831 research outputs found
Track train dynamics analysis and test program: Methodology development for the derailment safety analysis of six-axle locomotives
The operational safety of six axle locomotives is analyzed. A locomotive model with corresponding data on suspension characteristics, a method of track defect characterization, and a method of characterizing operational safety are used. A user oriented software package was developed as part of the methodology and was used to study the effect (on operational safety) of various locomotive parameters and operational conditions such as speed, tractive effort, and track curvature. The operational safety of three different locomotive designs was investigated
Multi-frequency Study of the LMC Supernova Remnant (SNR) B0513-692 and New SNR Candidate J051327-6911
We present a new multi-wavelength study of supernova remnant (SNR) B0513-692
in the Large Magellanic Cloud (LMC). The remnant also has a strong, superposed,
essentially unresolved, but unrelated radio source at its north-western edge,
J051324-691049. This is identified as a likely compact HII region based on
related optical imaging and spectroscopy. We use the Australia Telescope
Compact Array (ATCA) at 4790 and 8640 MHz to determine the large scale
morphology, spectral index and polarization characteristics of B0513-692 for
the first time. We detect a strongly polarized region (49%) in the remnant's
southern edge. Interestingly we also detect a small (~40 arcsec) moderately
bright, but distinct optical, circular shell in our Halpha imagery which is
adjacent to the compact HII region and just within the borders of the NE edge
of B0513-692. We suggest this is a separate new SNR candidate based on its
apparently distinct character in terms of optical morphology in 3 imaged
emission lines and indicative SNR optical spectroscopy (including enhanced
optical [SII] emission relative to Halpha).Comment: 12 page
The ionization mechanism of NGC 185: how to fake a Seyfert galaxy?
NGC 185 is a dwarf spheroidal satellite of the Andromeda galaxy. From
mid-1990s onwards it was revealed that dwarf spheroidals often display a varied
and in some cases complex star formation history. In an optical survey of
bright nearby galaxies, NGC 185 was classified as a Seyfert galaxy based on its
emission line ratios. However, although the emission lines in this object
formally place it in the category of Seyferts, it is probable that this galaxy
does not contain a genuine active nucleus. NGC 185 was not detected in radio
surveys either in 6 or 20 cm, or X-ray observations, which means that the
Seyfert-like line ratios may be produced by stellar processes. In this work, we
try to identify the possible ionization mechanisms for this galaxy. We
discussed the possibility of the line emissions being produced by planetary
nebulae (PNe), using deep spectroscopy observations obtained with GMOS-N, at
Gemini. Although the fluxes of the PNe are high enough to explain the
integrated spectrum, the line ratios are very far from the values for the
Seyfert classification. We then proposed that a mixture of supernova remnants
and PNe could be the source of the ionization, and we show that a composition
of these two objects do mimic Seyfert-like line ratios. We used chemical
evolution models to predict the supernova rates and to support the idea that
these supernova remnants should be present in the galaxy.Comment: 9 pages, 7 figures, accepted for publication in MNRA
Metal Abundances in the Magellanic Stream
We report on the first metallicity determination for gas in the Magellanic
Stream, using archival HST GHRS data for the background targets Fairall 9, III
Zw 2, and NGC 7469. For Fairall 9, using two subsequent HST revisits and new
Parkes Multibeam Narrowband observations, we have unequivocally detected the
MSI HI component of the Stream (near its head) in SII1250,1253 yielding a
metallicity of [SII/H]=-0.55+/-0.06(r)+/-0.2(s), consistent with either an SMC
or LMC origin and with the earlier upper limit set by Lu et al. (1994). We also
detect the saturated SiII1260 line, but set only a lower limit of
[SiII/H]>-1.5. We present serendipitous detections of the Stream, seen in
MgII2796,2803 absorption with column densities of (0.5-1)x10^13 cm^-2 toward
the Seyfert galaxies III Zw 2 and NGC 7469. These latter sightlines probe gas
near the tip of the Stream (80 deg down-Stream of Fairall 9). For III Zw 2, the
lack of an accurate HI column density and the uncertain MgIII ionization
correction limits the degree to which we can constrain [Mg/H]; a lower limit of
[MgII/HI]>-1.3 was found. For NGC 7469, an accurate HI column density
determination exists, but the extant FOS spectrum limits the quality of the
MgII column density determination, and we conclude that [MgII/HI]>-1.5.
