553 research outputs found
Dynamic heterogeneities in the out-of-equilibrium dynamics of simple spherical spin models
The response of spherical two-spin interaction models, the spherical
ferromagnet (s-FM) and the spherical Sherrington-Kirkpatrick (s-SK) model, is
calculated for the protocol of the so-called nonresonant hole burning
experiment (NHB) for temperatures below the respective critical temperatures.
It is shown that it is possible to select dynamic features in the
out-of-equilibrium dynamics of both models, one of the hallmarks of dynamic
heterogeneities. The behavior of the s-SK model and the s-FM in three
dimensions is very similar, showing dynamic heterogeneities in the long time
behavior, i.e. in the aging regime. The appearence of dynamic heterogeneities
in the s-SK model explicitly demonstrates that these are not necessarily
related to {\it spatial} heterogeneities. For the s-FM it is shown that the
nature of the dynamic heterogeneities changes as a function of dimensionality.
With incresing dimension the frequency selectivity of the NHB diminishes and
the dynamics in the mean-field limit of the s-FM model becomes homogeneous.Comment: 16 pages, 8 figure
Mapping Vesta: First Results from Dawn’s Survey Orbit
The geologic objectives of the Dawn Mission [1] are
to derive Vesta’s shape, map the surface geology,
understand the geological context and contribute to
the determination of the asteroids’ origin and
evolution.Geomorphology and distribution of surface features
will provide evidence for impact cratering, tectonic activity, volcanism, and regolith processes. Spectral
measurements of the surface will provide evidence of
the compositional characteristics of geological units.
Age information, as derived from crater sizefrequency
distributions, provides the stratigraphic
context for the structural and compositional mapping
results, thus revealing the geologic history of Vesta.
We present here the first results of the Dawn mission
from data collected during the approach to Vesta, and
its first discrete orbit phase – the Survey Orbit, which
lasts 21 days after the spacecraft had established a
circular polar orbit at a radius of ~3000 km with a
beta angle of 10°-15°
Magnetic Fields, Relativistic Particles, and Shock Waves in Cluster Outskirts
It is only now, with low-frequency radio telescopes, long exposures with
high-resolution X-ray satellites and gamma-ray telescopes, that we are
beginning to learn about the physics in the periphery of galaxy clusters. In
the coming years, Sunyaev-Zeldovich telescopes are going to deliver further
great insights into the plasma physics of these special regions in the
Universe. The last years have already shown tremendous progress with detections
of shocks, estimates of magnetic field strengths and constraints on the
particle acceleration efficiency. X-ray observations have revealed shock fronts
in cluster outskirts which have allowed inferences about the microphysical
structure of shocks fronts in such extreme environments. The best indications
for magnetic fields and relativistic particles in cluster outskirts come from
observations of so-called radio relics, which are megaparsec-sized regions of
radio emission from the edges of galaxy clusters. As these are difficult to
detect due to their low surface brightness, only few of these objects are
known. But they have provided unprecedented evidence for the acceleration of
relativistic particles at shock fronts and the existence of muG strength fields
as far out as the virial radius of clusters. In this review we summarise the
observational and theoretical state of our knowledge of magnetic fields,
relativistic particles and shocks in cluster outskirts.Comment: 34 pages, to be published in Space Science Review
The Geology of the Marcia Quadrangle of Asteroid Vesta: Assessing the Effects of Large, Young Craters
We used Dawn spacecraft data to identify and delineate geological units and landforms in the Marcia quadrangle of Vesta as a means to assess the role of the large, relatively young impact craters Marcia (approximately 63 kilometers diameter) and Calpurnia (approximately 53 kilometers diameter) and their surrounding ejecta field on the local geology. We also investigated a local topographic high with a dark-rayed crater named Aricia Tholus, and the impact crater Octavia that is surrounded by a distinctive diffuse mantle. Crater counts and stratigraphic relations suggest that Marcia is the youngest large crater on Vesta, in which a putative impact melt on the crater floor ranges in age between approximately 40 and 60 million years (depending upon choice of chronology system), and Marcia's ejecta blanket ranges in age between approximately 120 and 390 million years (depending upon choice of chronology system). We interpret the geologic units in and around Marcia crater to mark a major Vestan time-stratigraphic event, and that the Marcia Formation is one of the geologically youngest formations on Vesta. Marcia crater reveals pristine bright and dark material in its walls and smooth and pitted terrains on its floor. The smooth unit we interpret as evidence of flow of impact melts and (for the pitted terrain) release of volatiles during or after the impact process. The distinctive dark ejecta