1,019 research outputs found

### The Structure of Close Binaries in Two Dimensions

The structure and evolution of close binary stars has been studied using the
two-dimensional (2D) stellar structure algorithm developed by Deupree (1995).
We have calculated a series of solar composition stellar evolution sequences of
binary models, where the mass of the 2D model is 8Msun with a point-mass 5Msun
companion. We have also studied the structure of the companion in 2D, by
considering the zero-age main-sequence (ZAMS) structure of a 5Msun model with
an 8Msun point-mass companion. In all cases the binary orbit was assumed to be
circular and co-rotating with the rotation rate of the stars. We considered
binary models with three different initial separations, a = 10, 14 and 20Rsun.
These models were evolved through central hydrogen burning or until the more
massive star expanded to fill its critical potential surface or Roche lobe. The
calculations show that evolution of the deep interior quantities is only
slightly modified from those of single star evolution. Describing the model
surface as a Roche equipotential is also satisfactory until very close to the
time of Roche lobe overflow, when the self gravity of the model about to lose
mass develops a noticeable aspherical component and the surface time scale
becomes sufficiently short that it is conceivable that the actual surface is
not an equipotential.Comment: 22 pages, 10 figures, accepted by Ap

### Accretion flows: the Role of the Outer Boundary Condition

We investigate the influences of the outer boundary conditions(OBCs) on the
structure of an optically thin accretion flow. We find that OBC plays an
important role in determining the topological structure and the profiles of the
surface density and temperature of the solution, therefore it should be
regarded as a new parameter in the accretion disk model.Comment: 9 pages, 2 figures, to appear in ApJ Letters, Vol. 521, L5

### DIFFRACTION EFFECTS IN MICROLENSING OF Q2237+0305

Geometrical optics provides an excellent description for quasar images
crossing caustics which are formed by gravitational microlensing of objects
like Q2237+0305. Within this approximation the source size can be estimated
from the maximum magnification reached at caustic crossings. We evaluate the
limitations imposed by diffraction on caustics using the formalism developed by
Ulmer & Goodman (1995). Close to a caustic a new characteristic length, smaller
that the Fresnel length, enters the problem, limiting the angular resolution to
about 0.2 pico arcsecond, or equivalently about 3*10^9 cm at the source. To
achieve this resolution the brightness must be monitored at time intervals of a
few seconds. If a significant fraction of quasar luminosity comes from sources
smaller than those limits then interference effects would make the observed
intensity oscillate, in a close analogy with a two slit experiment. The
characteristic period of such oscillations is expected to be about one tenth of
a minute. If such oscillations are detected then photometry carried out at a
single site may permit the determination of the caustic transverse velocity,
and therefore may permit a direct conversion of the time units of brightness
variations to the linear units at the source. Subject headings: Gravitational
lensing - dark matter - quasars: structure -quasars: Q2237+0305Comment: 10 pages, plain TEX file, no figures. Submitted to ApJ postscript
available at by anonymous ftp at
ftp://astro.princeton.edu/library/preprint/pop615.ps.

### Gamma-ray burst beaming: a universal configuration with a standard energy reservoir?

We consider a gamma-ray burst (GRB) model based on an anisotropic fireball
with an axisymmetric power-law distribution of the energy per solid angle with
index -k, and allow for the observer's viewing direction being at an arbitrary
angle with respect to the jet axis. This model can reproduce the key features
expected from the conventional on-axis uniform jet models, with the novelty
that the achromatic break time in the broadband afterglow lightcurves
corresponds to the epoch when the relativistic beaming angle is equal to the
viewing angle rather than to the jet half opening angle. If all the GRB
fireballs have such a similar energy distribution form with 1.5 < k < (or \sim)
2, GRBs may be modeled by a quasi-universal beaming configuration, and an
approximately standard energy reservoir. The conclusion also holds for some
other forms of angular energy distributions, such as the Gaussian function.Comment: Slightly expanded version accepted for publication in Ap

### Neutrino oscillations and gamma-ray bursts

If the ordinary neutrinos oscillate into a sterile flavor in a manner
consistent with the Super-Kamiokande data on the zenith-angle dependence of
atmospheric mu-neutrino flux, an energy sufficient to power a typical cosmic
gamma-ray burst (GRB) (about 10^{52} erg) can be carried by sterile neutrinos
away from the source and deposited in a region relatively free of baryons.
Hence, ultra-relativistic bulk motion (required by the theory of and
observations of GRBs and their afterglows) can easily be achieved in the
vicinity of plausible sources of GRBs. Oscillations between sterile and
ordinary neutrinos would thus provide a solution to the ``baryon-loading
problem'' in the theory of GRBs

### A novel method to construct stationary solutions of the Vlasov-Maxwell system : the relativistic case

A method to derive stationary solutions of the relativistic Vlasov-Maxwell
system is explored. In the non-relativistic case, a method using the Hermite
polynomial series to describe the deviation from the Maxwell-Boltzmann
distribution is found to be successful in deriving a few stationary solutions
including two dimensional one. Instead of the Hermite polynomial series, two
special orthogonal polynomial series, which are appropriate to expand the
deviation from the Maxwell-J\"uttner distribution, are introduced in this
paper. By applying this method, a new two-dimensional equilibrium is derived,
which may provide an initial setup for investigations of three-dimensional
relativistic collisionless reconnection of magnetic fields.Comment: 15pages, 2 figures, to appear in Phys. Plasma

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