2,541 research outputs found
The physical determinants of the thickness of lamellar polymer crystals
Based upon kinetic Monte Carlo simulations of crystallization in a simple
polymer model we present a new picture of the mechanism by which the thickness
of lamellar polymer crystals is constrained to a value close to the minimum
thermodynamically stable thickness. This description contrasts with those given
by the two dominant theoretical approaches.Comment: 4 pages, 4 figures, revte
Pilot factors guidelines for the operational inspection of navigation systems
A computerized human engineered inspection technique is developed for use by FAA inspectors in evaluating the pilot factors aspects of aircraft navigation systems. The short title for this project is Nav Handbook. A menu-driven checklist, computer program and data base (Human Factors Design Criteria) were developed and merged to form a self-contained, portable, human factors inspection checklist tool for use in a laboratory or field setting. The automated checklist is tailored for general aviation navigation systems and can be expanded for use with other aircraft systems, transports or military aircraft. The Nav Handbook inspection concept was demonstrated using a lap-top computer and an Omega/VLF CDU. The program generates standardized inspection reports. Automated checklists for LORAN/C and R NAV were also developed. A Nav Handbook User's Guide is included
HI in four star-forming low-luminosity E/S0 and S0 galaxies
We present HI data cubes of four low-luminosity early-type galaxies which are
currently forming stars. These galaxies have absolute magnitudes in the range
M_B=-17.9 to -19.9 (H_o=50 km/s/Mpc). Their HI masses range between a few times
10^8 and a few times 10^9 M_sun and the corresponding values for M_HI/L_B are
between 0.07 and 0.42, so these systems are HI rich for their morphological
type. In all four galaxies, the HI is strongly centrally concentrated with high
central HI surface densities, in contrast to what is typically observed in more
luminous early-type galaxies. In two galaxies (NGC 802 and ESO 118-G34), the
kinematics of the HI suggests that the gas is in a strongly warped disk, which
we take as evidence for recent accretion of HI. In the other two galaxies (NGC
2328 and ESO 027-G21) the HI must have been part of the systems for a
considerable time. The HI properties of low-luminosity early-type galaxies
appear to be systematically different from those of many more luminous
early-type galaxies, and we suggest that these differences are due to a
different evolution of the two classes. The star formation history of these
galaxies remains unclear. Their UBV colours and Halpha emission-line strengths
are consistent with having formed stars at a slowly-declining rate for most of
the past 10^10 years. However, the current data do not rule out a small burst
of recent star formation overlaid on an older stellar population.Comment: To appear in AJ, LateX, figures in gif format, paper also available
at http://www.nfra.nl/~morganti/LowLu
The Evolution of Radio Galaxies at Intermediate Redshift
We describe a new estimate of the radio galaxy 1.4 GHz luminosity function
and its evolution at intermediate redshifts (z~0.4). Photometric redshifts and
color selection have been used to select Bj<23.5 early-type galaxies from the
Panoramic Deep Fields, a multicolor survey of two 25 sq deg fields.
Approximately 230 radio galaxies have then been selected by matching early-type
galaxies with NVSS radio sources brighter than 5 mJy. Estimates of the 1.4 GHz
luminosity function of radio galaxies measure significant evolution over the
observed redshift range. For an Omega_M=1 cosmology the evolution of the radio
power is consistent with luminosity evolution where P(z)=P(0)(1+z)^{k_L} and
3<k_L<5. The observed evolution is similar to that observed for UVX and X-ray
selected AGN and is consistent with the same physical process being responsible
for the optical and radio luminosity evolution of AGN.Comment: 26 pages, 9 Figures, Accepted for Publication in A
Hot Horizontal Branch Stars in the Galactic Bulge. I
We present the first results of a survey of blue horizontal branch (BHB)
stars in the Galactic bulge. 164 candidates with 15 < V < 17.5 in a field
7.5deg from the Galactic Center were observed in the blue at 2.4A FWHM
resolution with the AAT 2dF spectrograph. Radial velocities were measured for
all stars. For stars with strong Balmer lines, their profiles were matched to
theoretical spectrum calculations to determine stellar temperature Teff and
gravity log g; matches to metal lines yielded abundances. CTIO UBV photometry
then gave the reddening and distance to each hot star. Reddening was found to
be highly variable, with E(B-V) from 0.0 to 0.55 around a mean of 0.28.
