800 research outputs found
Exhaust emission reduction for intermittent combustion aircraft engines
Three concepts for optimizing the performance, increasing the fuel economy, and reducing exhaust emission of the piston aircraft engine were investigated. High energy-multiple spark discharge and spark plug tip penetration, ultrasonic fuel vaporization, and variable valve timing were evaluated individually. Ultrasonic fuel vaporization did not demonstrate sufficient improvement in distribution to offset the performance loss caused by the additional manifold restriction. High energy ignition and revised spark plug tip location provided no change in performance or emissions. Variable valve timing provided some performance benefit; however, even greater performance improvement was obtained through induction system tuning which could be accomplished with far less complexity
Polarimetric Properties of the Crab Pulsar between 1.4 and 8.4 GHz
New polarimetric observations of the Crab pulsar at frequencies between 1.4
and 8.4 GHz are presented. Additional pulse components discovered in earlier
observations (Moffett & Hankins 1996, astro-ph/9604163) are found to have high
levels of linear polarization, even at 8.4 GHz. No abrupt sweeps in position
angle are found within pulse components; however, the position angle and
rotational phase of the interpulse do change dramatically between 1.4 and 4.9
GHz. The multi-frequency profile morphology and polarization properties
indicate a non-standard origin of the emission. Several emission geometries are
discussed, but the one favored locates sites of emission both near the pulsar
surface and in the outer magnetosphere.Comment: 20 pages, 7 postscript figures, uses aaspp4 Latex style. To appear in
Volume 522 of The Astrophysical Journa
PYY (3-36) protects against high fat feeding induced changes of pancreatic islet and intestinal hormone content and morphometry
The Shape and Scale of Galactic Rotation from Cepheid Kinematics
A catalog of Cepheid variables is used to probe the kinematics of the
Galactic disk. Radial velocities are measured for eight distant Cepheids toward
l = 300; these new Cepheids provide a particularly good constraint on the
distance to the Galactic center, R_0. We model the disk with both an
axisymmetric rotation curve and one with a weak elliptical component, and find
evidence for an ellipticity of 0.043 +/- 0.016 near the Sun. Using these
models, we derive R_0 = 7.66 +/- 0.32 kpc and v_circ = 237 +/- 12 km/s. The
distance to the Galactic center agrees well with recent determinations from the
distribution of RR Lyrae variables, and disfavors most models with large
ellipticities at the solar orbit.Comment: 36 pages, LaTeX, 10 figure
Calibrating the projection factor for Galactic Cepheids
The projection factor (p), which converts the radial velocity to pulsational
velocity, is an important parameter in the Baade-Wesselink (BW) type analysis
and distance scale work. The p-factor is either adopted as a constant or
linearly depending on the logarithmic of pulsating periods. The aim of this
work is to calibrate the p-factor if a Cepheid has both the BW distance and an
independent distance measurement, and examine the p-factor for delta Cephei --
the prototype of classical Cepheids. We calibrated the p-factor for several
Galactic Cepheids that have both the latest BW distances and independent
distances either from Hipparcos parallaxes or main-sequence fitting distances
to Cepheid-hosted stellar clusters. Based on 25 Cepheids, the calibrated
p-factor relation is consistent with latest p-factor relation in literature.
The calibrated p-factor relation also indicates that this relation may not be
linear and may exhibit an intrinsic scatter. We also examined the discrepancy
of empirical p-factors for delta Cephei, and found that the reasons for this
discrepancy include the disagreement of angular diameters, the treatment of
radial velocity data, and the phase interval adopted during the fitting
procedure. Finally, we investigated the impact of the input p-factor in two BW
methodologies for delta Cephei, and found that different p-factors can be
adopted in these BW methodologies and yet result in the same angular diameters.Comment: 6 pages, 6 figures and 2 tables. A&A accepte
The fading of Cassiopeia A, and improved models for the absolute spectrum of primary radio calibration sources
Based on five years of observations with the 40-foot telescope at Green Bank
Observatory (GBO), Reichart & Stephens (2000) found that the radio source
Cassiopeia A had either faded more slowly between the mid-1970s and late 1990s
than Baars et al. (1977) had found it to be fading between the late 1940s and
mid-1970s, or that it had rebrightened and then resumed fading sometime between
the mid-1970s and mid-1990s, in L band (1.4 GHz). Here, we present 15
additional years of observations of Cas A and Cyg A with the 40-foot in L band,
and three and a half additional years of observations of Cas A, Cyg A, Tau A,
and Vir A with GBO's recently refurbished 20-meter telescope in L and X (9 GHz)
bands. We also present a more sophisticated analysis of the 40-foot data, and a
reanalysis of the Baars et al. (1977) data, which reveals small, but
non-negligible differences. We find that overall, between the late 1950s and
late 2010s, Cas A faded at an average rate of %/yr in L band,
consistent with Reichart & Stephens (2000). However, we also find, at the
6.3 credible level, that it did not fade at a constant rate. Rather,
Cas A faded at a faster rate through at least the late 1960s, rebrightened (or
at least faded at a much slower rate), and then resumed fading at a similarly
fast rate by, at most, the late 1990s. Given these differences from the
original Baars et al. (1977) analysis, and given the importance of their fitted
spectral and temporal models for flux-density calibration in radio astronomy,
we update and improve on these models for all four of these radio sources. In
doing so, we additionally find that Tau A is fading at a rate of
%/yr in L band.Comment: 17 pages, 12 figures, accepted to MNRA
The SAMI Galaxy Survey: Gas Streaming and Dynamical M/L in Rotationally Supported Systems
Line-of-sight velocities of gas and stars can constrain dark matter (DM)
within rotationally supported galaxies if they trace circular orbits
extensively. Photometric asymmetries may signify non-circular motions,
requiring spectra with dense spatial coverage. Our integral-field spectroscopy
of 178 galaxies spanned the mass range of the SAMI Galaxy Survey. We derived
circular speed curves (CSCs) of gas and stars from non-parametric Diskfit fits
out to . For 12/14 with measured H I profiles, ionized gas and H I
maximum velocities agreed. We fitted mass-follows-light models to 163 galaxies
by approximating the radial starlight profile as nested, very flattened mass
homeoids viewed as a S\'ersic form. Fitting broad-band SEDs to SDSS images gave
median stellar mass/light 1.7 assuming a Kroupa IMF vs. 2.6 dynamically.
