7,082 research outputs found
Mrk 1419 - a new distance determination
Water vapor megamasers from the center of active galaxies provide a powerful
tool to trace accretion disks at sub-parsec resolution and, through an entirely
geometrical method, measure direct distances to galaxies up to 200 Mpc. The
Megamaser Cosmology Project (MCP) is formed by a team of astronomers with the
aim of identifying new maser systems, and mapping their emission at high
angular resolution to determine their distance. Two types of observations are
necessary to measure a distance: single-dish monitoring to measure the
acceleration of gas in the disk, and sensitive VLBI imaging to measure the
angular size of the disk, measure the rotation curve, and model radial
displacement of the maser feature. The ultimate goal of the MCP is to make a
precise measurement of H0 by measuring such distances to at least 10 maser
galaxies in the Hubble flow. We present here the preliminary results from a new
maser system, Mrk 1419. Through a model of the rotation from the systemic
masers assuming a narrow ring, and combining these results with the
acceleration measurement from the Green Bank Telescope, we determine a distance
to Mrk 1419 of 81\pm10 Mpc. Given that the disk shows a significant warp that
may not be entirely traced by our current observations, more sensitive
observations and more sophisticated disk modeling will be essential to improve
our distance estimation to this galaxy.Comment: 5 pages, 3 figures, to appear in the proceedings of IAU Symposium 287
"Cosmic Masers- from OH to Ho", in Stellenbosch, S
Optimization of a bio-inspired sound localization sensor for high directional sensitivity
Miniaturization of sound localization sensors arrays is heavily constrained by the limited directional cues in intensity difference and phase difference available at the microscale. Micro-Electro Mechanical System (MEMS) sound localization sensors inspired by the auditory system of Ormia ochracea offer a potential solution to this problem by the apparent amplification of the available intensity and phase difference between the measurement points. An inherent limitation of these existing systems is that significant amplification of these cues is only available at or close to one of the resonant frequencies of the device, severely limiting it application as a directional microphone. Here we present the process of optimization of a sound localization sensor for the maximum amplification of directional cues across a narrow bandwidth, increasing the signal to noise ratio and the reading accuracy for sound localization measurements
Towards the development of a frequency agile MEMS acoustic sensor system
Designing acoustic sensors with adaptable frequency responses is of great interest in order to deal with diverse application requirements. A bio-inspired acoustic concept exploiting frequency agility using a MEMS microphone front-end is presented. Simulations and experimental results show adaptations of the microphone’s acoustic frequency response according to applied DC voltage potentials. Finally, the microphone is demonstrated as part of an integrated adaptive frequency sensor feedback loop. Such acoustic sensor systems can be used in many applications requiring high frequency discrimination and agile tuning
Active hearing mechanisms inspire adaptive amplification in an acoustic sensor system
Over many millions of years of evolution, nature has developed some of the most adaptable sensors and sensory systems possible, capable of sensing, conditioning and processing signals in a very power- and size-effective manner. By looking into biological sensors and systems as a source of inspiration, this paper presents the study of a bio-inspired concept of signal processing at the sensor level. By exploiting a feedback control mechanism between a front-end acoustic receiver and back-end neuronal based computation, a nonlinear amplification with hysteretic behavior is created. Moreover, the transient response of the front-end acoustic receiver can also be controlled and enhanced. A theoretical model is proposed and the concept is prototyped experimentally through an embedded system setup that can provide dynamic adaptations of a sensory system comprising a MEMS microphone placed in a closed-loop feedback system. It faithfully mimics the mosquito’s active hearing response as a function of the input sound intensity. This is an adaptive acoustic sensor system concept that can be exploit by sensor and system designers within acoustics and ultrasonic engineering fields
Bio-inspired active amplification in a MEMS microphone using feedback computation
Auditory signal processing relies on feedback mechanisms between mechanical and electrical systems that work together to enhance acoustic conditioning. In this paper a nonlinear amplification mechanism in the mosquito's auditory system is exploited as a model of inspiration. An acoustic system that provides active amplification of sound was developed using feedback computation integrated with a MEMS microphone to implement the concept. Experimental results generated by a purpose-built embedded system show signal amplification and hysteresis which replicate the response shown by the biological mosquito’s hearing system as a function of input sound intensity
A Flaring L5 Dwarf: The Nature of H\alpha Emission in Very Low Mass (Sub)Stellar Objects
Time series spectrophotometry of the L5 dwarf 2MASS 01443536-0716142 showed
strong H\alpha emission which declined by nearly 75% in four consecutive
exposures. The line was not detected in emission on a spectrum obtained eleven
months later. This behavior constrasts with that of 2MASSI J1315309-264951, an
L5 dwarf which has shown even stronger H\alpha emission on four separate
occasions. The observational database suggests that L dwarfs can be found in
such strong flares only occasionally, with a duty cycle of order 1%. In
contrast, the few, continuously-strong H\alpha emitters, including PC 0025+0447
and 2MASSI J1237392+652615, must either be (1) objects no older than 10-100
Myrs with continuously-active accretion and/or chromospheres, but which
apparently formed in isolation from known young stellar clusters and
associations, or (2) objects empowered by a different and unknown mechanism for
the H\alpha energy.Comment: 11 pages, 2 figures, Astronomical Journal in press -- Jan 2003 issu
Diffusion of Lithium Ions in Lithium-Argyrodite Solid-State Electrolytes from Equilibrium and Nonequilibrium Molecular Dynamics Simulations
The use of solid-state electrolytes to provide safer, next-generation
rechargeable batteries is becoming more feasible as new materials with greater
stability and higher ionic diffusion coefficients are designed. However,
accurate determination of diffusion coefficients in solids is problematic and
reliable calculations are highly sought-after. In this paper we compare
diffusion coefficients calculated using nonequilibrium and equilibrium ab
initio molecular dynamics simulations for highly diffusive solid-state
electrolytes for the first time, to demonstrate the accuracy that can be
obtained. Moreover, we show that ab initio nonequilibrium molecular dynamics
can be used to determine diffusion coefficients when the diffusion is too slow
for it to be feasible to obtain them using ab initio equilibrium simulations.