Ionization corrections associated with MgIII and HII suggest that the
corresponding [Mg/H] may range lower by 0.3-1.0 dex. However, an upward
revision of 0.5-1.0 dex would be expected under the assumption that the Stream
exhibits a dust depletion pattern similar to that seen in the Magellanic
Clouds. Remaining uncertainties do not allow us to differentiate between an LMC
versus SMC origin to the Stream gas.Comment: 30 pages, 8 figures, LaTeX (aaspp4), also available at
http://casa.colorado.edu/~bgibson/publications.html, accepted for publication
in The Astronomical Journa
Evidence for Extremely High Dust Polarization Efficiency in NGC 3184
Recent studies have found the Type II-plateau supernova (SN) 1999gi to be
highly polarized (p_max = 5.8%, where p_max is the highest degree of
polarization measured in the optical bandpass; Leonard & Filippenko 2001) and
minimally reddened (E[B-V] = 0.21 +/- 0.09 mag; Leonard et al. 2002). From
multiple lines of evidence, including the convincing fit of a ``Serkowski''
interstellar polarization (ISP) curve to the continuum polarization shape, we
conclude that the bulk of the observed polarization is likely due to dust along
the line of sight (l-o-s), and is not intrinsic to SN 1999gi. We present new
spectropolarimetric observations of four distant Galactic stars close to the
l-o-s to SN 1999gi (two are within 0.02 degrees), and find that all are null to
within 0.2%, effectively eliminating Galactic dust as the cause of the high
polarization. The high ISP coupled with the low reddening implies an
extraordinarily high polarization efficiency for the dust along this l-o-s in
NGC 3184: ISP / E(B-V) = 31^{+22}_{-9} % mag^{-1}. This is inconsistent with
the empirical Galactic limit (ISP / E[B-V] < 9% mag^{-1}), and represents the
highest polarization efficiency yet confirmed for a single sight line in either
the Milky Way or an external galaxy.Comment: 27 pages, accepted for publication by the Astronomical Journa
Why royalties ? Evidence from French distribution networks
Working paper GATE 2011-02This empirical note deals with the contractual design of relationships in distribution networks. In the framework of agency theory, I study the royalty rate as an incentive device for the upstream firm in maintaining brand-name value, using recent French data to estimate probit models. The results are consistent with the analytical framework
Alpha-band rhythms in visual task performance: phase-locking by rhythmic sensory stimulation
Oscillations are an important aspect of neuronal activity. Interestingly, oscillatory patterns are also observed in behaviour, such as in visual performance measures after the presentation of a brief sensory event in the visual or another modality. These oscillations in visual performance cycle at the typical frequencies of brain rhythms, suggesting that perception may be closely linked to brain oscillations. We here investigated this link for a prominent rhythm of the visual system (the alpha-rhythm, 8-12 Hz) by applying rhythmic visual stimulation at alpha-frequency (10.6 Hz), known to lead to a resonance response in visual areas, and testing its effects on subsequent visual target discrimination. Our data show that rhythmic visual stimulation at 10.6 Hz: 1) has specific behavioral consequences, relative to stimulation at control frequencies (3.9 Hz, 7.1 Hz, 14.2 Hz), and 2) leads to alpha-band oscillations in visual performance measures, that 3) correlate in precise frequency across individuals with resting alpha-rhythms recorded over parieto-occipital areas. The most parsimonious explanation for these three findings is entrainment (phase-locking) of ongoing perceptually relevant alpha-band brain oscillations by rhythmic sensory events. These findings are in line with occipital alpha-oscillations underlying periodicity in visual performance, and suggest that rhythmic stimulation at frequencies of intrinsic brain-rhythms can be used to reveal influences of these rhythms on task performance to study their functional roles
On the Exchange of Kinetic and Magnetic Energy Between Clouds and the Interstellar Medium
We investigate, through 2D MHD numerical simulations, the interaction of a
uniform magnetic field oblique to a moving interstellar cloud. In particular we
explore the transformation of cloud kinetic energy into magnetic energy as a
result of field line stretching. Some previous simulations have emphasized the
possible dynamical importance of a ``magnetic shield'' formed around clouds
when the magnetic field is perpendicular to the cloud motion (Jones et al.
