2,390 research outputs found
The Gaia reference frame for bright sources examined using VLBI observations of radio stars
Positions and proper motions of Gaia sources are expressed in a reference
frame that ideally should be non-rotating relative to distant extragalactic
objects, coincident with the International Celestial Reference System (ICRS),
and consistent across all magnitudes. For sources fainter than 16th magnitude
this is achieved thanks to Gaia's direct observations of quasars. At brighter
magnitudes it is difficult to validate the quality of the reference frame due
to the scarcity of comparison data. This paper examines the use of VLBI
observations of radio stars to determine the spin and orientation of the bright
reference frame of Gaia. Simultaneous estimation of the six spin and
orientation parameters makes optimal use of VLBI data and makes it possible to
include even single-epoch VLBI observations in the solution. The method is
applied to Gaia Data Release 2 (DR2) using published VLBI data for 41 radio
stars. Results for the 26 best-fitting sources indicate that the bright
reference frame of Gaia DR2 is rotating relative to the faint quasars at a rate
of about 0.1 mas/yr, significant at 2-sigma level. This supports a similar
conclusion based on a comparison with stellar positions in the Hipparcos frame.
The accuracy is currently limited by the small number of radio sources used, by
uncertainties in the Gaia DR2 proper motions, and by the astrophysical nature
of the radio stars. While the origin of the indicated rotation is understood
and can be avoided in future data releases, it remains important to validate
the bright reference frame of Gaia by independent observations. This can be
achieved using VLBI astrometry, which may require re-observing the old sample
of radio stars as well as measuring new objects. The unique historical value of
positional measurements is stressed and VLBI observers are urged to ensure that
relevant positional information is preserved for the future.Comment: 17 pages, 5 figures. Revised version incorporating a Corrigendum
published by A&A. Tables 2-3, Figures 3-5, and Sections 3-5 have been
substantially revise
A conjugate gradient algorithm for the astrometric core solution of Gaia
The ESA space astrometry mission Gaia, planned to be launched in 2013, has
been designed to make angular measurements on a global scale with
micro-arcsecond accuracy. A key component of the data processing for Gaia is
the astrometric core solution, which must implement an efficient and accurate
numerical algorithm to solve the resulting, extremely large least-squares
problem. The Astrometric Global Iterative Solution (AGIS) is a framework that
allows to implement a range of different iterative solution schemes suitable
for a scanning astrometric satellite. In order to find a computationally
efficient and numerically accurate iteration scheme for the astrometric
solution, compatible with the AGIS framework, we study an adaptation of the
classical conjugate gradient (CG) algorithm, and compare it to the so-called
simple iteration (SI) scheme that was previously known to converge for this
problem, although very slowly. The different schemes are implemented within a
software test bed for AGIS known as AGISLab, which allows to define, simulate
and study scaled astrometric core solutions. After successful testing in
AGISLab, the CG scheme has been implemented also in AGIS. The two algorithms CG
and SI eventually converge to identical solutions, to within the numerical
noise (of the order of 0.00001 micro-arcsec). These solutions are independent
of the starting values (initial star catalogue), and we conclude that they are
equivalent to a rigorous least-squares estimation of the astrometric
parameters. The CG scheme converges up to a factor four faster than SI in the
tested cases, and in particular spatially correlated truncation errors are much
more efficiently damped out with the CG scheme.Comment: 24 pages, 16 figures. Accepted for publication in Astronomy &
Astrophysic
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
Maximum likelihood estimation of local stellar kinematics
Context. Kinematical data such as the mean velocities and velocity
dispersions of stellar samples are useful tools to study galactic structure and
evolution. However, observational data are often incomplete (e.g., lacking the
radial component of the motion) and may have significant observational errors.
For example, the majority of faint stars observed with Gaia will not have their
radial velocities measured. Aims. Our aim is to formulate and test a new
maximum likelihood approach to estimating the kinematical parameters for a
local stellar sample when only the transverse velocities are known (from
parallaxes and proper motions). Methods. Numerical simulations using
synthetically generated data as well as real data (based on the
Geneva-Copenhagen survey) are used to investigate the statistical properties
(bias, precision) of the method, and to compare its performance with the much
simpler "projection method" described by Dehnen & Binney (1998). Results. The
maximum likelihood method gives more correct estimates of the dispersion when
observational errors are important, and guarantees a positive-definite
dispersion matrix, which is not always obtained with the projection method.
