9,026 research outputs found
Quasars can be used to verify the parallax zero-point of the Tycho-Gaia Astrometric Solution
Context. The Gaia project will determine positions, proper motions, and
parallaxes for more than one billion stars in our Galaxy. It is known that
Gaia's two telescopes are affected by a small but significant variation of the
basic angle between them. Unless this variation is taken into account during
data processing, e.g. using on-board metrology, it causes systematic errors in
the astrometric parameters, in particular a shift of the parallax zero-point.
Previously, we suggested an early reduction of Gaia data for the subset of
Tycho-2 stars (Tycho-Gaia Astrometric Solution; TGAS).
Aims. We aim to investigate whether quasars can be used to independently
verify the parallax zero-point already in early data reductions. This is not
trivially possible as the observation interval is too short to disentangle
parallax and proper motion for the quasar subset.
Methods. We repeat TGAS simulations but additionally include simulated Gaia
observations of quasars from ground-based surveys. All observations are
simulated with basic angle variations. To obtain a full astrometric solution
for the quasars in TGAS we explore the use of prior information for their
proper motions.
Results. It is possible to determine the parallax zero-point for the quasars
with a few {\mu}as uncertainty, and it agrees to a similar precision with the
zero-point for the Tycho-2 stars. The proposed strategy is robust even for
quasars exhibiting significant fictitious proper motion due to a variable
source structure, or when the quasar subset is contaminated with stars
misidentified as quasars.
Conclusions. Using prior information about quasar proper motions we could
provide an independent verification of the parallax zero-point in early
solutions based on less than one year of Gaia data.Comment: Astronomy & Astrophysics, accepted 25 October 2015, in press. Version
2 contains a few language improvements and a terminology change from
'fictitious proper motions' to 'spurious proper motions
Optical Flow in Mostly Rigid Scenes
The optical flow of natural scenes is a combination of the motion of the
observer and the independent motion of objects. Existing algorithms typically
focus on either recovering motion and structure under the assumption of a
purely static world or optical flow for general unconstrained scenes. We
combine these approaches in an optical flow algorithm that estimates an
explicit segmentation of moving objects from appearance and physical
constraints. In static regions we take advantage of strong constraints to
jointly estimate the camera motion and the 3D structure of the scene over
multiple frames. This allows us to also regularize the structure instead of the
motion. Our formulation uses a Plane+Parallax framework, which works even under
small baselines, and reduces the motion estimation to a one-dimensional search
problem, resulting in more accurate estimation. In moving regions the flow is
treated as unconstrained, and computed with an existing optical flow method.
The resulting Mostly-Rigid Flow (MR-Flow) method achieves state-of-the-art
results on both the MPI-Sintel and KITTI-2015 benchmarks.Comment: 15 pages, 10 figures; accepted for publication at CVPR 201
Mosaics from arbitrary stereo video sequences
lthough mosaics are well established as a compact and non-redundant representation of image sequences, their application still suffers from restrictions of the camera motion or has to deal with parallax errors. We present an approach that allows construction of mosaics from arbitrary motion of a head-mounted camera pair. As there are no parallax errors when creating mosaics from planar objects, our approach first decomposes the scene into planar sub-scenes from stereo vision and creates a mosaic for each plane individually. The power of the presented mosaicing technique is evaluated in an office scenario, including the analysis of the parallax error
Radial velocities for the Hipparcos-Gaia Hundred-Thousand-Proper-Motion project
(abridged) The Hundred-Thousand-Proper-Motion (HTPM) project will determine
the proper motions of ~113500 stars using a 23-year baseline. The proper
motions will use the Hipparcos data, with epoch 1991.25, as first epoch and the
first intermediate-release Gaia astrometry, with epoch ~2014.5, as second
epoch. The expected HTPM proper-motion standard errors are 30-190 muas/yr,
depending on stellar magnitude. Depending on the characteristics of an object,
in particular its distance and velocity, its radial velocity can have a
significant impact on the determination of its proper motion. The impact of
this perspective acceleration is largest for fast-moving, nearby stars. Our
goal is to determine, for each star in the Hipparcos catalogue, the
radial-velocity standard error that is required to guarantee a negligible
contribution of perspective acceleration to the HTPM proper-motion precision.
We employ two evaluation criteria, both based on Monte-Carlo simulations, with
which we determine which stars need to be spectroscopically (re-)measured. Both
criteria take the Hipparcos measurement errors into account. For each star in
the Hipparcos catalogue, we determine the confidence level with which the
available radial velocity and its standard error, taken from the XHIP
compilation catalogue, are acceptable. We find that for 97 stars, the radial
velocities available in the literature are insufficiently precise for a 68.27%
confidence level. We also identify 109 stars for which radial velocities are
currently unknown yet need to be acquired to meet the 68.27% confidence level.
To satisfy the radial-velocity requirements coming from our study will be a
daunting task consuming a significant amount of spectroscopic telescope time.
Fortunately, the follow-up spectroscopy is not time-critical since the HTPM
proper motions can be corrected a posteriori once (improved) radial velocities
become available.Comment: Accepted in A&
NASA ExoPAG Study Analysis Group 11: Preparing for the WFIRST Microlensing Survey
NASA's proposed WFIRST-AFTA mission will discover thousands of exoplanets
with separations from the habitable zone out to unbound planets, using the
technique of gravitational microlensing. The Study Analysis Group 11 of the
NASA Exoplanet Program Analysis Group was convened to explore scientific
programs that can be undertaken now, and in the years leading up to WFIRST's
launch, in order to maximize the mission's scientific return and to reduce
technical and scientific risk. This report presents those findings, which
include suggested precursor Hubble Space Telescope observations, a
ground-based, NIR microlensing survey, and other programs to develop and deepen
community scientific expertise prior to the mission.Comment: 35 pages, 5 Figures. A brief overview of the findings is presented in
the Executive Summary (2 pages
Three-dimensional structure of the Upper Scorpius association with the Gaia first data release
Using new proper motion data from recently published catalogs, we revisit the
membership of previously identified members of the Upper Scorpius association.
We confirmed 750 of them as cluster members based on the convergent point
method, compute their kinematic parallaxes and combined them with Gaia
parallaxes to investigate the 3D structure and geometry of the association
using a robust covariance method. We find a mean distance of ~pc
and show that the morphology of the association defined by the brightest (and
most massive) stars yields a prolate ellipsoid with dimensions of
~pc, while the faintest cluster members define a more
elongated structure with dimensions of ~pc. We
suggest that the different properties of both populations is an imprint of the
star formation history in this region.Comment: 5 pages, 1 figure, MNRAS letters (in press
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