2,362 research outputs found
UWB/GNSS-based cooperative positioning method for V2X applications
Limited availability of GNSS signals in urban canyons is a challenge for the implementation of many positioning-based traffic safety applications, and V2X technology provides an alternative solution to resolve this problem. As a key communication component in V2X technology, Dedicated Short Range Communication (DSRC) not only allows vehicles to exchange their position, but also traffic safety related information such as real-time congestion, up-to-date accident details, speed limits, etc. This position and traffic information could underpin various traffic safety applications - for instance, lane departure warnings, potential collision avoidance, and traffic congestion warnings. By taking advantage of DSRC, a vehicle in a GNSS denied environment is able to calculate its position using the assistance of other vehicles with sufficient GNSS signals to fix their locations. The concept of cooperative positioning, which is also called collaborative positioning, has been proposed to achieve this goal
The Radial Velocity Precision of Fiber-fed Spectrographs
We have measured the radial velocities of five 51 Peg-type stars and one star
with constant velocity. Our measurements, on 20 \AA centered at 3947 \AA, were
conventional using Th/Ar comparison spectra taken every 20 or 40 minutes
between the stellar exposures. Existing IRAF routines were used for the
reduction. We find 20 m s, provided 4 measurements
(out of 72) with residuals are neglected. The observations were
made with the CFHT Gecko spectrograph, fiber-fed with the CAFE system
(R110,000). 10 m s seems possible with
additional care. This study was incidental to the main program and so not
exhaustive but the small value of implies that the fiber
feed/image slicer system on Gecko + CAFE, essentially eliminates the long
standing problem of guiding errors in radial velocity measurements. We are not
suggesting this conventional approach for serious Doppler planet searches
(especially with Gecko which has such a small multiplex gain), but the
precision is valuable for observations made in spectral regions remote from
telluric lines or captive-gas fiducials. Instrument builders might consider the
advantages of the CAFE optics which incorporate agitation and invert the object
and pupil for slit and grating illumination in future spectrograph designs.Comment: 12 pages, 3 figure
Di-μ-sulfato-bis[diaqua(1H-imidazo[4,5-f][1,10]phenanthroline)iron(II)] dihydrate
The title dinuclear FeII complex, [Fe2(SO4)2(C13H8N4)2(H2O)4]·2H2O, is centrosymmetric. Two sulfate anions bridge two FeII cations to form the binuclear complex. Each FeII cation is coordinated by two N atoms from a 1H-imidazo[4,5-f][1,10]phenanthroline (IP) ligand, two O atoms from two sulfate anions and two water molecules in a distorted octahedral geometry. Extensive O—H⋯O, N—H⋯O and O—H⋯N hydrogen bonding is present in the crystal structure. Weak π–π stacking is observed between parallel IP ring systems, the face-to-face separation being 3.428 (14) Å
Galactic S Stars: Investigations of Color, Motion, and Spectral Features
Known bright S stars, recognized as such by their enhanced s-process
abundances and C/O ratio, are typically members of the asymptotic giant branch
(AGB) or the red giant branch (RGB). Few modern digital spectra for these
objects have been published, from which intermediate resolution spectral
indices and classifications could be derived. For published S stars we find
accurate positions using the Two-Micron All Sky Survey (2MASS), and use the
FAST spectrograph of the Tillinghast reflector on Mt. Hopkins to obtain the
spectra of 57 objects. We make available a digital S star spectral atlas
consisting of 14 spectra of S stars with diverse spectral features. We define
and derive basic spectral indices that can help distinguish S stars from
late-type (M) giants and carbon stars. We convolve all our spectra with the
SDSS bandpasses, and employ the resulting gri magnitudes together with 2MASS
JHK mags to investigate S star colors. S stars have colors similar to carbon
and M stars, and are therefore difficult to distinguish by color alone. Using
near and mid-infrared colors from IRAS and AKARI, we identify some of the stars
as intrinsic (AGB) or extrinsic (with abundances enhanced by past
mass-transfer). We also use V band and 2MASS magnitudes to calculate a
temperature index for stars in the sample. We analyze the proper motions and
parallaxes of our sample stars to determine upper and lower limit absolute
magnitudes and distances, and confirm that most are probably giants.Comment: 11 pages. Accepted for publication in ApJS July 19, 2011. Spectra
available as http://hea-www.harvard.edu/~pgreen/SStarAtlas.ta
Orbital Elements of Comet C/1490 Y1 and the Quadrantid shower
The Quadrantid shower, one of the most intense showers, has been observed at
the beginning of January each year. However, the origin of the meteors is still
unknown. It was Hasegawa (1979) who first suggested comet C/1490 Y1 to be the
likely origin of the shower based on the historical records of East Asia. We
analyse the records of Jo-Seon-Wang-Jo-Sil-Lok (the Annals of the Joseon
Dynasty in ancient Korea) and calculate the preliminary orbital elements of
comet C/1490 Y1 using a modified Gauss method. We find that comet C/1490 Y1 was
a periodic one and its orbital path was very similar to that of the Quadrantid
meteor stream. The determined orbital elements are perifocal passage time
Tp=2265652.2983 days (7.8 Jan. 1491 in UT), perifocal distance q=0.769 AU,
eccentricity e=0.747, semimajor axis a=3.04 AU, argument of the perifocus
