41,094 research outputs found
Formation of Relativistic Axion Stars
Axions and axion-like particles are compelling candidates for the missing
dark matter of the universe. As they undergo gravitational collapse, they can
form compact objects such as axion stars or even black holes. In this paper, we
study the formation and distribution of such objects. First, we simulate the
formation of compact axion stars using numerical relativity with aspherical
initial conditions that could represent the final stages of axion dark matter
structure formation. We show that the final states of such collapse closely
follow the known relationship of initial mass and axion decay constant .
Second, we demonstrate with a toy model how this information can be used to
scan a model density field to predict the number densities and masses of such
compact objects. In addition to being detectable by the LIGO/VIRGO
gravitational wave interferometer network for axion mass of eV, we show using peak statistics that for , there
exists a "mass gap" between the masses of axion stars and black holes formed
from collapse
Widely separated binary systems of very low mass stars
In this paper we review some recent detections of wide binary brown dwarf
systems and discuss them in the context of the multiplicity properties of very
low-mass stars and brown dwarfs.Comment: 2 pages, 1 figure (new version with minor corrections); to appear in
the proceedings of the workshop "Ultra-low mass star formation and
evolution", to be published in A
Intersubband transitions in nonpolar GaN/Al(Ga)N heterostructures in the short and mid-wavelength infrared regions
This paper assesses nonpolar m- and a-plane GaN/Al(Ga)N multi-quantum-wells
grown on bulk GaN for intersubband optoelectronics in the short- and
mid-wavelength infrared ranges. The characterization results are compared to
those for reference samples grown on the polar c-plane, and are verified by
self-consistent Schr\"odinger-Poisson calculations. The best results in terms
of mosaicity, surface roughness, photoluminescence linewidth and intensity, as
well as intersubband absorption are obtained from m-plane structures, which
display room-temperature intersubband absorption in the range from 1.5 to 2.9
um. Based on these results, a series of m-plane GaN/AlGaN multi-quantum-wells
were designed to determine the accessible spectral range in the mid-infrared.
These samples exhibit tunable room-temperature intersubband absorption from 4.0
to 5.8 um, the long-wavelength limit being set by the absorption associated
with the second order of the Reststrahlen band in the GaN substrates
Dynamic transition and Shapiro-step melting in a frustrated Josephson-junction array
We consider a two-dimensional fully frustrated Josephson-junction array
driven by combined direct and alternating currents. Interplay between the mode
locking phenomenon, manifested by giant Shapiro steps in the current-voltage
characteristics, and the dynamic phase transition is investigated at finite
temperatures. Melting of Shapiro steps due to thermal fluctuations is shown to
be accompanied by the dynamic phase transition, the universality class of which
is also discussed
Influence of blade aerodynamic model on prediction of helicopter rotor aeroacoustic signatures
Brown’s vorticity transport model has been used to investigate how the local blade aerodynamic model influences the quality of the prediction of the high-frequency airloads associated with blade–vortex interactions, and thus the accuracy with which the acoustic signature of a helicopter rotor can be predicted. The vorticity transport model can accurately resolve the structure of the wake of the rotor and allows significant flexibility in the way that the blade loading can be represented. The Second Higher-Harmonic Control Aeroacoustics Rotor Test was initiated to provide experimental insight into the acoustic signature of a rotor in cases of strong blade–vortex interaction. Predictions of two models for the local blade aerodynamics are compared with the test data. A marked improvement in accuracy of the predicted high-frequency airloads and acoustic signature is obtained when a lifting-chord model for the blade aerodynamics is used instead of a lifting-line-type approach. Errors in the amplitude and phase of the acoustic peaks are reduced, and the quality of the prediction is affected to a lesser extent by the computational resolution of the wake, with the lifting-chord model producing the best representation of the distribution of sound pressure below the rotor
Density dependence of the Ionization Avalanche in ultracold Rydberg gases
We report on the behaviour of the ionization avalanche in an ensemble of
ultracold 87Rb atoms coupled to a high lying Rydberg state and investigate
extensions to the current model by including the effects of three-body
recombination and plasma expansion. To separate the two effects we study the
time dependence of the plasma formation at various densities as well as for
different nS and nD states. At medium densities and low n we observe the onset
of the avalanche as has been reported in other experiments, as well as a
subsequent turn-off of the avalanche for longer excitation times, which we
associate with plasma expansion. At higher densities and for higher lying
Rydberg states we observe a disappearance of the avalanche signature, which we
attribute to three-body recombination.Comment: 5 pages, 4 figure
Discovery of Radio Emission from the Tight M8 Binary: LP 349-25
We present radio observations of 8 ultracool dwarfs with a narrow spectral
type range (M8-M9.5) using the Very Large Array at 8.5 GHz. Only the tight M8
binary LP 349-25 was detected. LP 349-25 is the tenth ultracool dwarf system
detected in radio and its trigonometric parallax pi = 67.6 mas, recently
measured by Gatewood et al., makes it the furthest ultracool system detected by
the Very Large Array to date, and the most radio-luminous outside of obvious
flaring activity or variability. With a separation of only 1.8 AU, masses of
the components of LP 349-25 can be measured precisely without any theoretical
assumptions (Forveille et al.), allowing us to clarify their fully-convective
status and hence the kind of magnetic dynamo in these components which may play
an important role to explain our detection of radio emission from these
objects. This also makes LP 349-25 an excellent target for further studies with
better constraints on the correlations between X-ray, radio emission and
stellar parameters such as mass, age, temperature, and luminosity in ultracool
dwarfs.Comment: accepted by ApJ, referee's comments included, typo in equation 1
correcte
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