Forty-seven BHB candidates were identified with Teff >= 7250K, of which seven
have the gravities of young stars, three are ambiguous, and 37 are HB stars.
They span a wide metallicity range, from solar to 1/300 solar. The warmer BHB's
are more metal-poor and loosely concentrated towards the Galactic center, while
the cooler ones are of somewhat higher metallicity and closer to the center.
Their red B-V colors overlap main-sequence stars, but the U-B vs. B-V diagram
separates them until E(B-V) > 0.5. We detect two cool solar-metallicity HB
stars in the bulge of our own Galaxy, the first such stars known. Still elusive
are their hot counterparts, the metal-rich sdB/O stars causing excess UV light
in metal-rich galaxies; they have V ~ 20.5 in the Bulge.Comment: 29 pages, 4 figures (the third with 4 panels, the fourth with 2
panels). To appear in the Astrophysical Journal v571n1, Jan. 20, 2000.
Abstract is shortened here, and figures compresse
ROSAT X-ray Colors and Emission Mechanisms in Early-Type Galaxies
The X-ray colors and X-ray-to-optical luminosity ratios (L_X/L_B) of 61
early- type galaxies observed with the ROSAT PSPC are determined. The colors
indicate that the X-ray spectral properties of galaxies vary as a function of
L_X/L_B. The brightest X-ray galaxies have colors consistent with thermal
emission from hot gas with roughly the same metallicity of 50% solar. The
spatial variation of the colors indicates that the gas temperature in these
galaxies increases radially. Galaxies with medium L_X/L_B also have spectral
properties consistent with emission from hot gas. If a simple one-component
thermal model is assumed to describe the 0.1-2.0 keV X-ray emission in these
galaxies, then one possible explanation for the progressive decrease in L_X/L_B
among galaxies of this class could be the progressive decrease in metal
abundance of the X-ray emitting contained by the galaxies. Galaxies with the
lowest L_X/L_B values appear to be lacking a hot interstellar component. Their
X-ray colors are consistent with those derived from the bulges of the spiral
galaxies M31 and NGC1291. In M31 the X-ray emission is resolved into discrete
sources, and is apparently due primarily to low mass X-ray binaries (LMXBs). We
therefore suggest that the bulk of the X-ray emission in the faintest
ellipticals is also due to LMXBs. Previously, the X-ray spectra of X-ray faint
galaxies had been found to be described by a hard component which was
attributed to LMXB emission, and a very soft component of unknown origin. We
show that the very soft component also likely results from LMXBs, as a very
soft component is seen in the X-ray spectra of the nearby LMXB Her X-1 and
LMXBs in the bulge of M31. (Abridged)Comment: 49 pages, 17 embedded Postscript figures, uses aaspp4.sty,
Astrophysical Journal, volume 499, in pres
Iron stored in ferritin is chemically reduced in the presence of aggregating Aβ(1-42).
Atypical low-oxidation-state iron phases in Alzheimer's disease (AD) pathology are implicated in disease pathogenesis, as they may promote elevated redox activity and convey toxicity. However, the origin of low-oxidation-state iron and the pathways responsible for its formation and evolution remain unresolved. Here we investigate the interaction of the AD peptide β-amyloid (Aβ) with the iron storage protein ferritin, to establish whether interactions between these two species are a potential source of low-oxidation-state iron in AD. Using X-ray spectromicroscopy and electron microscopy we found that the co-aggregation of Aβ and ferritin resulted in the conversion of ferritin's inert ferric core into more reactive low-oxidation-states. Such findings strongly implicate Aβ in the altered iron handling and increased oxidative stress observed in AD pathogenesis. These amyloid-associated iron phases have biomarker potential to assist with disease diagnosis and staging, and may act as targets for therapies designed to lower oxidative stress in AD tissue
Microjansky radio sources in DC0107-46 (Abell 2877)
The cluster DC0107-46 (Abell 2877) lies within the Phoenix Deep Survey, made
at 1.4 GHz with the Australia Telescope Compact Array. Of 89 known optical
cluster members, 70 lie within the radio survey area. Of these 70 galaxies, 15
(21%) are detected, with luminosities as faint as 10^20 W/Hz. Spectroscopic
observations are available for 14/15 of the radio-detected cluster galaxies.
