43,865 research outputs found
Confirmation of simultaneous p and g mode excitation in HD 8801 and Gamma Peg from time-resolved multicolour photometry of six candidate "hybrid" pulsators
We carried out a multi-colour time-series photometric study of six stars
claimed as "hybrid" p and g mode pulsators in the literature. Gamma Peg was
confirmed to show short-period oscillations of the Beta Cep type and
simultaneous long-period pulsations typical of Slowly Pulsating B (SPB) stars.
From the measured amplitude ratios in the Stromgren uvy passbands, the
stronger of the two short period pulsation modes was identified as radial; the
second is l=1. Three of the four SPB-type modes are most likely l=1 or 2.
Comparison with theoretical model calculations suggests that Gamma Peg is
either an 8.5 solar mass radial fundamental mode pulsator or a 9.6 solar mass
first radial overtone pulsator. HD 8801 was corroborated as a "hybrid" Delta
Sct Gamma Dor star; four pulsation modes of the Gamma Dor type were detected,
and two modes of the Delta Sct type were confirmed. Two pulsational signals
between the frequency domains of these two known classes of variables were
confirmed and another was newly detected. These are either previously unknown
types of pulsation, or do not originate from HD 8801. The O-type star HD 13745
showed small-amplitude slow variability on a time scale of 3.2 days. This
object may be related to the suspected new type of supergiant SPB stars, but a
rotational origin of its light variations cannot be ruled out at this point. 53
Psc is an SPB star for which two pulsation frequencies were determined and
identified with low spherical degree. The behaviour of 53 Ari and Iota Her is
consistent with non-variability during our observations, and we could not
confirm light variations of the comparison star 34 Psc previously suspected.
The use of signal-to-noise criteria in the analysis of data sets with strong
aliasing is critically discussed.Comment: 14 pages, 10 figures, accepted by MNRA
The 2MASS Wide-Field T Dwarf Search. IV Unting out T dwarfs with Methane Imaging
We present first results from a major program of methane filter photometry
for low-mass stars and brown dwarfs. The definition of a new methane filter
photometric system is described. A recipe is provided for the differential
calibration of methane imaging data using existing 2MASS photometry. We show
that these filters are effective in discriminating T dwarfs from other types of
stars, and demonstrate this with Anglo-Australian Telescope observations using
the IRIS2 imager. Methane imaging data and proper motions are presented for ten
T dwarfs identified as part of the 2MASS "Wide Field T Dwarf Search" -- seven
of them initially identified as T dwarfs using methane imaging.
We also present near-infrared moderate resolution spectra for five T dwarfs,
newly discovered by this technique. Spectral types obtained from these spectra
are compared to those derived from both our methane filter observations, and
spectral types derived by other observers. Finally, we suggest a range of
future programs to which these filters are clearly well suited: the winnowing
of T dwarf and Y dwarf candidate objects coming from the next generation of
near-infrared sky surveys; the robust detection of candidate planetary-mass
brown dwarfs in clusters; the detection of T dwarf companions to known L and T
dwarfs via deep methane imaging; and the search for rotationally-modulated
time-variable surface features on cool brown dwarfs.Comment: 20 pages. To appear in The Astronomical Journal, Nov. 200
Coherent receiver employing nonlinear coherence detection for carrier tracking
The concept of nonlinear coherence employed in carrier tracking to improve telecommunications efficiency is disclosed. A generic tracking loop for a coherent receiver is shown having seven principle feedback signals which may be selectively added and applied to a voltage controlled oscillator to produce a reference signal that is phase coherent with a received carrier. An eighth feedback signal whose nonrandom components are coherent with the phase detected and filtered carrier may also be added to exploit the sideband power of the received signal. A ninth feedback signal whose nonrandom components are also coherent with the quadrature phase detected and filtered carrier could be additionally or alternatively included in the composite feedback signal to the voltage controlled oscillator
Signal tracking beyond the time resolution of an atomic sensor by Kalman filtering
We study causal waveform estimation (tracking) of time-varying signals in a
paradigmatic atomic sensor, an alkali vapor monitored by Faraday rotation
probing. We use Kalman filtering, which optimally tracks known linear Gaussian
stochastic processes, to estimate stochastic input signals that we generate by
optical pumping. Comparing the known input to the estimates, we confirm the
accuracy of the atomic statistical model and the reliability of the Kalman
filter, allowing recovery of waveform details far briefer than the sensor's
intrinsic time resolution. With proper filter choice, we obtain similar
benefits when tracking partially-known and non-Gaussian signal processes, as
are found in most practical sensing applications. The method evades the
trade-off between sensitivity and time resolution in coherent sensing.Comment: 15 pages, 4 figure
Signal tracking beyond the time resolution of an atomic sensor by Kalman filtering
We study causal waveform estimation (tracking) of time-varying signals in a
paradigmatic atomic sensor, an alkali vapor monitored by Faraday rotation
probing. We use Kalman filtering, which optimally tracks known linear Gaussian
stochastic processes, to estimate stochastic input signals that we generate by
optical pumping. Comparing the known input to the estimates, we confirm the
accuracy of the atomic statistical model and the reliability of the Kalman
filter, allowing recovery of waveform details far briefer than the sensor's
intrinsic time resolution. With proper filter choice, we obtain similar
benefits when tracking partially-known and non-Gaussian signal processes, as
are found in most practical sensing applications. The method evades the
trade-off between sensitivity and time resolution in coherent sensing.Comment: 15 pages, 4 figure
Data compression techniques applied to high resolution high frame rate video technology
An investigation is presented of video data compression applied to microgravity space experiments using High Resolution High Frame Rate Video Technology (HHVT). An extensive survey of methods of video data compression, described in the open literature, was conducted. The survey examines compression methods employing digital computing. The results of the survey are presented. They include a description of each method and assessment of image degradation and video data parameters. An assessment is made of present and near term future technology for implementation of video data compression in high speed imaging system. Results of the assessment are discussed and summarized. The results of a study of a baseline HHVT video system, and approaches for implementation of video data compression, are presented. Case studies of three microgravity experiments are presented and specific compression techniques and implementations are recommended
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