Possible extensions and improvements of the method are discussed.Comment: 7 pages, 2 figures. Accepted for publication in Astronomy &
Astrophysic
The case for high precision in elemental abundances of stars in the era of large spectroscopic surveys
A number of large spectroscopic surveys of stars in the Milky Way are under
way or are being planned. In this context it is important to discuss the extent
to which elemental abundances can be used as discriminators between different
(known and unknown) stellar populations in the Milky Way. We aim to establish
the requirements in terms of precision in elemental abundances, as derived from
spectroscopic surveys of the Milky Way's stellar populations, in order to
detect interesting substructures in elemental abundance space. We present a
simple relation between the minimum number of stars needed to detect a given
substructure and the precision of the measurements. The results are in
agreement with recent small- and large-scale studies, with high and low
precision, respectively. Large-number statistics cannot fully compensate for
low precision in the abundance measurements and each survey should carefully
evaluate what the main science drivers are for the survey and ensure that the
chosen observational strategy will result in the precision necessary to answer
the questions posed.Comment: 6 pages, 6 figures. Accepted for publication in Astronomy &
Astrophysic
Astrometric signal profile fitting for Gaia
A tool for representation of the one-dimensional astrometric signal of Gaia
is described and investigated in terms of fit discrepancy and astrometric
performance with respect to number of parameters required. The proposed basis
function is based on the aberration free response of the ideal telescope and
its derivatives, weighted by the source spectral distribution. The influence of
relative position of the detector pixel array with respect to the optical image
is analysed, as well as the variation induced by the source spectral emission.
The number of parameters required for micro-arcsec level consistency of the
reconstructed function with the detected signal is found to be 11. Some
considerations are devoted to the issue of calibration of the instrument
response representation, taking into account the relevant aspects of source
spectrum and focal plane sampling. Additional investigations and other
applications are also suggested.Comment: 13 pages, 21 figures, Accepted by MNRAS 2010 January 29. Received
2010 January 28; in original form 2009 September 3
The Hipparcos Transit Data: What, why and how?
The Hipparcos Transit Data are a collection of partially reduced, fully
calibrated observations of (mostly) double and multiple stars obtained with the
ESA Hipparcos astrometry satellite. The data are publicly available, as part of
the CD-ROM set distributed with the Hipparcos and Tycho Catalogues (ESA
SP--1200, 1997), for about a third of the Hipparcos Catalogue entries including
all confirmed or suspected non-single stars. The Transit Data consist of signal
modulation parameters derived from the individual transits of the targets
across the Hipparcos focal grid. The Transit Data permit re-reduction of the
satellite data for individual objects, using arbitrarily complex object models
in which time-variable photometric as well as geometric characteristics may be
taken into account. We describe the structure and contents of the Transit Data
files and give examples of how the data can be used. Some of the applications
use standard astronomical software: Difmap or AIPS for aperture synthesis
imaging, and GaussFit for detailed model fitting. Fortran code converting the
data into formats suitable for these application programs has been made public
in order to encourage and facilitate the use of Hipparcos Transit Data.Comment: A&AS, accepted for publication, 17 pages, 9 figures, 1 Table,
Software available via http://www.astro.lu.se/~lennart/TD/index.html, Figures
4, 5, 6 and 7 need to copied separately, A complete postscript file can be
found at http://www.astro.lu.se/~lennart/TD/ds1699.ps.g
Rigorous treatment of barycentric stellar motion: Perspective and light-time effects in astrometric and radial velocity data
High-precision astrometric and radial-velocity observations require accurate
modelling of stellar motions in order to extrapolate measurements over long
time intervals, and to detect deviations from uniform motion caused for example
by unseen companions. We aim to explore the simplest possible kinematic model
