990 research outputs found
Closed Brayton Cycle power system with a high temperature pellet bed reactor heat source for NEP applications
Capitalizing on past and future development of high temperature gas reactor (HTGR) technology, a low mass 15 MWe closed gas turbine cycle power system using a pellet bed reactor heating helium working fluid is proposed for Nuclear Electric Propulsion (NEP) applications. Although the design of this directly coupled system architecture, comprising the reactor/power system/space radiator subsystems, is presented in conceptual form, sufficient detail is included to permit an assessment of overall system performance and mass. Furthermore, an attempt is made to show how tailoring of the main subsystem design characteristics can be utilized to achieve synergistic system level advantages that can lead to improved reliability and enhanced system life while reducing the number of parasitic load driven peripheral subsystems
Tertiary Subsurface Facies, Source Rocks and Hydrocarbon Reservoirs in the SW Part of the Pannonian Basin (Northern Croatia and South-Western Hungary)
The Neogene sedimentary successions of the Drava, Sava and SlavonijaāSrijem depressions in the SW part of the Pannonian Basin System are built up of three 2nd order megacycles separated by four major erosional unconformities. The first megacycle contains terrestrial to marine syn-rift and early post-rift sediments of Early to Mid-Miocene age. The second is built up of Late Miocene Lake Pannon deposits, while the third contains those sediments which were deposited in the remnants of Lake Pannon and in the subsequent fluvial systems, in areas of continuous subsidence associated with basin inversion from the Pliocene onwards. Most of the petroleum source rocks and reservoir rocks are of Miocene age and were formed during the first and second depositional megacycle. Conditions for the accumulation and preservation of large quantities of marine and terrigenous organic matter were most favourable during the Badenian, Sarmatian and Early Pannonian, in deep basin settings, partly associated with rifting. The generation of hydrocarbons was promoted by relatively high geothermal gradients during the initial and subsequent thermal subsidence. Various sedimentary environments produced deposits with good reservoir characteristics: e.g. fault-related talus breccia (mainly Lower Miocene), reefs (mainly Badenian), coastal, shallow marine (Karpatian, Badenian) and deltaic (PannonianāPontian) sand bodies or turbiditic sand lobes (mainly Pannonian). The hydrocarbon (HC) migration paths were often provided by the major unconformities bounding the three megacycles, as well as by faults, particularly around the basement highs
Measurement of the hyperfine structure of antihydrogen in a beam
A measurement of the hyperfine structure of antihydrogen promises one of the
best tests of CPT symmetry. We describe an experiment planned at the Antiproton
Decelerator of CERN to measure this quantity in a beam of slow antihydrogen
atoms.Comment: 5th International Symposium on Symmetries in Subatomic Physics
(SSP2012), Groningen (The Netherlands), June 18 to 22, 201
Systematic study of the decay rates of antiprotonic helium states
A systematic study of the decay rates of antiprotonic helium (\pbhef and
\pbhet) at CERN AD (Antiproton Decelerator) has been made by a laser
spectroscopic method. The decay rates of some of its short-lived states, namely
those for which the Auger rates are much larger than
their radiative decay rates ( s),
were determined from the time distributions of the antiproton annihilation
signals induced by laser beams, and the widths of the atomic resonance lines.
The magnitude of the decay rates, especially their relation with the transition
multipolarity, is discussed and compared with theoretical calculations.Comment: 6 pages, 5 figures, and 1 tabl
Dust evolution in protoplanetary disks around Herbig Ae/Be stars - The Spitzer view
In this paper we present mid-infrared spectra of a comprehensive set of
Herbig Ae/Be stars observed with the Spitzer Space Telescope. The
signal-to-noise ratio of these spectra is very high, ranging between about a
hundred and several hundreds. During the analysis of these data we tested the
validity of standard protoplanetary dust models and studied grain growth and
crystal formation. On the basis of the analyzed spectra, the major constituents
of protoplanetary dust around Herbig Ae/Be stars are amorphous silicates with
olivine and pyroxene stoichiometry, crystalline forsterite and enstatite and
silica. No other solid state features, indicating other abundant dust species,
are present in the Spitzer spectra. Deviations of the synthetic spectra from
the observations are most likely related to grain shape effects and
uncertainties in the iron content of the dust grains. Our analysis revealed
that larger grains are more abundant in the disk atmosphere of flatter disks
than in that of flared disks, indicating that grain growth and sedimentation
decrease the disk flaring. We did not find, however, correlations between the
value of crystallinity and any of the investigated system parameters. Our
analysis shows that enstatite is more concentrated toward the warm inner disk
than forsterite, in contrast to predictions of equilibrium condensation models.
