970 research outputs found
Bimodal chemical evolution of the Galactic disk and the Barium abundance of Cepheids
In order to understand the Barium abundance distribution in the Galactic disk
based on Cepheids, one must first be aware of important effects of the
corotation resonance, situated a little beyond the solar orbit. The thin disk
of the Galaxy is divided in two regions that are separated by a barrier
situated at that radius. Since the gas cannot get across that barrier, the
chemical evolution is independent on the two sides of it. The barrier is caused
by the opposite directions of flows of gas, on the two sides, in addition to a
Cassini-like ring void of HI (caused itself by the flows). A step in the
metallicity gradient developed at corotation, due to the difference in the
average star formation rate on the two sides, and to this lack of communication
between them. In connection with this, a proof that the spiral arms of our
Galaxy are long-lived (a few billion years) is the existence of this step. When
one studies the abundance gradients by means of stars which span a range of
ages, like the Cepheids, one has to take into account that stars, contrary to
the gas, have the possibility of crossing the corotation barrier. A few stars
born on the high metallicity side are seen on the low metallicity one, and
vice-versa. In the present work we re-discuss the data on Barium abundance in
Cepheids as a function of Galactic radius, taking into account the scenario
described above. The [Ba/H] ratio, plotted as a function of Galactic radius,
apparently presents a distribution with two branches in the external region
(beyond corotation). One can re-interpret the data and attribute the upper
branch to the stars that were born on the high metallicity side. The lower
branch, analyzed separately, indicates that the stars born beyond corotation
have a rising Barium metallicity as a function of Galactic radius.Comment: 6 pages, 7 figures, Proceedings of IAU Symposium 29
The role of surface roughness, albedo, and Bowen ratio on ecosystem energy balance in the Eastern United States
Land cover and land use influence surface climate through differences in biophysical surface properties, including partitioning of sensible and latent heat (e.g., Bowen ratio), surface roughness, and albedo. Clusters of closely spaced eddy covariance towers (e.g., \u3c10 \u3ekm) over a variety of land cover and land use types provide a unique opportunity to study the local effects of land cover and land use on surface temperature. We assess contributions albedo, energy redistribution due to differences in surface roughness and energy redistribution due to differences in the Bowen ratio using two eddy covariance tower clusters and the coupled (land-atmosphere) Variable-Resolution Community Earth System Model. Results suggest that surface roughness is the dominant biophysical factor contributing to differences in surface temperature between forested and deforested lands. Surface temperature of open land is cooler (−4.8 °C to −0.05 °C) than forest at night and warmer (+0.16 °C to +8.2 °C) during the day at northern and southern tower clusters throughout the year, consistent with modeled calculations. At annual timescales, the biophysical contributions of albedo and Bowen ratio have a negligible impact on surface temperature, however the higher albedo of snow-covered open land compared to forest leads to cooler winter surface temperatures over open lands (−0.4 °C to −0.8 °C). In both the models and observation, the difference in mid-day surface temperature calculated from the sum of the individual biophysical factors is greater than the difference in surface temperature calculated from radiative temperature and potential temperature. Differences in measured and modeled air temperature at the blending height, assumptions about independence of biophysical factors, and model biases in surface energy fluxes may contribute to daytime biases
Photoionization microscopy on magnesium atom and comparison with hydrogenic theory
International audienc
The Solar Neighborhood. XXXIX. Parallax Results from the CTIOPI and NOFS Programs: 50 New Members of the 25 Parsec White Dwarf Sample
We present 114 trigonometric parallaxes for 107 nearby white dwarf (WD)
systems from both the Cerro Tololo Inter-American Observatory Parallax
Investigation (CTIOPI) and the U. S. Naval Observatory Flagstaff Station (NOFS)
parallax programs. Of these, 76 parallaxes for 69 systems were measured by the
CTIOPI program and 38 parallaxes for as many systems were measured by the NOFS
program. A total of 50 systems are confirmed to be within the 25 pc horizon of
interest. Coupled with a spectroscopic confirmation of a common proper motion
companion to a Hipparcos star within 25 pc as well as confirmation parallax
determinations for two WD systems included in the recently released Tycho Gaia
Astrometric Solution (TGAS) catalog, we add 53 new systems to the 25 pc WD
sample a 42% increase. Our sample presented here includes four strong
candidate halo systems, a new metal-rich DAZ WD, a confirmation of a recently
discovered nearby short-period (P = 2.85 hr) double degenerate, a WD with a new
astrometric pertubation (long period, unconstrained with our data), and a new
triple system where the WD companion main-sequence star has an astrometric
perturbation (P 1.6 yr).Comment: 32 pages, 12 figures. Figure 4 in the manuscript is a representative
set of plots - plots for all WDs presented here are available
(allfits_photo.pdf, allfits_photo_DQ.pdf, and allfits_photo_DZ.pdf). Accepted
for publication in The Astronomical Journa
Polarization Variability Arising from Clumps in the Winds of Wolf-Rayet Stars
The polarimetric and photometric variability of Wolf-Rayet (WR) stars as
caused by clumps in the winds, is revisited. In the model which is improved
from Li et al. 2000, the radial expansion of the thickness is accounted for,