surrounding craters Marcia and Calpurnia is enriched in OH- or H-bearing phases and has a variable morphology, suggestive of a complex mixture of impact ejecta and impact melts including dark materials possibly derived from carbonaceous chondrite-rich material. Aricia Tholus, which was originally interpreted as a putative Vestan volcanic edifice based on lower resolution observations, appears to be a fragment of an ancient impact basin rim topped by a dark-rayed impact crater. Octavia crater has a cratering model formation age of approximately 280-990 million years based on counts of its ejecta field (depending upon choice of chronology system), and its ejecta field is the second oldest unit in this quadrangle. The relatively young craters and their related ejecta materials in this quadrangle are in stark contrast to the surrounding heavily cratered units that are related to the billion years old or older Rheasilvia and Veneneia impact basins and Vesta's ancient crust preserved on Vestalia Terra
Vesta in the Light of Dawn
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An in-depth study of Marcia Crater, Vesta
After visiting the second most massive asteroid Vesta from July 2011 to September 2012, the Dawn spacecraft is now on its way to asteroid Ceres. Dawn observed Vesta with three instruments: the German Framing Camera (FC), the Italian Visible and InfraRed mapping spectrometer (VIR), and the American Gamma Ray and Neutron Detector (GRaND) [1]. Marcia crater (190°E, 10°N; 68 x 58 km) is the largest of three adjacent impact structures: Marcia (youngest), Calpurnia, and Minucia (oldest). It is the largest well-preserved post-Rheasilvia impact crater, shows a complex geology [2], is young [2], exhibits evidence for gully-like mass wasting [3], contains the largest location of pitted terrain [4], has smooth impact melt ponds [5], shows enhanced spectral pyroxene signatures on its inner walls [2], and has low abundances of OH and H in comparison to the surrounding low-albedo terrain [6, 7]. Geophysically, the broad region of Marcia and Calpurnia craters is characterized by a higher Bouguer gravity, indicating denser material [9]. Williams et al. [2] have produced a detailed geologic map of Marcia crater and the surrounding terrain. They identified several units within Marcia crater, including bright crater material, pitted terrain, and smooth material. Units outside Marcia, include undivided crater ejecta material, bright lobate material, dark lobate material, and dark crater ray material [2]. Because of its extensive ejecta and fresh appearance, the Marcia impact defines a major stratigraphic event, postdating the Rheasilvia impact [2]. However, the exact age of Marcia crater is still under debate. Compositionally, Marcia crater is characterized by higher iron abundances, which were interpreted as more basaltic-eucrite-rich materials suggesting that this region has not been blanketed by diogenitic materials from large impact events [10, 11]. Using FC data, [13] identified "gray material" associated with the ejecta blanket of Marcia crater. This material is characterized by a 0.75-mm reflectance of ~15%, a shallow visible slope, and a weak R(0.75 µm)/R(0.92 µm) ratio [12], which is still high compared to immediately adjacent terrains. The most prominent thermal feature in Marcia is the pitted terrain on its floor [8]. Temperatures of the pitted floor of Marcia are significantly lower than in the surrounding terrains, when observed under similar solar illumination. Denevi et al. [4] argued that the morphology and geologic setting are consistent with rapid degassing of volatile-bearing materials following an impact, which would lead to an increased local density and/or a higher thermal conductivity [8]. References: [1] Russell et al. (2007), Earth Moon Planets 101; [2] Williams et al. (2014), submitted to Icarus; [3] Scully et al. (2013), LPSC 45; [4] Denevi et al. (2012), Science 338; [5] Williams, D.A., et al. (2013) PSS, in press, j.pss.2013.06.017 [6] De Sanctis et al. (2012b) Astrophys. J. Lett. 758; [7] Prettyman et al. (2012), Science 338; [8] Tosi et al. (2014), submitted to Icarus; [9] Konopliv et al. (2013) Icarus, in press; [10] Yamashita et al. (2013), Met. Planet. Sci. 48; [11] Prettyman et al. (2013), Met. Planet. Sci. 48; [12] Reddy et al. (2012), Science 33
Measurement of the Charged Multiplicities in b, c and Light Quark Events from Z0 Decays
Average charged multiplicities have been measured separately in , and
light quark () events from decays measured in the SLD experiment.
Impact parameters of charged tracks were used to select enriched samples of
and light quark events, and reconstructed charmed mesons were used to select
quark events. We measured the charged multiplicities:
,
, from
which we derived the differences between the total average charged
multiplicities of or quark events and light quark events: and . We compared
these measurements with those at lower center-of-mass energies and with
perturbative QCD predictions. These combined results are in agreement with the
QCD expectations and disfavor the hypothesis of flavor-independent
fragmentation.Comment: 19 pages LaTex, 4 EPS figures, to appear in Physics Letters
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