7,828 research outputs found
The Outer Edges of Dwarf Irregular Galaxies: Stars and Gas
We have in recent years come to view dwarf galaxy evolution in the broader
context of the cosmic evolution of large-scale structure. Dwarf galaxies, as
the putative building blocks of hierarchical galaxy formation, and also as the
most numerous galaxies in the Universe, play a central role in cosmic
evolution. In particular, the interplay of galactic and intergalactic material
around dwarf irregulars must be more extensive than in more massive disk
galaxies because of their lower gravitational potential and lower interstellar
pressures. The outer regions of dwarf irregular galaxies therefore yield vital
clues to the dominant processes in this interaction zone.
The Workshop addressed a number of questions related to the role of the outer
regions in the evolution of dwarf galaxies and broader consequences. On-line
Workshop Proceedings are at http://www.lowell.edu/Workshops/Lowell02/Comment: Summary of the 2002 Lowell Observatory Workshop, to appear in PASP
Conference Highlights; 6 pp, uses aaspp4.sty. On-line Proceedings at
http://www.lowell.edu/Workshops/Lowell02
Evaluation of hot corrosion behavior of thermal barrier coatings
Calcium silicate and yttria stabilized zirconia/MCrAlY thermal barrier coating systems on air-cooled specimens were exposed to sodium plus vanadium doped Mach 0.3 combustion gases. Thermal barrier coating endurance was determined to be a strong inverse function of ceramic coating thickness. Coating system durability was increased through the use of higher Cr + Al NiCrAl and CoCrAlY bond coatings. Chemical and electron microprobe analyses supported the predictions of condensate compositions and the determination of their roles in causing spalling of the ceramic coatings
Review of NASA progress in thermal barrier coatings for stationary gas turbines
Ceramic thermal barrier coatings for industrial/utility gas turbines were investigated. In burner rig tests of a zirconia yttria/nickel chromium aluminum yttrium ZrO2-12w/0Y2O3/NiCrAlY coating system on air cooled superalloy specimens, ceramic coating life (spallation) was sensitive to Na and V concentration in the fuel. The locations of coating spallation correspond to areas where combustion products were predicted to condense. Three new thermal barrier coating systems were identified. These are based on calcium silicate, ZrO2-8w/0Y2O3, and a MgO-NiCrAlY cermet. The spall resistance can be increased by reducing the ceramic layer thickness from 0.038 to 0.013 cm and by the use of more oxidation/corrosion resistant bond coats
The Stellar Populations of NGC 3109: Another Dwarf Irregular Galaxy with a Population II Stellar Halo
We have obtained V and I-band photometry for about 17500 stars in the field
of the dwarf irregular galaxy NGC3109, located in the outskirts of the Local
Group. The photometry allows us to study the stellar populations present inside
and outside the disk of this galaxy. From the VI color-magnitude diagram we
infer metallicities and ages for the stellar populations in the main body and
in the halo of NGC3109. The stars in the disk of this galaxy have a wide
variety of ages, including very young stars with approximately 10^7 yr. Our
main result is to establish the presence of a halo consisting of population II
stars, extending out to about 4.5 arcmin (or 1.8 kpc) above and below the plane
of this galaxy. For these old stars we derive an age of > 10 Gyr and a
metallicity of [Fe/H] = -1.8 +/- 0.2. We construct a deep luminosity function,
obtaining an accurate distance modulus (m-M)_0 = 25.62 +/- 0.1 for this galaxy
based on the I-magnitude of the red giant branch (RGB) tip and adopting E(V-I)
= 0.05.Comment: Accepted for publication in the Astronomical Journal 23 pages, latex,
12 Figures (Fig 1 not available in electronic format
4U 1626-67 as seen by Suzaku before and after the 2008 torque reversal
Aims. The accretion-powered pulsar 4U 1626-67 experienced a new torque
reversal at the beginning of 2008, after about 18 years of steadily spinning
down. The main goal of the present work is to study this recent torque reversal
that occurred in 2008 February.
Methods. We present a spectral analysis of this source using two pointed
observations performed by Suzaku in 2006 March and in 2010 September.
Results. We confirm with Suzaku the presence of a strong emission-line
complex centered on 1 keV, with the strongest line being the hydrogen-like Ne
Ly-alpha at 1.025(3) keV. We were able to resolve this complex with up to seven
emission lines. A dramatic increase of the intensity of the Ne Ly-alpha line
after the 2008 torque reversal occurred, with the equivalent width of this line
reaching almost the same value measured by ASCA in 1993. We also report on the
detection of a cyclotron line feature centered at ~37 keV. In spite of the fact
that an increase of the X-ray luminosity (0.5-100 keV) of a factor of ~2.8
occurred between these two observations, no significant change in the energy of
the cyclotron line feature was observed. However, the intensity of the ~1 keV
line complex increased by an overall factor of ~8.
Conclusions. Our results favor a scenario in which the neutron star in 4U
1626-67 accretes material from a geometrically thin disk during both the
spin-up and spin-down phases.Comment: 7 pages, 5 figures and 2 tables. Accepted in A&
The Glass Transition Temperature of Water: A Simulation Study
We report a computer simulation study of the glass transition for water. To
mimic the difference between standard and hyperquenched glass, we generate
glassy configurations with different cooling rates and calculate the
dependence of the specific heat on heating. The absence of crystallization
phenomena allows us, for properly annealed samples, to detect in the specific
heat the simultaneous presence of a weak pre-peak (``shadow transition''), and
an intense glass transition peak at higher temperature.
We discuss the implications for the currently debated value of the glass
transition temperature of water. We also compare our simulation results with
the Tool-Narayanaswamy-Moynihan phenomenological model.Comment: submitted to Phys. Re
Star complexes and stellar populations in NGC 6822 - Comparison with the Magellanic Clouds
The star complexes (large scale star forming regions) of NGC 6822 were traced
and mapped and their size distribution was compared with the size distribution
of star complexes in the Magellanic Clouds (MCs). Furthermore, the spatial
distributions of different age stellar populations were compared with each
other. The star complexes of NGC 6822 were determined by using the isopleths,
based on star counts, of the young stars of the galaxy, using a statistical
cutoff limit in density. In order to map them and determine their geometrical
properties, an ellipse was fitted to every distinct region satisfying this
minimum limit. The Kolmogorov-Smirnov statistical test was used to study
possible patterns in their size distribution. Isopleths were also used to study
the stellar populations of NGC 6822. The star complexes of NGC 6822 were
detected and a list of their positions and sizes was produced. Indications of
hierarchical star formation, in terms of spatial distribution, time evolution
and preferable sizes were found in NGC 6822 and the MCs. The spatial
distribution of the various age stellar populations has indicated traces of an
interaction in NGC 6822, dated before 350 +/- 50 Myr.Comment: 10 pages, 7 figures, accepted by A&A; minor typeface correction
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