but we retain the dependence on the beta velocity law, stellar occultation
effects. We again search for parameters that can yield results consistent with
observations in regards to the mean polarization, the ratio of polarimetric to
photometric variability, and the volume filling factor. Clump generation and
spatial distribution are randomized by the Monte Carlo method so as to produce
clumps which are, in the mean, distributed uniformly in space and have time
intervals with a Gaussian distribution. The generated clumps move radially
outward with a velocity law determined by a beta index, and the angular size of
the clumps is assumed to keep fixed. By fitting the observational results and
the volume filling factor, the clump velocity law index beta and clump ejection
rate are inferred, and are found to be well constrained. In addition, the
subpeak features on broad emission lines seem to support the clump ejection
rate. Meanwhile, the fraction of the total mass loss rate that is contained in
the clumps is obtained by fitting the observed polarization. We conclude that
this picture for the clump properties produces a valuable diagnostic of WR wind
structure.Comment: 28 pages, 13 figures, accepted for publication in RA
The First Hypervelocity Star from the LAMOST Survey
We report the first hypervelocity star (HVS) discovered from the LAMOST
spectroscopic survey. It is a B-type star with a heliocentric radial velocity
about 620 km/s, which projects to a Galactocentric radial velocity component of
~477 km/s. With a heliocentric distance of ~13 kpc and an apparent magnitude of
~13 mag, it is the nearest bright HVS currently known. With a mass of ~9Msun,
it is one of the three most massive HVSs discovered so far. The star is
clustered on the sky with many other known HVSs, with the position suggesting a
possible connection to Galactic center structures. With the current
poorly-determined proper motion, a Galactic center origin of this HVS remains
consistent with the data at the 1sigma level, while a disk run-away origin
cannot be excluded. We discuss the potential of the LAMOST survey to discover a
large statistical sample of HVSs of different types.Comment: 5 pages, 5 figures, accepted for publication in ApJL, updated contour
plot for the ejection positions after correcting a mistake in the calculatio
An Automated Machine-Learning Approach for Road Pothole Detection Using Smartphone Sensor Data.
Road surface monitoring and maintenance are essential for driving comfort, transport safety and preserving infrastructure integrity. Traditional road condition monitoring is regularly conducted by specially designed instrumented vehicles, which requires time and money and is only able to cover a limited proportion of the road network. In light of the ubiquitous use of smartphones, this paper proposes an automatic pothole detection system utilizing the built-in vibration sensors and global positioning system receivers in smartphones. We collected road condition data in a city using dedicated vehicles and smartphones with a purpose-built mobile application designed for this study. A series of processing methods were applied to the collected data, and features from different frequency domains were extracted, along with various machine-learning classifiers. The results indicated that features from the time and frequency domains outperformed other features for identifying potholes. Among the classifiers tested, the Random Forest method exhibited the best classification performance for potholes, with a precision of 88.5% and recall of 75%. Finally, we validated the proposed method using datasets generated from different road types and examined its universality and robustness
Coupled-Channel Theory of Photoionization Microscopy
We develop a quantum-mechanical coupled-channel theory to simulate spatial distributions of electron current densities, produced in photoionization for nonhydrogenic atoms in the presence of a uniform external electric field. The coupled Schrodinger equations are numerically solved using the renormalized Numerov method. The expression for the outgoing wave function for photoelectrons ejected from the nonhydrogenic atomic source is derived. The theory is applied to investigations of photoionization for ground-state Li atoms. The distributions of electron current densities are computed and compared to the corresponding experimental images. Excellent agreement is obtained. It is, furthermore, found that the presence of the nonhydrogenic residual ion significantly changes the differential cross sections and/or electron current densities with respect to the hydrogenic case. Finally, the implications of the presence of the atomic core for quantum resonance tunneling are also analyzed
The UKIDSS-2MASS Proper Motion Survey I: Ultracool dwarfs from UKIDSS DR4
The UKIRT Infrared Deep Sky Survey (UKIDSS) is the first of a new generation
of infrared surveys. Here we combine the data from two UKIDSS components, the
Large Area Survey (LAS) and the Galactic Cluster Survey (GCS), with 2MASS data
to produce an infrared proper motion survey for low mass stars and brown
dwarfs. In total we detect 267 low mass stars and brown dwarfs with significant
proper motions. We recover all ten known single L dwarfs and the one known T
dwarf above the 2MASS detection limit in our LAS survey area and identify eight
additional new candidate L dwarfs. We also find one new candidate L dwarf in
our GCS sample. Our sample also contains objects from eleven potential common
proper motion binaries. Finally we test our proper motions and find that while
the LAS objects have proper motions consistent with absolute proper motions,
the GCS stars may have proper motions which are significantly under-estimated.
This is due possibly to the bulk motion of some of the local astrometric
reference stars used in the proper motion determination.Comment: Accepted for publication in MNRA
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