omega=164.03 degrees, longitude of ascending node Omega=283.00 degrees, and
inclination i=70.22 degrees for the epoch of J2000.0. We, therefore, conclude
that our result verifies the suggestion that the comet C/1490 Y1 is the origin
of the Quandrantid meteor shower, but was a periodic comet. We dicuss a
possible link between this comet and the asteroid 2003 EH1 as well.Comment: 7 pages, 2 figures, accepted for publication in MNRA
A Vibrational Approach to Node Centrality and Vulnerability in Complex Networks
We propose a new measure of vulnerability of a node in a complex network. The
measure is based on the analogy in which the nodes of the network are
represented by balls and the links are identified with springs. We define the
measure as the node displacement, or the amplitude of vibration of each node,
under fluctuation due to the thermal bath in which the network is supposed to
be submerged. We prove exact relations among the thus defined node
displacement, the information centrality and the Kirchhoff index. The relation
between the first two suggests that the node displacement has a better
resolution of the vulnerability than the information centrality, because the
latter is the sum of the local node displacement and the node displacement
averaged over the entire network.Comment: 27 page
Global patterns of woody residence time and its influence on model simulation of aboveground biomass
Woody residence time (τw) is an important parameter that expresses the balance between mature forest recruitment/growth and mortality. Using field data collected from the literature, this study explored the global forest τw and investigated its influence on model simulations of aboveground biomass (AGB) at a global scale. Specifically, τw was found to be related to forest age, annual temperature, and precipitation at a global scale, but its determinants were different among various plant function types. The estimated global forest τw based on the filed data showed large spatial heterogeneity, which plays an important role in model simulation of AGB by a dynamic global vegetation model (DGVM). The τw could change the resulting AGB in tenfold based on a site-level test using the Monte Carlo method. At the global level, different parameterization schemes of the Integrated Biosphere Simulator using the estimated τw resulted in a twofold change in the AGB simulation for 2100. Our results highlight the influences of various biotic and abiotic variables on forest τw. The estimation of τw in our study may help improve the model simulations and reduce the parameter\u27s uncertainty over the projection of future AGB in the current DGVM or Earth System Models. A clearer understanding of the responses of τw to climate change and the corresponding sophisticated description of forest growth/mortality in model structure is also needed for the improvement of carbon stock prediction in future studies
A Planetary Companion to gamma Cephei A
We report on the detection of a planetary companion in orbit around the
primary star of the binary system Cephei. High precision radial
velocity measurements using 4 independent data sets spanning the time interval
1981--2002 reveal long-lived residual radial velocity variations superimposed
on the binary orbit that are coherent in phase and amplitude with a period or
2.48 years (906 days) and a semi-amplitude of 27.5 m s. We performed a
careful analysis of our Ca II H & K S-index measurements, spectral line
bisectors, and {\it Hipparcos} photometry. We found no significant variations
in these quantities with the 906-d period. We also re-analyzed the Ca II
8662 {\AA} measurements of Walker et al. (1992) which showed possible
periodic variations with the ``planet'' period when first published. This
analysis shows that periodic Ca II equivalent width variations were only
present during 1986.5 -- 1992 and absent during 1981--1986.5. Furthermore, a
refined period for the Ca II 8662 {\AA} variations is 2.14 yrs,
significantly less than residual radial velocity period. The most likely
explanation of the residual radial velocity variations is a planetary mass
companion with sin = 1.7 and an orbital semi-major axis
of 2.13 AU. This supports the planet hypothesis for the residual
radial velocity variations for Cep first suggested by Walker et al.
(1992). With an estimated binary orbital period of 57 years Cep is the
shortest period binary system in which an extrasolar planet has been found.
This system may provide insights into the relationship between planetary and
binary star formation.Comment: 19 pages, 15 figures, accepted in Ap. J. Includes additional data and
improved orbital solutio
An orthorhombic polymorph of pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile
The title compound, C16H6N6, is a polymorph of the previously reported structure [Kozlov & Goldberg (2008 ▶). Acta Cryst. C64, o498–o501]. Unlike the previously reported monoclinic polymorph (space group P21/c, Z = 8), the title compound reveals orthorhombic symmetry (space group Pnma, Z = 4). The molecule shows crystallographic mirror symmetry, while the previously reported structure exhibits two independent molecules per asymmetric unit. In the title compound, adjacent molecules are essentially parallel along the c axis and tend to be vertical along the b axis with dihedral angles of 72.02 (6)°. However, in the reported polymorph, the entire crystal structure shows an antiparallel arrangement of adjacent columns related by inversion centers and the two independent molecules are nearly parallel with a dihedral angle of 2.48 (6)°
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