4,354 research outputs found
Perspective acceleration and gravitational redshift. Measuring masses of individual white dwarfs using Gaia + SIM astrometry
According to current plans, the SIM/NASA mission will be launched just after
the end of operations for the Gaia/ESA mission. This is a new situation which
enables long term astrometric projects that could not be achieved by either
mission alone. Using the well-known perspective acceleration effect on
astrometric measurements, the true heliocentric radial velocity of a nearby
star can be measured with great precision if the time baseline of the
astrometric measurements is long enough. Since white dwarfs are compact
objects, the gravitational redshift can be quite large (40-80 km/s), and is the
predominant source of any shift in wavelength. The mismatch of the true radial
velocity with the spectroscopic shift thus leads to a direct measure of the
Mass--Radius relation for such objects. Using available catalog information
about the known nearby white dwarfs, we estimate how many masses/gravitational
redshift measurements can be obtained with an accuracy better than 2%. Nearby
white dwarfs are relatively faint objects (10 < V < 15), which can be easily
observed by both missions. We also briefly discuss how the presence of a long
period planet can mask the astrometric signal of perspective acceleration.Comment: 3 pages, 2 Figures. Proceedings of the IAU Symposium 261 : Relativity
in Fundamental Astronomy. 27 April - 1 May 2009, Virginia Beach, VA, USA.
refereed and accepted versio
Astrometric Light-Travel Time signature of sources in nonlinear motion
Context:Very precise planned space astrometric missions and recent
improvements on imaging capabilities require a detailed review of the
assumptions of classical astrometric modeling.
Aims:We show that Light-Travel Time must be taken into account to model the
kinematics of astronomical objects in nonlinear motion, even at stellar
distances.
Methods:A closed expression to include Light-Travel Time in the actual
astrometric models with nonlinear motion is provided. Using a perturbative
approach the expression of the Light-Travel Time signature is derived. We
propose a practical form of the astrometric modelling to be applied in
astrometric data reduction of sources at stellar distances().
Results :We show that the Light-Travel Time signature is relevant at \muas
accuracy (or even at ) depending on the time span of the astrometric
measurements. We explain how information about the radial motion of a source
can be obtained. Some estimative numbers are provided for known nearby binary
systems
Conclusions :In the light of the obtained results, it is clear that this
effect must be taken into account to interpret any kind of precise astrometric
measurements. The effect is particularly interesting in measurements performed
by the planned astrometric space missions (GAIA, SIM, JASMINE, TPF/DARWIN).
Finally an objective criterion is provided to quickly evaluate whether the
Light-Travel Time modeling is required for a given source or system.Comment: 10 pages, 2 figure
High Angular Resolution Radio Observations of the HL/XZ Tau Region: Mapping the 50 AU Protoplanetary Disk around HL Tau and Resolving XZ Tau S into a 13 AU Binary
We present new 7 mm and archive 1.3 cm high angular resolution observations
of the HL/XZ Tau region made with the VLA. At 7 mm, the emission from HL Tau
seems to be arising in a clumpy disk with radius of order 25 AU. The 1.3 cm
emission from XZ Tau shows the emission from a binary system with 0"3 (42 AU)
separation, known from previous optical/IR observations. However, at 7 mm, the
southern radio component resolves into a binary with 0"09 (13 AU) separation,
suggesting that XZ Tau is actually a triple star system. We suggest that the
remarkable ejection of gas from the XZ Tau system observed with the HST may be
related to a periastron passage of this newly discovered close binary system.Comment: Accepted by The Astrophysical Journal Letters (23 Jan 2009
Regular triangulations of dynamic sets of points
The Delaunay triangulations of a set of points are a class of
triangulations which play an important role in a variety of
different disciplines of science. Regular triangulations are a
generalization of Delaunay triangulations that maintain both their
relationship with convex hulls and with Voronoi diagrams. In regular
triangulations, a real value, its weight, is assigned to each point.
In this paper a simple data structure is presented that allows
regular triangulations of sets of points to be dynamically updated,
that is, new points can be incrementally inserted in the set and old
points can be deleted from it. The algorithms we propose for
insertion and deletion are based on a geometrical interpretation of
the history data structure in one more dimension and use lifted
flips as the unique topological operation. This results in rather
simple and efficient algorithms. The algorithms have been
implemented and experimental results are given.Postprint (published version
Optimal Fourier filtering of a function that is strictly confined within a sphere
We present an alternative method to filter a distribution, that is strictly
confined within a sphere of given radius , so that its Fourier transform
is optimally confined within another sphere of radius .
In electronic structure methods, it can be used to generate optimized
pseudopotentials, pseudocore charge distributions, and pseudo atomic orbital
basis sets.Comment: 3 pages, 2 figure
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