Six galaxies show only absorption features and are typical low-luminosity AGN
radio sources. One galaxy hosts a Seyfert 2 nucleus, two are star-forming
galaxies, and the remaining five may be star-forming galaxies, AGNs, or both.Comment: 14 pages, 3 figures, Accepted by ApJS (v128n2p JUN 2000 issue
The Sub-parsec Scale Radio Properties of Southern Starburst Galaxies. I. Supernova Remnants, the Supernova Rate, and the Ionised Medium in the NGC 253 Starburst
Wide-field, very long baseline interferometry (VLBI) observations of the
nearby starburst galaxy NGC 253, obtained with the Australian Long Baseline
Array (LBA), have produced a 2.3 GHz image with a maximum angular resolution of
15 mas (0.3 pc). Six sources were detected, all corresponding to sources
identified in higher frequency (>5 GHz) VLA images. One of the sources,
supernova remnant 5.48-43.3, is resolved into a shell-like structure
approximately 90 mas (1.7 pc) in diameter. From these data and data from the
literature, the spectra of 20 compact radio sources in NGC 253 were modelled
and found to be consistent with free-free absorbed power laws. Broadly, the
free-free opacity is highest toward the nucleus but varies significantly
throughout the nuclear region (tau_0 ~ 1->20), implying that the overall
structure of the ionised medium is clumpy. Of the 20 sources, nine have flat
intrinsic spectra associated with thermal radio emission and the remaining 11
have steep intrinsic spectra, associated with synchrotron emission from
supernova remnants. A supernova rate upper limit of 2.4 yr^-1 is determined for
the inner 320 pc region of the galaxy at the 95% confidence level, based on the
lack of detection of new sources in observations spanning almost 17 years and a
simple model for the evolution of supernova remnants. A supernova rate of >0.14
(v/10^4) yr^-1 is implied from estimates of supernova remnant source counts,
sizes and expansion rates, where v is the radial expansion velocity of the
supernova remnant in km s^-1. A star formation rate of 3.4 (v/10^4) <
SFR(M<=5Msun) < 59 Msun yr^-1 has been estimated directly from the supernova
rate limits and is of the same order of magnitude as rates determined from
integrated FIR and radio luminosities.Comment: Accepted by the Astronomical Journal. 34 pages, 6 figures; fixed
typos in assumed expansion velocit
The properties of extragalactic radio sources selected at 20 GHz
We present some first results on the variability, polarization and general
properties of radio sources selected in a blind survey at 20 GHz, the highest
frequency at which a sensitive radio survey has been carried out over a large
area of sky. Sources with flux densities above 100 mJy in the AT20G Pilot
Survey at declination -60 to -70 were observed at up to three epochs during
2002-4, including near-simultaneous measurements at 5, 8 and 18 GHz in 2003. Of
the 173 sources detected, 65% are candidate QSOs, BL Lac objects or blazars,
20% galaxies and 15% faint (b > 22 mag) optical objects or blank fields.
On a 1-2 year timescale, the general level of variability at 20 GHz appears
to be low. For the 108 sources with good-quality measurements in both 2003 and
2004, the median variability index at 20 GHz was 6.9% and only five sources
varied by more than 30% in flux density.
Most sources in our sample show low levels of linear polarization (typically
1-5%), with a median fractional polarization of 2.3% at 20 GHz. There is a
trend for fainter sources to show higher fractional polarization.
At least 40% of sources selected at 20GHz have strong spectral curvature over
the frequency range 1-20 GHz. We use a radio `two-colour diagram' to
characterize the radio spectra of our sample, and confirm that the radio-source
population at 20 GHz (which is also the foreground point-source population for
CMB anisotropy experiments like WMAP and Planck) cannot be reliably predicted
by extrapolating the results of surveys at lower frequencies. As a result,
direct selection at 20 GHz appears to be a more efficient way of identifying 90
GHz phase calibrators for ALMA than the currently-proposed technique of
extrapolation from all-sky surveys at 1-5 GHz.Comment: 14-page paper plus 5-page data table. Replaced with published versio
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