25,042 research outputs found
A New Photometric Model of the Galactic Bar using Red Clump Giants
We present a study of the luminosity density distribution of the Galactic bar
using number counts of red clump giants (RCGs) from the OGLE-III survey. The
data were recently published by Nataf et al. (2013) for 9019 fields towards the
bulge and have RC stars over a viewing area of . The data include the number counts, mean distance modulus
(), dispersion in and full error matrix, from which we fit the data
with several tri-axial parametric models. We use the Markov Chain Monte Carlo
(MCMC) method to explore the parameter space and find that the best-fit model
is the model, with the distance to the GC is 8.13 kpc, the ratio of
semi-major and semi-minor bar axis scale lengths in the Galactic plane
, and vertical bar scale length , is (close to being prolate). The scale length of the stellar
density profile along the bar's major axis is 0.67 kpc and has an angle
of , slightly larger than the value obtained from a similar study
based on OGLE-II data. The number of estimated RC stars within the field of
view is , which is systematically lower than the observed
value. We subtract the smooth parametric model from the observed counts and
find that the residuals are consistent with the presence of an X-shaped
structure in the Galactic centre, the excess to the estimated mass content is
. We estimate the total mass of the bar is . Our results can be used as a key ingredient to construct new density
models of the Milky Way and will have implications on the predictions of the
optical depth to gravitational microlensing and the patterns of hydrodynamical
gas flow in the Milky Way.Comment: 15 pages, 6 figures, 4 tables. MNRAS accepte
Delay-dependent robust stability of stochastic delay systems with Markovian switching
In recent years, stability of hybrid stochastic delay systems, one of the important issues in the study of stochastic systems, has received considerable attention. However, the existing results do not deal with the structure of the diffusion but estimate its upper bound, which induces conservatism. This paper studies delay-dependent robust stability of hybrid stochastic delay systems. A delay-dependent criterion for robust exponential stability of hybrid stochastic delay systems is presented in terms of linear matrix inequalities (LMIs), which exploits the structure of the diffusion. Numerical examples are given to verify the effectiveness and less conservativeness of the proposed method
Measurements in liquid fuel sprays
A ground test facility is being established at NASA Lewis Research Center to simulate the environmental and flight conditions needed to study adverse weather effects. One of the most important components is the water spray system which consists of many nozzles fitted on spray bars. Water is injected through air-assisted atomizers to generate uniform size drops to simulate icing in clouds. The primary objective is to provide experimental data on drop size distribution over a wide range of operating conditions. Correlation equations for mean drop size and initial injection parameters are being determined to assist in the design and modification of the Altitude Wind Tunnel. Special emphasis is being placed on the study of the aerodynamic structure of the air-assisted atomizer sprays. Detailed measurements of the variation of drop size distribution and velocity as a function of time and space are being made. Accurate initial and boundary conditions are being provided for computer model evaluation
Site evaluation for laser satellite-tracking stations
Twenty-six locations for potential laser satellite-tracking stations, four of them actually already occupied in this role, are reviewed in terms of their known local and regional geology and geophysics. The sites are also considered briefly in terms of weather and operational factors. Fifteen of the sites qualify as suitable for a stable station whose motions are likely to reflect only gross plate motion. The others, including two of the present laser station sites (Arequipa and Athens), fail to qualify unless extra monitoring schemes can be included, such as precise geodetic surveying of ground deformation
Photometric properties and luminosity function of nearby massive early-type galaxies
We perform photometric analyses for a bright early-type galaxy (ETG) sample
with 2949 galaxies ( mag) in the redshift range of 0.05 to
0.15, drawn from the SDSS DR7 with morphological classification from Galaxy Zoo
1. We measure the Petrosian and isophotal magnitudes, as well as the
corresponding half-light radius for each galaxy. We find that for brightest
galaxies ( mag), our Petrosian magnitudes, and isophotal
magnitudes to 25 and 1\% of the sky brightness are on
average 0.16 mag, 0.20 mag, and 0.26 mag brighter than the SDSS Petrosian
values, respectively. In the first case the underestimations are caused by
overestimations in the sky background by the SDSS PHOTO algorithm, while the
latter two are also due to deeper photometry. Similarly, the typical half-light
