64,955 research outputs found
The observed spiral structure of the Milky Way
The spiral structure of the Milky Way is not yet well determined. The keys to
understanding this structure are to increase the number of reliable spiral
tracers and to determine their distances as accurately as possible. HII
regions, giant molecular clouds (GMCs), and 6.7-GHz methanol masers are closely
related to high mass star formation, and hence they are excellent spiral
tracers. We update the catalogs of Galactic HII regions, GMCs, and 6.7-GHz
methanol masers, and then outline the spiral structure of the Milky Way. We
collected data for more than 2500 known HII regions, 1300 GMCs, and 900 6.7-GHz
methanol masers. If the photometric or trigonometric distance was not yet
available, we determined the kinematic distance using a Galaxy rotation curve
with the current IAU standard, = 8.5 kpc and = 220 km
s, and the most recent updated values of = 8.3 kpc and
= 239 km s, after we modified the velocities of tracers with the adopted
solar motions. With the weight factors based on the excitation parameters of
HII regions or the masses of GMCs, we get the distributions of these spiral
tracers. The distribution of tracers shows at least four segments of arms in
the first Galactic quadrant, and three segments in the fourth quadrant. The
Perseus Arm and the Local Arm are also delineated by many bright HII regions.
The arm segments traced by massive star forming regions and GMCs are able to
match the HI arms in the outer Galaxy. We found that the models of three-arm
and four-arm logarithmic spirals are able to connect most spiral tracers. A
model of polynomial-logarithmic spirals is also proposed, which not only
delineates the tracer distribution, but also matches the observed tangential
directions.Comment: 22 Pages, 16 Figures, 7 Tables, updated to match the published
versio
Understanding of the Retarded Oxidation Effects in Silicon Nanostructures
In-depth understanding of the retarded oxidation phenomenon observed during
the oxidation of silicon nanostructures is proposed. The wet thermal oxidation
of various silicon nanostructures such as nanobeams, concave/convex nanorings
and nanowires exhibits an extremely different and complex behavior. Such
effects have been investigated by the modeling of the mechanical stress
generated during the oxidation process explaining the retarded regime. The
model describes the oxidation kinetics of silicon nanowires down to a few
nanometers while predicting reasonable and physical stress levels at the
Si/SiO interface by correctly taking into account the relaxation effects
in silicon oxide through plastic flow
Mean magnetic field generation in sheared rotators
A generalized mean magnetic field induction equation for differential
rotators is derived, including a compressibility, and the anisotropy induced on
the turbulent quantities from the mean magnetic field itself and a mean
velocity shear. Derivations of the mean field equations often do not emphasize
that there must be anisotropy and inhomogeneity in the turbulence for mean
field growth. The anisotropy from shear is the source of a term involving the
product of the mean velocity gradient and the cross-helicity correlation of the
isotropic parts of the fluctuating velocity and magnetic field,
\lb{\bfv}\cdot{\bfb}\rb^{(0)}. The full mean field equations are derived to
linear order in mean fields, but it is also shown that the cross-helicity term
survives to all orders in the velocity shear. This cross-helicity term can
obviate the need for a pre-existing seed mean magnetic field for mean field
growth: though a fluctuating seed field is necessary for a non-vanishing
cross-helicity, the term can produce linear (in time) mean field growth of the
toroidal field from zero mean field. After one vertical diffusion time, the
cross-helicity term becomes sub-dominant and dynamo exponential
amplification/sustenance of the mean field can subsequently ensue. The
cross-helicity term should produce odd symmetry in the mean magnetic field, in
contrast to the usually favored even modes of the dynamo amplification in
sheared discs. This may be important for the observed mean field geometries of
spiral galaxies. The strength of the mean seed field provided by the cross-
helicity depends linearly on the magnitude of the cross-helicity.Comment: 15 pages, LaTeX, matches version accepted to ApJ, minor revision
Effect of polymer concentration and length of hydrophobic end block on the unimer-micelle transition broadness in amphiphilic ABA symmetric triblock copolymer solutions
The effects of the length of each hydrophobic end block N_{st} and polymer
concentration \bar{\phi}_{P} on the transition broadness in amphiphilic ABA
symmetric triblock copolymer solutions are studied using the self-consistent
field lattice model. When the system is cooled, micelles are observed, i.e.,the
homogenous solution (unimer)-micelle transition occurs. When N_{st} is
increased, at fixed \bar{\phi}_{P}, micelles occur at higher temperature, and
the temperature-dependent range of micellar aggregation and half-width of
specific heat peak for unimer-micelle transition increase monotonously.
Compared with associative polymers, it is found that the magnitude of the
transition broadness is determined by the ratio of hydrophobic to hydrophilic
blocks, instead of chain length. When \bar{\phi}_{P} is decreased, given a
large N_{st}, the temperature-dependent range of micellar aggregation and
half-width of specific heat peak initially decease, and then remain nearly
constant. It is shown that the transition broadness is concerned with the
changes of the relative magnitudes of the eductions of nonstickers and solvents
from micellar cores.Comment: 8 pages, 4 figure
Searching for sub-millisecond pulsars from highly polarized radio sources
Pulsars are among the most highly polarized sources in the universe. The NVSS
has catalogued 2 million radio sources with linear polarization measurements,
from which we have selected 253 sources, with polarization percentage greater
than 25%, as targets for pulsar searches. We believe that such a sample is not
biased by selection effects against ultra-short spin or orbit periods. Using
the Parkes 64m telescope, we conducted searches with sample intervals of 0.05
ms and 0.08 ms, sensitive to submillisecond pulsars. Unfortunately we did not
find any new pulsars.Comment: 2 pages 1 figure. To appear in "Young Neutron Stars and Their
Environments" (IAU Symposium 218, ASP Conference Proceedings), eds F. Camilo
and B. M. Gaensle
A Sino-German 6cm polarisation survey of the Galactic plane IX. HII regions
Large-scale radio continuum surveys provide data to get insights into the
physical properties of radio sources. HII regions are prominent radio sources
produced by thermal emission of ionised gas around young massive stars. We
identify and analyse HII regions in the Sino-German 6cm polarisation survey of
the Galactic plane. Objects with flat radio continuum spectra together with
infrared and/or Halpha emission were identified as HII regions. For HII regions
with small apparent sizes, we cross-matched the 6cm small-diameter source
catalogue with the radio HII region catalogue compiled by Paladini and the
infrared HII region catalogue based on the WISE data. Extended HII regions were
identified by eye by overlaying the Paladini and the WISE HII regions onto the
6cm survey images for coincidences. The TT-plot method was employed for
spectral index verification. A total of 401 HII regions were identified and
their flux densities were determined with the Sino-German 6cm survey data. In
the surveyed area, 76 pairs of sources are found to be duplicated in the
Paladini HII region catalogue, mainly due to the non-distinction of previous
observations with different angular resolutions, and 78 objects in their
catalogue are misclassified as HII regions, being actually planetary nebulae,
supernova remnants or extragalactic sources that have steep spectra. More than
30 HII regions and HII region candidates from our 6cm survey data, especially
extended ones, do not have counterparts in the WISE HII region catalogue, of
which 9 are identified for the first time. Based on the newly derived radio
continuum spectra and the evidence of infrared emission, the previously
identified SNRs G11.1-1.0, G20.4+0.1 and G16.4-0.5 are believed to be HII
regions.Comment: version after some minor corrections and language editing, full Table
2 - 5 will appear in CDS, accepted for publication in A&
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