9,647 research outputs found
Composition of Jupiter irregular satellites sheds light on their origin
Irregular satellites of Jupiter with their highly eccentric, inclined and
distant orbits suggest that their capture took place just before the giant
planet migration. We aim to improve our understanding of the surface
composition of irregular satellites of Jupiter to gain insight into a narrow
time window when our Solar System was forming. We observed three Jovian
irregular satellites, Himalia, Elara, and Carme, using a medium-resolution
0.8-5.5 micro m spectrograph on the National Aeronautics and Space
Administration (NASA) Infrared Telescope Facility (IRTF). Using a linear
spectral unmixing model we have constrained the major mineral phases on the
surface of these three bodies. Our results confirm that the surface of Himalia,
Elara, and Carme are dominated by opaque materials such as those seen in
carbonaceous chondrite meteorites. Our spectral modeling of NIR spectra of
Himalia and Elara confirm that their surface composition is the same and
magnetite is the dominant mineral. A comparison of the spectral shape of
Himalia with the two large main C-type asteroids, Themis (D 176 km) and Europa
(D 352 km), suggests surface composition similar to Europa. The NIR spectrum of
Carme exhibits blue slope up to 1.5 microm and is spectrally distinct from
those of Himalia and Elara. Our model suggests that it is compositionally
similar to amorphous carbon. Himalia and Elara are compositionally similar but
differ significantly from Carme. These results support the hypotheses that the
Jupiter irregular satellites are captured bodies that were subject to further
breakup events and clustered as families based on their similar physical and
surface compositions
Analysis of the exciton-exciton interaction in semiconductor quantum wells
The exciton-exciton interaction is investigated for quasi-two-dimensional
quantum structures. A bosonization scheme is applied including the full spin
structure. For generating the effective interaction potentials, the
Hartree-Fock and Heitler-London approaches are improved by a full two-exciton
calculation which includes the van der Waals effect. With these potentials the
biexciton formation in bilayer systems is investigated. For coupled quantum
wells the two-body scattering matrix is calculated and employed to give a
modified relation between exciton density and blue shift. Such a relation is of
central importance for gauging exciton densities in experiments which pave the
way toward Bose-Einstein condensation of excitons
CHANDRA reveals galaxy cluster with the most massive nearby cooling core, RXCJ1504.1-0248
A CHANDRA follow-up observation of an X-ray luminous galaxy cluster with a
compact appearance, RXCJ1504.1-0248 discovered in our REFLEX Cluster Survey,
reveals an object with one of the most prominent cluster cooling cores. With a
core radius of ~30 kpc smaller than the cooling radius with ~140 kpc more than
70% of the high X-ray luminosity of Lbol = 4.3 10e45 erg s-1 of this cluster is
radiated inside the cooling radius. A simple modeling of the X-ray morphology
of the cluster leads to a formal mass deposition rate within the classical
cooling flow model of 1500 - 1900 Msun yr-1 (for h=0.7), and 2300 - 3000 Msun
yr-1 (for h=0.5). The center of the cluster is marked by a giant elliptical
galaxy which is also a known radio source. Thus it is very likely that we
observe one of the interaction systems where the central cluster AGN is heating
the cooling core region in a self-regulated way to prevent a massive cooling of
the gas, similar to several such cases studied in detail in more nearby
clusters. The interest raised by this system is then due to the high power
recycled in RXCJ1504-0248 over cooling time scales which is about one order of
magnitude higher than what occurs in the studied, nearby cooling core clusters.
