137,991 research outputs found
Newly discovered brown dwarfs not seen in microlensing time scale frequency distribution?
The 2-Micron All Sky Survey (2MASS) (Skrutskie et al. 1997) and the DEep Near
Infrared Survey of the southern sky (DENIS) (Epchtein et al. 1997) have
revealed a heretofore unknown population of free brown dwarfs that has extended
the local mass function down to as small as 0.01M_sun (Reid et al. 1999). If
this local proportion of brown dwarfs extends throughout the Galaxy---in
particular in the Galactic bulge---one expects an increase in the predicted
fraction of short time scale microlensing events in directions toward the
Galactic bulge. Zhao et al.(1996) have indicated that a mass function with
30-60% of the lens mass in brown dwarfs is not consistent with empirical
microlensing data. Here we show that even the much lower mass fraction (~ 10%)
of brown dwarfs inferred from the new discoveries appears inconsistent with the
data. The added brown dwarfs do indeed increase the expected number of short
time scale events, but they appear to drive the peak in the time scale
frequency distribution to time scales smaller than that observed, and do not
otherwise match the observed distribution. A reasonably good match to the
empirical data (Alcock et al. 1996) is obtained by increasing the fraction of
stars in the range 0.08<m<0.7M_sun considerably above that deduced from several
star counts. However, all inferences from microlensing about the appropriate
stellar mass function must be qualified by the meagerness of the microlensing
data and the uncertainties in the Galactic model.Comment: 5 pages, 1 figure. PS file using aas2pp4.sty. To appear in ApJ
Letter
Probability of Detecting a Planetary Companion during a Microlensing Event
The probability of detecting a planetary companion of a lensing star during a
microlensing event toward the Galactic center, averaged over all relevant event
and galactic parameters, when the planet-star mass ratio has a
maximum exceeding 10% at an orbit semimajor axis near 1.5 AU for a uniform
distribution of impact parameters. The maximum probability is raised to more
than 20% for a distribution of source-lens impact parameters that is determined
by the efficiency of event detection. The averaging procedures are carefully
defined, and they determinine the dependence of the detection probabilities on
several properties of the Galaxy. The probabilities scale approximately as
. A planet is assumed detectable if the perturbation of the single
lens light curve exceeds for at least 20 consecutive photometric
points sometime during the event. Two meter telescopes with 60 second
integrations in I-band with high time resolution photometry throughout the
duration of an ongoing event are assumed. The probabilities are derived as a
function of , where they remain significant for AU. Dependence of
the detection probabilities on the lens mass function, luminosity function of
the source stars as modified by extinction, distribution of source-lens impact
parameters, and the line of sight to the source are also determined, and the
probabilities are averaged over the distribution of the projected planet
position, the lens mass function, the distribution of impact parameters, the
lens and source distances as weighted by their distributions along the line of
sight and over the -band apparent luminosity function of the sources. The
extraction of the probabilility as a function of for a particular from
empirical data is indicated.Comment: 32 pages, 20 figures, In Press, ApJ, Latex format with aas2pp4 forma
Fitting functions for dark matter density profiles
We present a unified parameterization of the fitting functions suitable for
density profiles of dark matter haloes or elliptical galaxies. A notable
feature is that the classical Einasto profile appears naturally as the
continuous limiting case of the cored subfamily amongst the double power-law
profiles of Zhao (1996). Based on this, we also argue that there is basically
no qualitative difference between halo models well-fitted by the Einasto
profile and the standard NFW model. This may even be the case quantitatively
unless the resolutions of simulations and the precisions of fittings are
sufficiently high to make meaningful distinction possible.Comment: 13 pages (6 pages main text + 5 pages appendices + 2 pages full
tables) including 5 figures and 7 tables. submitted to MNRA
A simple approach to determination of stiffness characteristics of unidirectional composites
The test data on a carbon fiber reinforced epoxy composite material were processed using a simplified method. The results show that this simplified approach is feasible, practical, and economical. Basic details of the method are given