radii () measured by the SDSS algorithm are smaller than our
measurements. As a result, the bright-end of the -band luminosity function
is found to decline more slowly than previous works. Our measured luminosity
densities at the bright end are more than one order of magnitude higher than
those of Blanton et al. (2003), and the stellar mass densities at and are a few tenths
and a factor of few higher than those of Bernardi et al. (2010). These results
may significantly alleviate the tension in the assembly of massive galaxies
between observations and predictions of the hierarchical structure formation
model.Comment: 43 pages, 14 figures, version accepted for publication in the
Astrophysical Journa
Clearing residual planetesimals by sweeping secular resonances in transitional disks: a lone-planet scenario for the wide gaps in debris disks around Vega and Fomalhaut
Extended gaps in the debris disks of both Vega and Fomalhaut have been
observed. These structures have been attributed to tidal perturbations by
multiple super-Jupiter gas giant planets. Within the current observational
limits, however, no such massive planets have been detected. Here we propose a
less stringent `lone-planet' scenario to account for the observed structure
with a single eccentric gas giant and suggest that clearing of these wide gaps
is induced by its sweeping secular resonance. During the depletion of the disk
gas, the planet's secular resonance propagates inward and clears a wide gap
over an extended region of the disk. Although some residual intermediate-size
planetesimals may remain in the gap, their surface density is too low to either
produce super-Earths or lead to sufficiently frequent disruptive collisions to
generate any observable dusty signatures. The main advantage of this
lone-planet sweeping-secular-resonance model over the previous multiple gas
giant tidal truncation scenario is the relaxed requirement on the number of gas
giants. The observationally inferred upper mass limit can also be satisfied
provided the hypothetical planet has a significant eccentricity. A significant
fraction of solar or more massive stars bear gas giant planets with significant
eccentricities. If these planets acquired their present-day kinematic
properties prior to the depletion of their natal disks, their sweeping secular
resonance would effectively impede the retention of neighboring planets and
planetesimals over a wide range of orbital semi-major axes.Comment: 20 pages, 12 figures. Accepted for publication in Ap
The absolute radiometric calibration of the advanced very high resolution radiometer
The early results of an absolute radiometric calibration of the NOAA-9 AVHRR sensor indicate significant degradations in the response of bands 1 and 2 compared to prelaunch values. The results are currently in the process of being verified and it may be that refinements of the methodology will be in order as additional data sets are analyzed. The LANDSAT TM calibration used in this approach is known to be very precise and the Herman radiative transfer code, supplemented by the 5-S code for gaseous transmission, is reliable as well. The extent to which other steps in the analysis procedure give rise to uncertainties in the results is currently under investigation. Particular attention is being given to the geometric matching of the AVHRR and TM imagery, as well as to the spectral redistribution procedure. By taking advantage of a reasonably precise calibration of TM imagery acquired on the same day as the AVHRR data at White Sands, a promising approach to the in-orbit calibration of AVHRR sensors is being developed. Current efforts involve primarily the examination of additional test cases and the investigation of possible simplifications in the procedure through judicious use of atmospheric models
Free-floating planets from core accretion theory: microlensing predictions
We calculate the microlensing event rate and typical time-scales for the
free-floating planet (FFP) population that is predicted by the core accretion
theory of planet formation. The event rate is found to be ~
of that for the stellar population. While the stellar microlensing event
time-scale peaks at around 20 days, the median time-scale for FFP events (~0.1
day) is much shorter. Our values for the event rate and the median time-scale
are significantly smaller than those required to explain the \cite{Sum+11}
result, by factors of ~13 and ~16, respectively. The inclusion of planets at
wide separations does not change the results significantly. This discrepancy
may be too significant for standard versions of both the core accretion theory
and the gravitational instability model to explain satisfactorily. Therefore,
either a modification to the planet formation theory is required, or other
explanations to the excess of short-time-scale microlensing events are needed.
Our predictions can be tested by ongoing microlensing experiment such as
KMTNet, and by future satellite missions such as WFIRST and Euclid.Comment: 6 pages, 5 figures, MNRAS in pres
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