The cluster is also found to be very massive, with a global X-ray temperature
of about 10.5 keV and a total mass of about 1.7 10e15 Msun inside 3 Mpc.Comment: accepted for publication in Astrophys. Journal, 10 figure
Density of states in graphene with vacancies: midgap power law and frozen multifractality
The density of states (DoS), , of graphene is investigated
numerically and within the self-consistent T-matrix approximation (SCTMA) in
the presence of vacancies within the tight binding model. The focus is on
compensated disorder, where the concentration of vacancies, and
, in both sub-lattices is the same. Formally, this model belongs to
the chiral symmetry class BDI. The prediction of the non-linear sigma-model for
this class is a Gade-type singularity . Our numerical data is compatible with this
result in a preasymptotic regime that gives way, however, at even lower
energies to , . We take this finding as an evidence that similar to the case
of dirty d-wave superconductors, also generic bipartite random hopping models
may exhibit unconventional (strong-coupling) fixed points for certain kinds of
randomly placed scatterers if these are strong enough. Our research suggests
that graphene with (effective) vacancy disorder is a physical representative of
such systems.Comment: References updated onl
Low temperature phase diagram of condensed para-Hydrogen in two dimensions
Extensive Path Integral Monte Carlo simulations of condensed para-Hydrogen in
two dimensions at low temperature have been carried out. In the zero
temperature limit, the system is a crystal at equilibrium, with a triangular
lattice structure. No metastable liquid phase is observed, as the system
remains a solid down to the spinodal density, and breaks down into solid
clusters at lower densities. The equilibrium crystal is found to melt at a
temperature close to 7 K
Mass-Temperature Relation of Galaxy Clusters: A Theoretical Study
Combining conservation of energy throughout nearly-spherical collapse of
galaxy clusters with the virial theorem, we derive the mass-temperature
relation for X-ray clusters of galaxies . The normalization factor
and the scatter of the relation are determined from first principles with
the additional assumption of initial Gaussian random field. We are also able to
reproduce the recently observed break in the M-T relation at T \sim 3 \keV,
based on the scatter in the underlying density field for a low density
CDM cosmology. Finally, by combining observational data of high
redshift clusters with our theoretical formalism, we find a semi-empirical
temperature-mass relation which is expected to hold at redshifts up to unity
with less than 20% error.Comment: 43 pages, 13 figures, One figure is added and minor changes are made.
Accepted for Publication in Ap
On the possibility to supercool molecular hydrogen down to superfluid transition
Recent calculations by Vorobev and Malyshenko (JETP Letters, 71, 39, 2000)
show that molecular hydrogen may stay liquid and superfluid in strong electric
fields of the order of . I demonstrate that strong local
electric fields of similar magnitude exist beneath a two-dimensional layer of
electrons localized in the image potential above the surface of solid hydrogen.
Even stronger local fields exist around charged particles (ions or electrons)
if surface or bulk of a solid hydrogen crystal is statically charged.
Measurements of the frequency shift of the photoresonance transition
in the spectrum of two-dimensional layer of electrons above positively or
negatively charged solid hydrogen surface performed in the temperature range 7
- 13.8 K support the prediction of electric field induced surface melting. The
range of surface charge density necessary to stabilize the liquid phase of
molecular hydrogen at the temperature of superfluid transition is estimated.Comment: 5 pages, 2 figure
The effect of partner-directed emotion in social exchange decision-making
Despite the prevalence of studies examining economic decision-making as a purely rational phenomenon, common sense suggests that emotions affect our decision-making particularly in a social context. To explore the influence of emotions on economic decision-making, we manipulated opponent-directed emotions prior to engaging participants in two social exchange decision-making games (the Trust Game and the Prisoner's Dilemma). Participants played both games with three different (fictional) partners and their tendency to defect was measured. Prior to playing each game, participants exchanged handwritten âessaysâ with their partners, and subsequently exchanged evaluations of each essay. The essays and evaluations, read by the participant, were designed to induce either anger, sympathy, or a neutral emotional response toward the confederate with whom they would then play the social exchange games. Galvanic skin conductance level (SCL) showed enhanced physiological arousal during anger induction compared to both the neutral and sympathy conditions. In both social exchange games, participants were most likely to defect against their partner after anger induction and least likely to defect after sympathy induction, with the neutral condition eliciting intermediate defection rates. This pattern was found to be strongest in participants exhibiting low cognitive control (as measured by a Go/no-Go task). The findings indicate that emotions felt toward another individual alter how one chooses to interact with them, and that this influence depends both on the specific emotion induced and the cognitive control of the individual
A Pilot Study to Measure Upper Extremity H-reflexes Following Neuromuscular Electrical Stimulation Therapy after Stroke
Upper extremity (UE) hemiparesis persists after stroke, limiting hand function. Neuromuscular electrical stimulation (NMES) is an effective intervention to improve UE recovery, although the underlying mechanisms are not fully understood. Our objective was to establish a reliable protocol to measure UE agonistâantagonist forearm monosynaptic reflexes in a pilot study to determine if NMES improves wrist function after stroke. We established the between-day reliability of the H-reflex in the extensor carpi radialis longus (ECRL) and flexor carpi radialis (FCR) musculature for individuals with prior stroke (n = 18). The same-day generation of ECRL/FCR H-reflex recruitment curves was well tolerated, regardless of age or UE spasticity. The between-day reliability of the ECRL H-reflex was enhanced above FCR, similar to healthy subjects [20], with the Hmax the most reliable parameter quantified in both muscles. H-reflex and functional measures following NMES show the potential for NMES-induced increases in ECRL Hmax, but confirmation requires a larger clinical study. Our initial results support the safe, easy, and efficacious use of in-home NMES, and establish a potential method to measure UE monosynaptic reflexes after stroke
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