Anomalous Hall effect in L10-MnAl films with controllable orbital two-channel Kondo effect
The anomalous Hall effect (AHE) in strongly disordered magnetic systems has
been buried in persistent confusion despite its long history. We report the AHE
in perpendicularly magnetized L10-MnAl epitaxial films with variable orbital
two-channel Kondo (2CK) effect arising from the strong coupling of conduction
electrons and the structural disorders of two-level systems. The AHE is
observed to excellently scale with pAH/f=a0pxx0+bpxx2 at high temperatures
where phonon scattering prevails. In contrast, significant deviation occurs at
low temperatures where the orbital 2CK effect becomes important, suggesting a
negative AHE contribution. The deviation of the scaling agrees with the orbital
2CK effect in the breakdown temperatures and deviation magnitudes
Investigation of transition between spark ignition and controlled auto-ignition combustion in a V6 direct-injection engine with cam profile switching
Controlled auto-ignition (CAI) combustion, also known as Homogeneous Charge Compression Ignition (HCCI) can be achieved by trapping residuals with early exhaust valve closure in a direct fuel injection in-cylinder four-stroke gasoline engines (through the employment of low-lift cam profiles). Due to the operating region being limited to low and mid-load operation for CAI combustion with a low-lift cam profile, it is important to be able to operate SI combustion at high-load with a normal cam profile. A 3.0L prototype engine was modified to achieve CAI combustion, using a Cam Profile Switching mechanism which has the capability to switch between high and low-lift cam-profiles. A strategy was used where a high-profile could be used for SI combustion and a low-lift profile was used for CAI combustion. Initial analysis showed that for transitioning from SI to CAI combustion, misfire occurred on the first CAI transitional cycle. Subsequent experiments showed that the throttle opening position and switching time could be controlled avoiding misfire. Further work investigated transitioning at different loads and from CAI to SI combustion
Topological Weyl and Node-Line Semimetals in Ferromagnetic Vanadium-Phosphorous-Oxide -VOPO Compound
We propose that the topological semimetal features can co-exist with
ferromagnetic ground state in vanadium-phosphorous-oxide -VOPO
compound from first-principles calculations. In this magnetic system with
inversion symmetry, the direction of magnetization is able to manipulate the
symmetric protected band structures from a node-line type to a Weyl one in the
presence of spin-orbital-coupling. The node-line semimetal phase is protected
by the mirror symmetry with the reflection-invariant plane perpendicular to
magnetic order. Within mirror symmetry breaking due to the magnetization along
other directions, the gapless node-line loop will degenerate to only one pair
of Weyl points protected by the rotational symmetry along the magnetic axis,
which are largely separated in momentum space. Such Weyl semimetal phase
provides a nice candidate with the minimum number of Weyl points in a condensed
matter system. The results of surface band calculations confirm the non-trivial
topology of this proposed compound. This findings provide a realistic candidate
for the investigation of topological semimetals with time-reversal symmetry
breaking, particularly towards the realization of quantum anomalous Hall effect
in Weyl semimetals.Comment: 5 pages, 4 figure
Mixed Power Control Strategies for Cognitive Radio Networks under SINR and Interference Temperature Constraints
Without consideration of the minimum signal-to-interference-plus-noise ratio (SINR) and frequent information exchange, traditional power control algorithms can not always satisfy SINR requirements of secondary users (SUs) and primary users (PUs) in cognitive radio networks. In this paper, a distributed power control problem for maximizing total throughput of SUs is studied subject to the SINR constraints of SUs and the interference constraints of PUs. To reduce message exchange among SUs, two improved methods are obtained by dual decomposition approaches. For a large-scale network, an average interference constraint is presented at the cost of performance degradation. For a small-scale network, a weighted interference constraint with fairness consideration is proposed to obtain good performance. Simulation results demonstrate that the proposed algorithm is superior to ADCPC and TPCG algorithms
- …