65,713 research outputs found
Binary evolution using the theory of osculating orbits: conservative Algol evolution
Our aim is to calculate the evolution of Algol binaries within the framework
of the osculating orbital theory, which considers the perturbing forces acting
on the orbit of each star arising from mass exchange via Roche lobe overflow
(RLOF). The scheme is compared to results calculated from a `classical'
prescription. Using our stellar binary evolution code BINSTAR, we calculate the
orbital evolution of Algol binaries undergoing case A and case B mass transfer,
by applying the osculating scheme. The velocities of the ejected and accreted
material are evaluated by solving the restricted three-body equations of
motion, within the ballistic approximation. This allows us to determine the
change of linear momentum of each star, and the gravitational force applied by
the mass transfer stream. Torques applied on the stellar spins by tides and
mass transfer are also considered. Using the osculating formalism gives shorter
post-mass transfer orbital periods typically by a factor of 4 compared to the
classical scheme, owing to the gravitational force applied onto the stars by
the mass transfer stream. Additionally, during the rapid phase of mass
exchange, the donor star is spun down on a timescale shorter than the tidal
synchronization timescale, leading to sub-synchronous rotation. Consequently,
between 15 and 20 per cent of the material leaving the inner-Lagrangian point
is accreted back onto the donor (so-called `self-accretion'), further enhancing
orbital shrinkage. Self-accretion, and the sink of orbital angular momentum
which mass transfer provides, may potentially lead to more contact binaries.
Even though Algols are mainly considered, the osculating prescription is
applicable to all types of interacting binaries, including those with eccentric
orbits.Comment: A&A in press. Minor typos correcte
Exobiology and Future Mars Missions
Scientific questions associated with exobiology on Mars were considered and how these questions should be addressed on future Mars missions was determined. The mission that provided a focus for discussions was the Mars Rover/Sample Return Mission
Development of a surface isolation estimation technique suitable for application of polar orbiting satellite data
A technique is developed for the estimation of total daily insolation on the basis of data derivable from operational polar-orbiting satellites. Although surface insolation and meteorological observations are used in the development, the algorithm is constrained in application by the infrequent daytime polar-orbiter coverage
Fabrication and surface characterization of composite refractory compounds suitable for thermionic converters
Thermal faceting was observed for the high index planes of LaB6. The (100), (110), and (111) planes were found to be the most thermodynamically stable faces in vacuum in a study of electrode materials for thermionic emitters. The properties of adsorbed carbon, cesium, and cesium-oxygen layers were investigated on LaB6 single crystal surfaces as well as on Zr/0/W(100) and W(100). Cesium was found to increase electron reflection near the collision threshold on LaB6(100) and W(100) and to decrease the reflection on Zr/0/W(100). This difference may be explained by the unusually high threshold reflection coefficient of Zr/0/W without adsorbed cesium
Mariner Mars 1971 spacecraft destruct unit
Shaped charge destruct unit for Mariner Mars 1971 spacecraf
Cosmological Perturbations of Quantum-Mechanical Origin and Anisotropy of the Microwave Background
Cosmological perturbations generated quantum-mechanically (as a particular
case, during inflation) possess statistical properties of squeezed quantum
states. The power spectra of the perturbations are modulated and the angular
distribution of the produced temperature fluctuations of the CMBR is quite
specific. An exact formula is derived for the angular correlation function of
the temperature fluctuations caused by squeezed gravitational waves. The
predicted angular pattern can, in principle, be revealed by the COBE-type
observations.Comment: 9 pages, WUGRAV-92-17 Accepted for Publication in Phys. Rev. Letters
(1993
Fine asymptotic behavior in eigenvalues of random normal matrices: Ellipse Case
We consider the random normal matrices with quadratic external potentials
where the associated orthogonal polynomials are Hermite polynomials and the
limiting support (called droplet) of the eigenvalues is an ellipse. We
calculate the density of the eigenvalues near the boundary of the droplet up to
the second subleading corrections and express the subleading corrections in
terms of the curvature of the droplet boundary. From this result we
additionally get the expected number of eigenvalues outside the droplet. We
also obtain the asymptotics of the kernel and found that, in the bulk, the
correction term is exponentially small. This leads to the vanishing of certain
Cauchy transform of the orthogonal polynomial in the bulk of the droplet up to
an exponentially small error.Comment: 39 pages, 5 figures. Extended version: Theorem 1.2, Theorem 1.4,
Section 6 and Section 7.3 are ne
Joint design for improved fatigue life of diffusion-bonded box-stiffened panels
Simple photoelastic models were used to identify a cross-section geometry that would eliminate the severe stress concentrations at the bond line between box stiffeners diffusion bonded to a panel skin. Experimental fatigue-test data from titanium test specimens quantified the allowable stress in terms of cycle life for various joint geometries. It is shown that the effect of stress concentration is reduced and an acceptable fatigue life is achieved
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