None of the three crystal formation mechanisms proposed so far can alone
explain all our findings. It is very likely that all three play at least some
role in the formation of crystalline silicates.Comment: 56 pages, 21 figures, accepted for publication in Ap
On the interplay between flaring and shadowing in disks around Herbig Ae/Be stars
Based on the SED, Herbig stars have been categorized into two observational
groups, reflecting their overall disk structure: group I members have disks
with a higher degree of flaring than their group II counterparts. We
investigate the 5-35 um Spitzer IRS spectra of a sample of 13 group I sources
and 20 group II sources. We focus on the continuum emission to study the
underlying disk geometry. We have determined the [30/13.5] and [13.5/7]
continuum flux ratios. The 7-um flux excess with respect to the stellar
photosphere is measured, as a marker for the strength of the near-IR emission
produced by the inner disk. We have compared our data to self-consistent
passive-disk model spectra, for which the same quantities were derived. We
confirm the literature result that the difference in continuum emission between
group I and II sources can largely be explained by a different amount of small
dust grains. However, we report a strong correlation between the [30/13.5] and
[13.5/7] flux ratios for Meeus group II sources. Moreover, the [30/13.5] flux
ratio decreases with increasing 7-um excess for all targets in the sample. To
explain these correlations with the models, we need to introduce an artificial
scaling factor for the inner disk height. In roughly 50% of the Herbig Ae/Be
stars in our sample, the inner disk must be inflated by a factor 2 to 3 beyond
what hydrostatic calculations predict. The total disk mass in small dust grains
determines the degree of flaring. We conclude, however, that for any given disk
mass in small dust grains, the shadowing of the outer (tens of AU) disk is
determined by the scale height of the inner disk (1 AU). The inner disk
partially obscures the outer disk, reducing the disk surface temperature. Here,
for the first time, we prove these effects observationally.Comment: 4 pages, 3 figures, accepted by A&
Evolution of dust and ice features around FU Orionis objects
(abridged) We present spectroscopy data for a sample of 14 FUors and 2 TTauri
stars observed with the Spitzer Space Telescope or with the Infrared Space
Observatory (ISO). Based on the appearance of the 10 micron silicate feature we
define 2 categories of FUors. Objects showing the silicate feature in
absorption (Category 1) are still embedded in a dusty and icy envelope. The
shape of the 10 micron silicate absorption bands is compared to typical dust
compositions of the interstellar medium and found to be in general agreement.
Only one object (RNO 1B) appears to be too rich in amorphous pyroxene dust, but
a superposed emission feature can explain the observed shape. We derive optical
depths and extinction values from the silicate band and additional ice bands at
6.0, 6.8 and 15.2 micron. In particular the analysis of the CO_2 ice band at
15.2 micron allows us to search for evidence for ice processing and constrains
whether the absorbing material is physically linked to the central object or in
the foreground. For objects showing the silicate feature in emission (Category
2), we argue that the emission comes from the surface layer of accretion disks.
Analyzing the dust composition reveals that significant grain growth has
already taken place within the accretion disks, but no clear indications for
crystallization are present. We discuss how these observational results can be
explained in the picture of a young, and highly active accretion disk. Finally,
a framework is proposed as to how the two categories of FUors can be understood
in a general paradigm of the evolution of young, low-mass stars. Only one
object (Parsamian 21) shows PAH emission features. Their shapes, however, are
often seen toward evolved stars and we question the object's status as a FUor
and discuss other possible classifications.Comment: accepted for publication in ApJ; 63 pages preprint style including 8
tables and 24 figure
Phase diagram of two-lane driven diffusive systems
We consider a large class of two-lane driven diffusive systems in contact
with reservoirs at their boundaries and develop a stability analysis as a
method to derive the phase diagrams of such systems. We illustrate the method
by deriving phase diagrams for the asymmetric exclusion process coupled to
various second lanes: a diffusive lane; an asymmetric exclusion process with
advection in the same direction as the first lane, and an asymmetric exclusion
process with advection in the opposite direction. The competing currents on the
two lanes naturally lead to a very rich phenomenology and we find a variety of
phase diagrams. It is shown that the stability analysis is equivalent to an
`extremal current principle' for the total current in the two lanes. We also
point to classes of models where both the stability analysis and the extremal
current principle fail
Radiative lifetime measurements of rubidium Rydberg states
We have measured the radiative lifetimes of ns, np and nd Rydberg states of
rubidium in the range 28 < n < 45. To enable long-lived states to be measured,
our experiment uses slow-moving Rb atoms in a magneto-optical trap (MOT). Two
experimental techniques have been adopted to reduce random and systematic
errors. First, a narrow-bandwidth pulsed laser is used to excite the target
Rydberg state, resulting in minimal shot-to-shot variation in the initial state
population. Second, we monitor the target state population as a function of
time delay from the laser pulse using a short-duration, millimetre-wave pulse
that is resonant with a one- or two-photon transition. We then selectively
field ionize the monitor state, and detect the resulting electrons with a
micro-channel plate. This signal is an accurate mirror of the target state
population, and is uncontaminated by contributions from other states which are
populated by black body radiation. Our results are generally consistent with
other recent experimental results obtained using a less sensitive method, and
are also in excellent agreement with theory.Comment: 27 pages,6 figure
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