminosities of approximately 1.0 x
10^30 erg s^-1. Combining calculated luminosities with an estimated contact
binary space density, we find that contact binaries do not have strong enough
x-ray emission to account for a significant portion of the galactic x-ray
background.Comment: 19 pages, 5 figures, accepted by A
A relation between moduli space of D-branes on orbifolds and Ising model
We study D-branes transverse to an abelian orbifold C^3/Z_n Z_n. The moduli
space of the gauge theory on the D-branes is analyzed by combinatorial
calculation based on toric geometry. It is shown that the calculation is
related to a problemto count the number of ground states of an
antiferromagnetic Ising model. The lattice on which the Ising model is defined
is a triangular one defined on the McKay quiver of the orbifold.Comment: 20 pages, 13 figure
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On Birthing Dancing Stars: The Need for Bounded Chaos in Information Interaction
While computers causing chaos is acommon social trope, nearly the entirety of the history of computing is dedicated to generating order. Typical interactive information retrieval tasks ask computers to support the traversal and exploration of large, complex information spaces. The implicit assumption is that they are to support users in simplifying the complexity (i.e. in creating order from chaos). But for some types of task, particularly those that involve the creative application or synthesis of knowledge or the creation of new knowledge, this assumption may be incorrect. It is increasingly evident that perfect orderâand the systems we create with itâsupport highly-structured information tasks well, but provide poor support for less-structured tasks.We need digital information environments that help create a little more chaos from order to spark creative thinking and knowledge creation. This paper argues for the need for information systems that offerwhat we term âbounded chaosâ, and offers research directions that may support the creation of such interface
The Evolving Activity of the Dynamically Young Comet C/2009 P1 (Garradd)
We used the UltraViolet-Optical Telescope on board Swift to observe the
dynamically young comet C/2009 P1 (Garradd) from a heliocentric distance of 3.5
AU pre-perihelion until 4.0 AU outbound. At 3.5 AU pre-perihelion, comet
Garradd had one of the highest dust-to-gas ratios ever observed, matched only
by comet Hale-Bopp. The evolving morphology of the dust in its coma suggests an
outburst that ended around 2.2 AU pre-perihelion. Comparing slit-based
measurements and observations acquired with larger fields of view indicated
that between 3 AU and 2 AU pre-perihelion a significant extended source started
producing water in the coma. We demonstrate that this source, which could be
due to icy grains, disappeared quickly around perihelion. Water production by
the nucleus may be attributed to a constantly active source of at least 75
km, estimated to be more than 20 percent of the surface. Based on our
measurements, the comet lost kg of ice and dust during this
apparition, corresponding to at most a few meters of its surface.Even though
this was likely not Garradd's first passage through the inner solar system, the
activity of the comet was complex and changed significantly during the time it
was observed
Ultrahigh energy neutrino scattering: an update
We update our estimates of charged and neutral current neutrino total cross
sections on isoscalar nucleons at ultrahigh energies using a global (x, Q^2)
fit, motivated by the Froissart bound, to the F_2 (electron-proton) structure
function utilizing the most recent analysis of the complete ZEUS and H1 data
sets from HERA I. Using the large Q^2, small Bjorken-x limits of the "wee"
parton model, we connect the ultrahigh energy neutrino cross sections directly
to the large Q^2, small-x extrapolation of our new fit, which we assume
saturates the Froissart bound. We compare both to our previous work, which
utilized only the smaller ZEUS data set, as well as to recent results of a
calculation using the ZEUS-S based global perturbative QCD parton distributions
using the combined HERA I results as input. Our new results substantiate our
previous conclusions, again predicting significantly smaller cross sections
than those predicted by extrapolating pQCD calculations to neutrino energies
above 10^9 GeV.Comment: 8 pages, 1 figure, 3 table
Extension of the sun-synchronous Orbit
Through careful consideration of the orbit perturbation force due to the oblate nature of the primary body a secular variation of the ascending node angle of a near-polar orbit can be induced without expulsion of propellant. Resultantly, the orbit perturbations can be used to maintain the orbit plane in, for example, a near-perpendicular (or at any other angle) alignment to the Sun-line throughout the full year of the primary body; such orbits are normally termed Sun-synchronous orbits [1, 2]. Sun-synchronous orbits about the Earth are typically near-circular Low-Earth Orbits (LEOs), with an altitude of less than 1500 km. It is normal to design a LEO such that the orbit period is synchronised with the rotation of the Earthâs surface over a given period, such that a repeating ground-track is established. A repeating ground-track, together with the near-constant illumination conditions of the ground-track when observed from a Sun-synchronous orbit, enables repeat observations of a target over an extended period under similar illumination conditions [1, 2]. For this reason, Sun-synchronous orbits are extensively used by Earth Observation (EO) platforms, including currently the Environmental Satellite (ENVISAT), the second European Remote Sensing satellite (ERS-2) and many more. By definition, a given Sun-synchronous orbit is a finite resource similar to a geostationary orbit. A typical characterising parameter of a Sun-synchronous orbit is the Mean Local Solar Time (MLST) at descending node, with a value of 1030 hours typical. Note that ERS-1 and ERS-2 used a MLST at descending node of 1030 hours ± 5 minutes, while ENVISAT uses a 1000 hours ± 5 minutes MLST at descending node [3]. Following selection of the MLST at descending node and for a given desired repeat ground-track, the orbit period and hence the semi-major axis are fixed, thereafter assuming a circular orbit is desired it is found that only a single orbit inclination will enable a Sun-synchronous orbit [2]. As such, only a few spacecraft can populate a given repeat ground-track Sun-synchronous orbit without compromise, for example on the MLST at descending node. Indeed a notable feature of on-going studies by the ENVISAT Post launch Support Office is the desire to ensure sufficient propellant remains at end-of-mission for re-orbiting to a graveyard orbit to ensure the orbital slot is available for future missions [4]. An extension to the Sun-synchronous orbit is considered using an undefined, non-orientation constrained, low-thrust propulsion system. Initially the low-thrust propulsion system will be considered for the free selection of orbit inclination and altitude while maintaining the Sun-synchronous condition. Subsequently the maintenance of a given Sun-synchronous repeat-ground track will be considered, using the low-thrust propulsion system to enable the free selection of orbit altitude. An analytical expression will be developed to describe these extensions prior to then validating the analytical expressions within a numerical simulation of a spacecraft orbit. Finally, an analysis will be presented on transfer and injection trajectories to these orbits
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Beagle to the Moon: nn experiment package to measure polar ice and volatiles in permanently shadowed areas or beneath the lunar surface
The Beagle Science Package is a flight qualified set of instruments which should be deployed to the lunar surface to answer the questions about water and volatiles present in permanently shadowed regions and/or beneath the surface
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