1,607 research outputs found
A Carbon-Cycle Based Stochastic Cellular Automata Climate Model
In this article a stochastic cellular automata model is examined, which has
been developed to study a "small" world, where local changes may noticeably
alter global characteristics. This is applied to a climate model, where global
temperature is determined by an interplay between atmospheric carbon dioxide
and carbon stored by plant life. The latter can be relased by forest fires,
giving rise to significant changes of global conditions within short time.Comment: 17 pages, 8 figure
On the gravitational redshift
The study of the gravitational redshift\,---\,a relative wavelength increase
of was predicted for solar radiation by Einstein in
1908\,---\,is still an important subject in modern physics. In a dispute
whether or not atom interferometry experiments can be employed for
gravitational redshift measurements, two research teams have recently disagreed
on the physical cause of the shift. Regardless of any discussion on the
interferometer aspect\,---\,we find that both groups of authors miss the
important point that the ratio of gravitational to the electrostatic forces is
generally very small. For instance, the gravitational force acting on an
electron in a hydrogen atom situated in the Sun's photosphere to the
electrostatic force between the proton and the electron is approximately . A comparison of this ratio with the predicted and observed
solar redshift indicates a discrepancy of many orders of magnitude. Here we
show, with Einstein's early assumption of the frequency of spectral lines
depending only on the generating ion itself as starting point, that a solution
can be formulated based on a two-step process in analogy with Fermi's treatment
of the Doppler effect. It provides a sequence of physical processes in line
with the conservation of energy and momentum resulting in the observed shift
and does not employ a geometric description. The gravitational field affects
the release of the photon and not the atomic transition. The control parameter
is the speed of light. The atomic emission is then contrasted with the
gravitational redshift of matter-antimatter annihilation events.Comment: 19 Pages; 2 Table
The Boundary Layer in compact binaries
Disk accretion onto stars leads to the formation of a Boundary Layer (BL)
near the stellar surface where the disk makes contact with the star. Albeit a
large fraction of the total luminosity of the system originates from this tiny
layer connecting the accretion disk and the accreting object, its structure has
not been fully understood yet. It is the aim of this work, to obtain more
insight into the Boundary Layer around the white dwarf in compact binary
systems. There are still many uncertainties concerning the extent and
temperature of the BL and the rotation rate of the white dwarf. We perform
numerical hydrodynamical simulations, where the problem is treated in a
one-dimensional, radial approximation (slim disk). The turbulence is described
by the alpha parameter viscosity. We include both cooling from the disk
surfaces and radial radiation transport. The radiation energy is treated in the
one-temperature approximation. For a given M_dot our results show a strong
dependence on the stellar mass and rotation rate. Both, the midplane and the
effective temperature rise considerably with increasing stellar mass or
decreasing stellar rotation rate. Our simulations further show, that the
radiation energy and pressure are indeed important in the BL. However, some
models show a low optical depth in the BL, making it necessary to find a better
representation for optically thin regions. The combination of a high mass and a
small radius, such as in white dwarfs, can lead to an enormous energy release
in the BL, provided the WD rotates slowly. Since the radial extent of BLs is
typically very small (about 0.02 to 0.05 R_star), this leads to surface
temperatures of a few hundred thousand Kelvin. All of our models showed
subsonic infall velocities with Mach numbers of < 0.4 at most.Comment: 13 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
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