102 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
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
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
Astrometric radial velocities. I. Non-spectroscopic methods for measuring stellar radial velocity
High-accuracy astrometry permits the determination of not only stellar
tangential motion, but also the component along the line-of-sight. Such
non-spectroscopic (i.e. astrometric) radial velocities are independent of
stellar atmospheric dynamics, spectral complexity and variability, as well as
of gravitational redshift. Three methods are analysed: (1) changing annual
parallax, (2) changing proper motion and (3) changing angular extent of a
moving group of stars. All three have significant potential in planned
astrometric projects. Current accuracies are still inadequate for the first
method, while the second is marginally feasible and is here applied to 16
stars. The third method reaches high accuracy (<1 km/s) already with present
data, although for some clusters an accuracy limit is set by uncertainties in
the cluster expansion rate.Comment: 13 pages, 2 figures. Accepted for publication in Astronomy &
Astrophysics (main journal
The Gaia inertial reference frame and the tilting of the Milky Way disk
While the precise relationship between the Milky Way disk and the symmetry
planes of the dark matter halo remains somewhat uncertain, a time-varying disk
orientation with respect to an inertial reference frame seems probable.
Hierarchical structure formation models predict that the dark matter halo is
triaxial and tumbles with a characteristic rate of ~2 rad/Hubble time (~30
muas/yr). These models also predict a time-dependent accretion of gas, such
that the angular momentum vector of the disk should be misaligned with that of
the halo. These effects, as well as tidal effects of the LMC, will result in
the rotation of the angular momentum vector of the disk population with respect
to the quasar reference frame. We assess the accuracy with which the positions
and proper motions from Gaia can be referred to a kinematically non-rotating
system, and show that the spin vector of the transformation from any rigid
self-consistent catalog frame to the quasi-inertial system defined by quasars
should be defined to better than 1 muas/yr. Determination of this inertial
frame by Gaia will reveal any signature of the disk orientation varying with
time, improve models of the potential and dynamics of the Milky Way, test
theories of gravity, and provide new insights into the orbital evolution of the
Sagittarius dwarf galaxy and the Magellanic Clouds.Comment: 16 pages; accepted for publication in Ap
Error characterization of the Gaia astrometric solution I. Mathematical basis of the covariance expansion model
Context. Accurate characterization of the astrometric errors in the forthcoming Gaia Catalogue will be essential for making optimal use of the data. This includes the correlations among the estimated astrometric parameters of the stars as well as their standard uncertainties, i.e., the complete (variance-)covariance matrix of the relevant astrometric parameters. Aims. Because a direct computation of the covariance matrix is infeasible due to the large number of parameters, approximate methods must be used. The aim of this paper is to provide a mathematical basis for estimating the variance-covariance of any pair of astrometric parameters, and more generally the covariance matrix for multidimensional functions of the astrometric parameters. The validation of this model by means of numerical simulations will be considered in a forthcoming paper. Methods. Based on simplifying assumptions (in particular that calibration errors can be neglected), we derive and analyse a series expansion of the covariance matrix of the least-squares solution. A recursive relation for successive terms is derived and interpreted in terms of the propagation of errors from the stars to the attitude and back. We argue that the expansion should converge rapidly to useful precision. The recursion is vastly simplified by using a kinematographic (step-wise) approximation of the attitude model. Results. Low-order approximations of arbitrary elements from the covariance matrix can be computed efficiently in terms of a limited amount of pre-computed data representing compressed observations and the structural relationships among them. It is proposed that the user interface to the Gaia Catalogue should provide the tools necessary for such computations
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