17,250 research outputs found
SN1987A - a Testing Ground for the KARMEN Anomaly
We show, that SN1987A can serve as an astrophysical laboratory for testing
the viability of the assertion that a new massive neutral fermion is implied by
the KARMEN data. We show that a wide range of the parameters characterizing the
proposed particle is ruled out by the above constraints making this
interpretation very unlikely.Comment: 12 pages, 1 eps figure embedded, to appear in Phys. Lett.
Effects of Extreme Obliquity Variations on the Habitability of Exoplanets
We explore the impact of obliquity variations on planetary habitability in
hypothetical systems with high mutual inclination. We show that large
amplitude, high frequency obliquity oscillations on Earth-like exoplanets can
suppress the ice-albedo feedback, increasing the outer edge of the habitable
zone. We restrict our exploration to hypothetical systems consisting of a
solar-mass star, an Earth-mass planet at 1 AU, and 1 or 2 larger planets. We
verify that these systems are stable for years with N-body simulations,
and calculate the obliquity variations induced by the orbital evolution of the
Earth-mass planet and a torque from the host star. We run a simplified energy
balance model on the terrestrial planet to assess surface temperature and ice
coverage on the planet's surface, and we calculate differences in the outer
edge of the habitable zone for planets with rapid obliquity variations. For
each hypothetical system, we calculate the outer edge of habitability for two
conditions: 1) the full evolution of the planetary spin and orbit, and 2) the
eccentricity and obliquity fixed at their average values. We recover previous
results that higher values of fixed obliquity and eccentricity expand the
habitable zone, but also find that obliquity oscillations further expand
habitable orbits in all cases. Terrestrial planets near the outer edge of the
habitable zone may be more likely to support life in systems that induce rapid
obliquity oscillations as opposed to fixed-spin planets. Such planets may be
the easiest to directly characterize with space-borne telescopes.Comment: 46 pages, 12 Figures, 5 Table
Possible Implications of Asymmetric Fermionic Dark Matter for Neutron Stars
We consider the implications of fermionic asymmetric dark matter for a "mixed
neutron star" composed of ordinary baryons and dark fermions. We find examples,
where for a certain range of dark fermion mass -- when it is less than that of
ordinary baryons -- such systems can reach higher masses than the maximal
values allowed for ordinary ("pure") neutron stars. This is shown both within a
simplified, heuristic Newtonian analytic framework with non-interacting
particles and via a general relativistic numerical calculation, under certain
assumptions for the dark matter equation of state. Our work applies to various
dark fermion models such as mirror matter models and to other models where the
dark fermions have self interactions.Comment: 20 pages, 6 figure
Current-induced spin polarization in InGaAs and GaAs epilayers with varying doping densities
The current-induced spin polarization and momentum-dependent spin-orbit field
were measured in InGaAs epilayers with varying indium
concentrations and silicon doping densities. Samples with higher indium
concentrations and carrier concentrations and lower mobilities were found to
have larger electrical spin generation efficiencies. Furthermore,
current-induced spin polarization was detected in GaAs epilayers despite the
absence of measurable spin-orbit fields, indicating that the extrinsic
contributions to the spin polarization mechanism must be considered.
Theoretical calculations based on a model that includes extrinsic contributions
to the spin dephasing and the spin Hall effect, in addition to the intrinsic
Rashba and Dresselhaus spin-orbit coupling, are found to qualitatively agree
with the experimental results.Comment: 16 pages, 8 figure
Description of the SERT 2 spacecraft and mission
Performance and orbital flight plan of SERT 2 ion thruster spacecraf
Generation and remote detection of THz sound using semiconductor superlattices
The authors introduce a novel approach to study the propagation of high
frequency acoustic phonons in which the generation and detection involves two
spatially separated superlattices apart. Propagating modes
of frequencies up to escape from the superlattice where they
are generated and reach the second superlattice where they are detected. The
measured frequency spectrum reveals finite size effects, which can be accounted
for by a continuum elastic model.Comment: Submitted to Applied Physics Letter
Stationary state volume fluctuations in a granular medium
A statistical description of static granular material requires ergodic
sampling of the phase space spanned by the different configurations of the
particles. We periodically fluidize a column of glass beads and find that the
sequence of volume fractions phi of post-fluidized states is history
independent and Gaussian distributed about a stationary state. The standard
deviation of phi exhibits, as a function of phi, a minimum corresponding to a
maximum in the number of statistically independent regions. Measurements of the
fluctuations enable us to determine the compactivity X, a temperature-like
state variable introduced in the statistical theory of Edwards and Oakeshott
[Physica A {\bf 157}, 1080 (1989)].Comment: published with minor change
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