Thereby, using ab initio nonequilibrium molecular dynamics simulations we are
able to obtain accurate estimates of the diffusion coefficients of Li ions in
LiPSCl and LiPSCl, two promising electrolytes for
all-solid-state batteries. Furthermore, these calculations show that the
diffusion coefficient of lithium ions in LiPSCl is higher than many
other potential all-solid-state electrolytes, making it promising for future
technologies. The reasons for variation in conductivities determined using
computational and experimental methods are also discussed. It is demonstrated
that small degrees of disorder and vacancies can result in orders of magnitude
differences in diffusivities of Li ions in LiPSCl, and these factors
are likely to contribute to inconsistencies observed in experimentally reported
values. Notably, the introduction of Li-vacancies and disorder can enhance the
ionic conductivity of LiPSCl.Comment: 32 pages, 8 figures, 2 table
Four nearby L dwarfs
We present spectroscopic, photometric and astrometric observations of four
bright L dwarfs identified in the course of the 2MASS near-infrared survey. Our
spectroscopic data extend to wavelengths shortward of 5000\AA in the L0 dwarf
2MASSJ0746+2000 and the L4 dwarf 2MASSJ0036+1840, allowing the identification
of absorption bands due to MgH and CaOH. The atomic resonance lines Ca I
4227\AA and Na I 5890/5896\AA are extremely strong, with the latter having an
equivalent width of 240\AA in the L4 dwarf. By spectral type L5, the D lines
extend over \AA and absorb a substantial fraction of the flux emitted
in the V band, with a corresponding effect on the (V-I) broadband colour. The
KI resonance doublet at 7665/7699\AA increases in equivalent width from
spectral type M3 to M7, but decreases in strength from M7 to L0 before
broadening substantially at later types. These variations are likely driven by
dust formation in these cool atmospheres.Comment: to appear in AJ, January 2000; 27 pages, including 3 tables and 7
figures embedded in the tex
Evolution of directional hearing in moths via conversion of bat detection devices to asymmetric pressure gradient receivers
Small animals typically localize sound sources by means of complex internal connections and baffles that effectively increase time or intensity differences between the 2 ears. But some miniature acoustic species achieve directional hearing without such devices, indicating that other mechanisms have evolved. Using 3D laser vibrometry to measure tympanum deflection, we show that female lesser waxmoths (Achroia grisella) can orient toward the 100-kHz male song because each ear functions independently as an asymmetric pressure gradient receiver that responds sharply to high-frequency sound arriving from an azimuth angle 30° contralateral to the animal's midline. We found that females presented with a song stimulus while running on a locomotion compensation sphere follow a trajectory 20° - 40° to the left or right of the stimulus heading but not directly toward it, movement consistent with the tympanum deflections and suggestive of a monaural mechanism of auditory tracking. Moreover, females losing their track typically regain it by auditory scanning – sudden, wide deviations in their heading – and females initially facing away from the stimulus quickly change their general heading toward it, orientation indicating superior ability to resolve the front-rear ambiguity in source location. X-ray CT scans of the moths did not reveal any internal coupling between the 2 ears, confirming for the first time that an acoustic insect can localize a sound source based solely on the distinct features of each ear
Extreme call amplitude from near-field acoustic wave coupling in the stridulating water insect Micronecta scholtzi (Micronectinae)
Underwater acoustic transducers, particularly at low frequencies, are beset by problems of scale and inefficiency due to the large wavelengths of sound in water. In insect mating calls, a high call volume is usually desirable, increasing the range of signal transmission and providing a form of advertisement of the signaller's quality to a potential mate; however, the strength of the call is constrained by body size and by the need to avoid predators who may be listening in. Male crickets and water boatmen avoid some of the limitations of body size by exploiting resonant structures, which produce sharply tuned species specific songs, but call frequency and volume remain linked to body size. Recently, the water boatman Micronecta scholtzi was found to circumvent this rule, producing a louder mating call than that of similar, but much larger, Corixa. The resonant structure in Corixidae and Micronectinae is believed to be the trapped air reserves around the insect as it dives, driven by a stridulatory apparatus. However, the method by which energy is transferred from the striated area to the bubble is unknown. Here, we present modelling of a system of near-field coupling of acoustic sources to bubbles showing an exponential increase in sound power gain with decreasing distance that provides a simple solution to the stimulus of the air bubbles in Corixidae and Micronectinae and explains the discrepancy of M. scholtzi's extreme call volume. The findings suggest a possible route to engineered systems using near-field coupling to overcome size constraints in low-frequency (less than 500 Hz) underwater transducers, where the input efficiency of a piezoelectric device can be coupled through the hydrodynamic field to the high radiative efficiency of a near-ideal monopole emitter
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