Two-thirds of the dynamical mass/light measures were consistent with
star+remnant IMFs. One-fifth required upscaled starlight to fit, hence
comparable mass of unobserved baryons and/or DM distributed similarly across
the SAMI aperture that came to dominate motions as the starlight CSC declined
rapidly. The rest had mass distributed differently from starlight. Subtracting
fits of S\'ersic profiles to 13 VIKING Z-band images revealed residual weak
bars. Near the bar PA, we assessed m = 2 streaming velocities, and found
deviations usually <30 km/s from the CSC; three showed no deviation. Thus,
asymmetries rarely influenced our CSCs despite co-located shock-indicating,
emission-line flux ratios in more than 2/3.Comment: 21 pages, 15 figures. Accepted to MNRA
Pulsed electromagnetic energy treatment offers no clinical benefit in reducing the pain of knee osteoarthritis: a systematic review
Background
The rehabilitation of knee osteoarthritis often includes electrotherapeutic modalities as well as advice and exercise. One commonly used modality is pulsed electromagnetic field therapy (PEMF). PEMF uses electro magnetically generated fields to promote tissue repair and healing rates. Its equivocal benefit over placebo treatment has been previously suggested however recently a number of randomised controlled trials have been published that have allowed a systematic review to be conducted.
Methods
A systematic review of the literature from 1966 to 2005 was undertaken. Relevant computerised bibliographic databases were searched and papers reviewed independently by two reviewers for quality using validated criteria for assessment. The key outcomes of pain and functional disability were analysed with weighted and standardised mean differences being calculated.
Results
Five randomised controlled trials comparing PEMF with placebo were identified. The weighted mean differences of the five papers for improvement in pain and function, were small and their 95% confidence intervals included the null.
Conclusion
This systematic review provides further evidence that PEMF has little value in the management of knee osteoarthritis. There appears to be clear evidence for the recommendation that PEMF does not significantly reduce the pain of knee osteoarthritis
The extra-galactic Cepheid distance scale from LMC and Galactic period-luminosity relations
In this paper, we recalibrate the Cepheid distance to some nearby galaxies observed by the HST Key Project and the Sandage-Tammann-Saha group. We use much of the Key Project methodology in our analysis but apply new techniques, based on Fourier methods to estimate the mean of a sparsely sampled Cepheid light curve, to published extra-galactic Cepheid data. We also apply different calibrating PL relations to estimate Cepheid distances, and investigate the sensitivity of the distance moduli to the adopted calibrating PL relation. We re-determine the OGLELMC PL relations using a more conservative approach and also study the effect of using Galactic PL relations on the distance scale. For the Key Project galaxies after accounting for charge transfer effects, we find good agreement with an average discrepancy of -0.002 and 0.075 mag when using the LMC and Galaxy, respectively, as a calibrating PL relation. For NGC 4258 which has a geometric distance of 29.28 mag, we find a distance modulus of 29. 44 ± 0.06(random) mag, after correcting for metallicity. In addition we have calculated the Cepheid distance to 8 galaxies observed by the Sandage-Tammann-Saha group and find shorter distance moduli by -0.178 mag (mainly due to the use of different LMC PL relations) and -0.108 mag on average again when using the LMC and Galaxy, respectively, as a calibrating PL relation. However care must be taken to extrapolate these changed distances to changes in the resulting values of the Hubble constant because STS also use distances to NGC 3368 and 4414 and because STS calibration of SN la is often decoupled from the distance to the host galaxy through their use of differential extinction arguments. We also calculate the distance to all these galaxies using PL relations at maximum light and find very good agreement with mean light PL distances. However, after correcting for metallicity effects, the difference between the distance moduli obtained using the two sets of calibrating PL relations becomes negligible. This suggests that Cepheids in the LMC and Galaxy do follow different PL relations and constrains the sign for the coefficient of the metallicity correction, γ, to be negative, at least at the median period log(P) approximately equals 1.4, of the target galaxies
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