1996, Miniati et al. 1998). It was not clear, however, how dependent those
findings were to the assumed field configuration and cloud properties. To
expand our understanding of this effect, we examine several new cases by varing
the magnetic field orientation angle with respect to the cloud motion (\theta),
the cloud-background density contrast, and the cloud Mach number.
We show that in 2D and with \theta large enough, the magnetic field tension
can become dominant in the dynamics of the motion of high density contrast, low
Mach number clouds. In such cases a significant fraction of cloud kinetic
energy can be transformed into magnetic energy with the magnetic pressure at
the cloud nose exceeding the ram pressure of the impinging flow. We derive a
characteristic timescale for this process of energy ``conversion''. We find
also that unless the cloud motion is highly aligned to the magnetic field,
reconnection through tearing mode instabilities in the cloud wake limit the
formation of a strong flux rope feature following the cloud. Finally we attempt
to interpret some observational properties of the magnetic field in view of our
results.Comment: 24 pages in aaspp4 Latex and 7 figures. Accepted for publication in
The Astrophysical Journa
Comparing leakage currents and dark count rates in Geiger-mode avalanche photodiodes
This letter presents an experimental study of dark count rates and leakage current in Geiger-mode avalanche photodiodes (GM APD). Experimental results from circular diodes over a range of areas (20-500 mum diam), exhibit leakage current levels orders of magnitude higher than anticipated from dark count rates. Measurements of the area and peripheral components of the leakage current indicate that the majority of the current in reverse bias does not enter the high-field region of the diode, and therefore, does not contribute to the dark count rate. Extraction of the area leakage current term from large-area devices (500 mum) corresponds well with the measured dark count rates on smaller devices (20 mum). Finally, the work indicates how dark count measurements represent 10(-18) A levels of leakage current detection in GM APDs. (C) 2002 American Institute of Physics. (DOI: 10.1063/1.1483119
A Search for Intrinsic Polarization in O Stars with Variable Winds
New observations of 9 of the brightest northern O stars have been made with
the Breger polarimeter on the 0.9~m telescope at McDonald Observatory and the
AnyPol polarimeter on the 0.4~m telescope at Limber Observatory, using the
Johnson-Cousins UBVRI broadband filter system. Comparison with earlier
measurements shows no clearly defined long-term polarization variability. For
all 9 stars the wavelength dependence of the degree of polarization in the
optical range can be fit by a normal interstellar polarization law. The
polarization position angles are practically constant with wavelength and are
consistent with those of neighboring stars. Thus the simplest conclusion is
that the polarization of all the program stars is primarily interstellar.
The O stars chosen for this study are generally known from ultraviolet and
optical spectroscopy to have substantial mass loss rates and variable winds, as
well as occasional circumstellar emission. Their lack of intrinsic polarization
in comparison with the similar Be stars may be explained by the dominance of
radiation as a wind driving force due to higher luminosity, which results in
lower density and less rotational flattening in the electron scattering inner
envelopes where the polarization is produced. However, time series of
polarization measurements taken simultaneously with H-alpha and UV spectroscopy
during several coordinated multiwavelength campaigns suggest two cases of
possible small-amplitude, periodic short-term polarization variability, and
therefore intrinsic polarization, which may be correlated with the more widely
recognized spectroscopic variations.Comment: LaTeX2e, 22 pages including 11 tables; 12 separate gif figures; uses
aastex.cls preprint package; accepted by The Astronomical Journa
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