9,128 research outputs found
BUGS system clock distributor
A printed circuit board which will provide external clocks and precisely measure the time at which events take place was designed for the Bristol University Gas Spectrometer (BUGS). The board, which was designed to interface both mechanically and electrically to the Computer Automated Measurement and Control (CAMAC) system, has been named the BUGS system clock control. The board's design and use are described
Reversal and amplification of zonal flows by boundary enforced thermal wind
Zonal flows in rapidly-rotating celestial objects such as the Sun, gas or ice
giants form in a variety of surface patterns and amplitudes. Whereas the
differential rotation on the Sun, Jupiter and Saturn features a super-rotating
equatorial region, the ice giants, Neptune and Uranus harbour an equatorial jet
slower than the planetary rotation. Global numerical models covering the
optically thick, deep-reaching and rapidly rotating convective envelopes of gas
giants reproduce successfully the prograde jet at the equator. In such models,
convective columns shaped by the dominant Coriolis force typically exhibit a
consistent prograde tilt. Hence angular momentum is pumped away from the
rotation axis via Reynolds stresses. Those models are found to be strongly
geostrophic, hence a modulation of the zonal flow structure along the axis of
rotation, e.g. introduced by persistent latitudinal temperature gradients,
seems of minor importance. Within our study we stimulate these thermal
gradients and the resulting ageostrophic flows by applying an axisymmetric and
equatorially symmetric outer boundary heat flux anomaly () with
variable amplitude and sign. Such a forcing pattern mimics the thermal effect
of intense solar or stellar irradiation. Our results suggest that the
ageostrophic flows are linearly amplified with the forcing amplitude
leading to a more pronounced dimple of the equatorial jet (alike Jupiter). The
geostrophic flow contributions, however, are suppressed for weak , but
inverted and re-amplified once exceeds a critical value. The inverse
geostrophic differential rotation is consistently maintained by now also
inversely tilted columns and reminiscent of zonal flow profiles observed for
the ice giants. Analysis of the main force balance and parameter studies
further foster these results
Short-term variability and mass loss in Be stars II. Physical taxonomy of photometric variability observed by the Kepler spacecraft
Context: [abbreviated]
Aims: Kepler data of three known Be stars are re-visited to establish their
pulsational nature and assess the properties of additional, non-pulsational
variations. The three program stars turned out to be one inactive Be star, one
active, continuously outbursting Be star, and one Be star transiting from a
non-outbursting into an outbursting phase, thus forming an excellent sample to
distill properties of Be stars in the various phases of their life-cycle.
Methods: [abbreviated]
Results: The short-term photometric variability of Be stars must be
disentangled into a stellar and a circumstellar part. The stellar part is on
the whole not different from what is seen in non-Be stars. However, some of the
observed phenomena might be to be due to resonant mode coupling, a mechanism
not typically considered for B-type stars. Short-term circumstellar variability
comes in the form of either a group of relatively well-defined, short-lived
frequencies during outbursts, which are called \v{S}tefl frequencies, and broad
bumps in the power spectra, indicating aperiodic variability on a time scale
similar to typical low-order -mode pulsation frequencies, rather than true
periodicity.
Conclusions: From a stellar pulsation perspective, Be stars are rapidly
rotating SPB stars, that is they pulsate in low order -modes, even if the
rapid rotation can project the observed frequencies into the traditional
high-order -mode regime above about 4 c/d. However, when a circumstellar
disk is present, Be star power spectra are complicated by both cyclic, or
periodic, and aperiodic circumstellar phenomena, possibly even dominating the
power spectrum.Comment: Accepted by Astronomy and Astrophysic
Localization under the effect of randomly distributed decoherence
Electron transport through disordered quasi one-dimensional quantum systems
is studied. Decoherence is taken into account by a spatial distribution of
virtual reservoirs, which represent local interactions of the conduction
electrons with their environment. We show that the decoherence distribution has
observable effects on the transport. If the decoherence reservoirs are
distributed randomly without spatial correlations, a minimal degree of
decoherence is necessary to obtain Ohmic conduction. Below this threshold the
system is localized and thus, a decoherence driven metal-insulator transition
is found. In contrast, for homogenously distributed decoherence, any finite
degree of decoherence is sufficient to destroy localization. Thus, the presence
or absence of localization in a disordered one-dimensional system may give
important insight about how the electron phase is randomized.Comment: 8 pages, 5 figure
Statistical model for the effects of phase and momentum randomization on electron transport
A simple statistical model for the effects of dephasing on electron transport
in one-dimensional quantum systems is introduced, which allows to adjust the
degree of phase and momentum randomization independently. Hence, the model is
able to describe the transport in an intermediate regime between classic and
quantum transport. The model is based on B\"uttiker's approach using fictitious
reservoirs for the dephasing effects. However, in contrast to other models, at
the fictitious reservoirs complete phase randomization is assumed, which
effectively divides the system into smaller coherent subsystems, and an
ensemble average over randomly distributed dephasing reservoirs is calculated.
This approach reduces not only the computation time but allows also to gain new
insight into system properties. In this way, after deriving an efficient
formula for the disorder-averaged resistance of a tight-binding chain, it is
shown that the dephasing-driven transition from localized-exponentially to
ohmic-linear behavior is not affected by the degree of momentum randomizing
dephasing.Comment: 6 pages, 7 figure
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