of stellar motions, namely that of uniform rectilinear motion relative to the
Solar System Barycentre, in terms of observable quantities including error
propagation. The apparent path equation for uniform rectilinear motion is
solved analytically in a classical (special-relativistic) framework, leading to
rigorous expressions which relate the (apparent) astrometric parameters and
radial velocity to the (true) kinematic parameters of the star in the
barycentric reference system. We present rigorous and explicit formulae for the
transformation of stellar positions, parallaxes, proper motions, and radial
velocities from one epoch to another, assuming uniform rectilinear motion and
taking into account light-time effects. The Jacobian matrix of the
transformation is also given, allowing accurate and reversible propagation of
errors over arbitrary time intervals. The light-time effects are generally very
small but exceeds 0.1 mas or 0.1 m/s over 100 yr for at least 33 stars in the
Hipparcos Catalogue. For high-velocity stars within a few tens of pc from the
Sun light-time effects are generally more important than the effects of the
curvature of their orbits in the Galactic potential.Comment: Accepted for publication in A&
Astrometry and exoplanets in the Gaia era: a Bayesian approach to detection and parameter recovery
(abridged) We develop Bayesian methods and detection criteria for orbital
fitting, and revise the detectability of exoplanets in light of the in-flight
properties of Gaia. Limiting ourselves to one-planet systems as a first step of
the development, we simulate Gaia data for exoplanet systems over a grid of
S/N, orbital period, and eccentricity. The simulations are then fit using
Markov chain Monte Carlo methods. We investigate the detection rate according
to three information criteria and the delta chi^2. For the delta chi^2, the
effective number of degrees of freedom depends on the mission length. We find
that the choice of the Markov chain starting point can affect the quality of
the results; we therefore consider two limit possibilities: an ideal case, and
a very simple method that finds the starting point assuming circular orbits.
Using Jeffreys' scale of evidence, the fraction of false positives passing a
strong evidence criterion is < ~0.2% (0.6%) when considering a 5 yr (10 yr)
mission and using the Akaike information criterion or the Watanabe-Akaike
information criterion, and <0.02% (<0.06%) when using the Bayesian information
criterion. We find that there is a 50% chance of detecting a planet with a
minimum S/N=2.3 (1.7). This sets the maximum distance to which a planet is
detectable to ~70 pc and ~3.5 pc for a Jupiter-mass and Neptune-mass planet,
respectively, assuming a 10 yr mission, a 4 au semi-major axis, and a 1 M_sun
star. The period is the orbital parameter that can be determined with the best
accuracy, with a median relative difference between input and output periods of
4.2% (2.9%) assuming a 5 yr (10 yr) mission. The median accuracy of the
semi-major axis of the orbit can be recovered with a median relative error of
7% (6%). The eccentricity can also be recovered with a median absolute accuracy
of 0.07 (0.06).Comment: 18 pages, 11 figures. New version accepted by A&A for publicatio
The Tycho-Gaia astrometric solution. How to get 2.5 million parallaxes with less than one year of Gaia data
Context. The first release of astrometric data from Gaia will contain the
mean stellar positions and magnitudes from the first year of observations, and
proper motions from the combination of Gaia data with Hipparcos prior
information (HTPM).
Aims. We study the potential of using the positions from the Tycho-2
Catalogue as additional information for a joint solution with early Gaia data.
We call this the Tycho-Gaia astrometric solution (TGAS).
Methods. We adapt Gaia's Astrometric Global Iterative Solution (AGIS) to
incorporate Tycho information, and use simulated Gaia observations to
demonstrate the feasibility of TGAS and to estimate its performance.
Results. Using six to twelve months of Gaia data, TGAS could deliver
positions, parallaxes and annual proper motions for the 2.5 million Tycho-2
stars, with sub-milliarcsecond accuracy. TGAS overcomes some of the limitations
of the HTPM project and allows its execution half a year earlier. Furthermore,
if the parallaxes from Hipparcos are not incorporated in the solution, they can
be used as a consistency check of the TGAS/HTPM solution.Comment: Accepted for publication in A&A, 24 Dec 201
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