245,257 research outputs found
Uranium isotopes quantitatively determined by modified method of atomic absorption spectrophotometry
Hollow-cathode discharge tubes determine the quantities of uranium isotopes in a sample by using atomic absorption spectrophotometry. Dissociation of the uranium atoms allows a large number of ground state atoms to be produced, absorbing the incident radiation that is different for the two major isotopes
A Theory of Gamma-Ray Bursts
We present a specific scenario for the link between GRB and hypernovae, based
on Blandford-Znajek extraction of black-hole spin energy. Such a mechanism
requires a high angular momentum in the progenitor object. The observed
association of gamma-ray bursts with type Ibc supernovae leads us to consider
massive helium stars that form black holes at the end of their lives as
progenitors. We combine the numerical work of MacFadyen & Woosley with analytic
calculations, to show that about 1E53 erg each are available to drive the fast
GRB ejecta and the supernova. The GRB ejecta are driven by the power output
through the open field lines, whereas the supernova is powered by closed filed
lines and jet shocks. We also present a much simplified approximate derivation
of these energetics.
Helium stars that leave massive black-hole remnants in special ways, namely
via soft X-ray transients or very massive WNL stars. Since binaries naturally
have high angular momentum, we propose a link between black-hole transients and
gamma-ray bursts. Recent observations of one such transient, GRO J1655-40/Nova
Scorpii 1994, explicitly support this connection: its high space velocity
indicates that substantial mass was ejected in the formation of the black hole,
and the overabundance of alpha-nuclei, especially sulphur, indicates that the
explosion energy was extreme, as in SN 1998bw/GRB 980425. (abstract shortened)Comment: 32 pages, 8 figures, accepted for publication in New Astronom
Motion of a sphere in the presence of a plane interface. Part 2. An exact solution in bipolar co-ordinates
A general solution for Stokesâ equation in bipolar co-ordinates is derived, and then applied to the arbitrary motion of a sphere in the presence of a plane fluid/fluid interface. The drag force and hydrodynamic torque on the sphere are then calculated for four specific motions of the sphere; namely, translation perpendicular and parallel to the interface and rotation about an axis which is perpendicular and parallel, respectively, to the interface. The most significant result of the present work is the comparison between these numerically exact solutions and the approximate solutions from part 1. The latter can be generalized to a variety of particle shapes, and it is thus important to assess their accuracy for this case of spherical particles where an exact solution can be obtained. In addition to comparisons with the approximate solutions, we also examine the predicted changes in the velocity, pressure and vorticity fields due to the presence of the plane interface. One particularly interesting feature of the solutions is the fact that the direction of rotation of a freely suspended sphere moving parallel to the interface can either be the same as for a sphere rolling along the interface (as might be intuitively expected), or opposite depending upon the location of the sphere centre and the ratio of viscosities for the two fluids
The Formation of High-Mass Black Holes in Low Mass X-ray Binaries
In this note we suggest that high-mass black holes; i.e., black holes of
several solar masses, can be formed in binaries with low-mass main-sequence
companions, provided that the hydrogen envelope of the massive star is removed
in common envelope evolution which begins only after the massive star has
finished He core burning. That is, the massive star is in the supergiant stage,
which lasts only years, so effects of mass loss by He winds are
small. Since the removal of the hydrogen envelope of the massive star occurs so
late, it evolves essentially as a single star, rather than one in a binary.
Thus, we can use evolutionary calculations of Woosley & Weaver (1995) of single
stars. We find that the black holes in transient sources can be formed from
stars with ZAMS masses in the interval 20-35\msun. The black hole mass is
only slightly smaller than the He core mass, typically \sim 7\msun.Comment: 19 pages, substantial changes, accepted in New Astronom
Large-N Yang-Mills Theory as Classical Mechanics
To formulate two-dimensional Yang-Mills theory with adjoint matter fields in
the large-N limit as classical mechanics, we derive a Poisson algebra for the
color-invariant observables involving adjoint matter fields. We showed
rigorously in J. Math. Phys. 40, 1870 (1999) that different quantum orderings
of the observables produce essentially the same Poisson algebra. Here we
explain, in a less precise but more pedagogical manner, the crucial topological
graphical observations underlying the formal proof.Comment: 8 pages, 3 eps figues, LaTeX2.09, aipproc macros needed; conference
proceeding of MRST '99 (10-12 May, 1999, Carleton University, Canada
Evolution and Merging of Binaries with Compact Objects
In the light of recent observations in which short gamma-ray bursts are
interpreted as arising from black-hole(BH), neutron-star(NS) or NS-NS mergings
we would like to review our research on the evolution of compact binaries,
especially those containing NS's. These were carried out with predictions for
LIGO in mind, but are directly applicable to short gamma-ray bursts in the
interpretation above.
Most important in our review is that we show that the standard scenario for
evolving NS-NS binaries always ends up with a low-mass BH (LMBH), NS binary.
Bethe and Brown (1998) showed that this fate could be avoided if the two giants
in the progenitor binary burned He at the same time, and that in this way the
binary could avoid the common envelope evolution of the NS with red giant
companion which sends the first born NS into a BH in the standard scenario. The
burning of He at the same time requires, for the more massive giants such as
the progenitors of the Hulse-Taylor binary NS that the two giants be within 4%
of each other in ZAMS mass. Applying this criterion to all binaries results in
a factor 5 of LMBH-NS binaries as compared with NS-NS binaries.
Our scenario of NS-NS binaries as having been preceded by a double He-star
binary is collecting observational support in terms of the nearly equal NS
masses within a given close binary.Comment: 32 pages, 1 figure, substantial changes from v
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