63,498 research outputs found
Rationality of moduli of vector bundles on curves
The moduli space M(r,d) of stable, rank r, degree d vector bundles on a
smooth projective curve of genus g>1 is shown to be birational to M(h,0) x A,
where h=hcf(r,d) and A is affine space of dimension (r^2-h^2)(g-1). The
birational isomorphism is compatible with fixing determinants in M(r,d) and
M(h,0) and we obtain as a corollary that the moduli space of bundles of rank r
and fixed determinant of degree d is rational, when r and d are coprime. A key
ingredient in the proof is the use of a naturally defined Brauer class for the
function field of M(r,d).Comment: 21 pages, Latex2e (with AMS packages
Electronic gating circuit and ultraviolet laser excitation permit improved dosimeter sensitivity
Standard dosimeter reader, modified by adding an electronic gating circuit to trigger the intensity level photomultiplier, increases readout sensitivity of photoluminescent dosimeter systems. The gating circuit is controlled by a second photomultiplier which senses a short ultraviolet pulse from a laser used to excite the dosimeter
Transonic separated flow predictions based on a mathematically simple, nonequilibrium turbulence closure model
A mathematically simple, turbulence closure model designed to treat transonic airfoil flows even with massive separation is described. Numerical solutions of the Reynolds-averaged, Navier-Stokes equations obtained with this closure model are shown to agree well with experiments over a broad range of test conditions
Separated transonic airfoil flow calculations with a nonequilibrium turbulence model
Navier-Stokes transonic airfoil calculations based on a recently developed nonequilibrium, turbulence closure model are presented for a supercritical airfoil section at transonic cruise conditions and for a conventional airfoil section at shock-induced stall conditions. Comparisons with experimental data are presented which show that this nonequilibrium closure model performs significantly better than the popular Baldwin-Lomax and Cebeci-Smith equilibrium algebraic models when there is boundary-layer separation that results from the inviscid-viscous interactions
Cygnus X-2, super-Eddington mass transfer, and pulsar binaries
We consider the unusual evolutionary state of the secondary star in Cygnus
X-2. Spectroscopic data give a low mass (M_2 \simeq 0.5 - 0.7\msun) and yet a
large radius (R_2 \simeq 7\rsun) and high luminosity (L_2 \simeq 150\lsun).
We show that this star closely resembles a remnant of early massive Case B
evolution, during which the neutron star ejected most of the \sim 3\msun
transferred from the donor (initial mass M_{\rm 2i}\sim 3.6\msun) on its
thermal time-scale yr. As the system is far too wide to result from
common-envelope evolution, this strongly supports the idea that a neutron star
efficiently ejects the excess inflow during super--Eddington mass transfer.
Cygnus X-2 is unusual in having had an initial mass ratio in a narrow critical range near . Smaller lead to long-period systems with the former donor near the Hayashi line,
and larger to pulsar binaries with shorter periods and relatively
massive white dwarf companions. The latter naturally explain the surprisingly
large companion masses in several millisecond pulsar binaries. Systems like
Cygnus X-2 may thus be an important channel for forming pulsar binaries.Comment: 9 pages, 4 encapsulated figures, LaTeX, revised version with a few
typos corrected and an appendix added, accepted by MNRA
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