766 research outputs found
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&
Gaia astrometry for stars with too few observations - a Bayesian approach
Gaia's astrometric solution aims to determine at least five parameters for
each star, together with appropriate estimates of their uncertainties and
correlations. This requires at least five distinct observations per star. In
the early data reductions the number of observations may be insufficient for a
five-parameter solution, and even after the full mission many stars will remain
under-observed, including faint stars at the detection limit and transient
objects. In such cases it is reasonable to determine only the two position
parameters. Their formal uncertainties would however grossly underestimate the
actual errors, due to the neglected parallax and proper motion. We aim to
develop a recipe to calculate sensible formal uncertainties that can be used in
all cases of under-observed stars. Prior information about the typical ranges
of stellar parallaxes and proper motions is incorporated in the astrometric
solution by means of Bayes' rule. Numerical simulations based on the Gaia
Universe Model Snapshot (GUMS) are used to investigate how the prior influences
the actual errors and formal uncertainties when different amounts of Gaia
observations are available. We develop a criterion for the optimum choice of
priors, apply it to a wide range of cases, and derive a global approximation of
the optimum prior as a function of magnitude and galactic coordinates. The
feasibility of the Bayesian approach is demonstrated through global astrometric
solutions of simulated Gaia observations. With an appropriate prior it is
possible to derive sensible positions with realistic error estimates for any
number of available observations. Even though this recipe works also for
well-observed stars it should not be used where a good five-parameter
astrometric solution can be obtained without a prior. Parallaxes and proper
motions from a solution using priors are always biased and should not be used.Comment: Revised version, accepted 21st of August 2015 for publication in A&
Impact of basic angle variations on the parallax zero point for a scanning astrometric satellite
Determination of absolute parallaxes by means of a scanning astrometric
satellite such as Hipparcos or Gaia relies on the short-term stability of the
so-called basic angle between the two viewing directions. Uncalibrated
variations of the basic angle may produce systematic errors in the computed
parallaxes. We examine the coupling between a global parallax shift and
specific variations of the basic angle, namely those related to the satellite
attitude with respect to the Sun. The changes in observables produced by small
perturbations of the basic angle, attitude, and parallaxes are calculated
analytically. We then look for a combination of perturbations that has no net
effect on the observables. In the approximation of infinitely small fields of
view, it is shown that certain perturbations of the basic angle are
observationally indistinguishable from a global shift of the parallaxes. If
such perturbations exist, they cannot be calibrated from the astrometric
observations but will produce a global parallax bias. Numerical simulations of
the astrometric solution, using both direct and iterative methods, confirm this
theoretical result. For a given amplitude of the basic angle perturbation, the
parallax bias is smaller for a larger basic angle and a larger solar aspect
angle. In both these respects Gaia has a more favourable geometry than
Hipparcos. In the case of Gaia, internal metrology is used to monitor basic
angle variations. Additionally, Gaia has the advantage of detecting numerous
quasars, which can be used to verify the parallax zero point.Comment: 8 pages, 2 figures; Accepted for publication in Astronomy &
Astrophysic
Electrode level Monte Carlo model of radiation damage effects on astronomical CCDs
Current optical space telescopes rely upon silicon Charge Coupled Devices
(CCDs) to detect and image the incoming photons. The performance of a CCD
detector depends on its ability to transfer electrons through the silicon
efficiently, so that the signal from every pixel may be read out through a
single amplifier. This process of electron transfer is highly susceptible to
the effects of solar proton damage (or non-ionizing radiation damage). This is
because charged particles passing through the CCD displace silicon atoms,
introducing energy levels into the semi-conductor bandgap which act as
localized electron traps. The reduction in Charge Transfer Efficiency (CTE)
leads to signal loss and image smearing. The European Space Agency's
astrometric Gaia mission will make extensive use of CCDs to create the most
complete and accurate stereoscopic map to date of the Milky Way. In the context
of the Gaia mission CTE is referred to with the complementary quantity Charge
Transfer Inefficiency (CTI = 1-CTE). CTI is an extremely important issue that
threatens Gaia's performances. We present here a detailed Monte Carlo model
which has been developed to simulate the operation of a damaged CCD at the
pixel electrode level. This model implements a new approach to both the charge
density distribution within a pixel and the charge capture and release
probabilities, which allows the reproduction of CTI effects on a variety of
measurements for a large signal level range in particular for signals of the
order of a few electrons. A running version of the model as well as a brief
documentation and a few examples are readily available at
http://www.strw.leidenuniv.nl/~prodhomme/cemga.php as part of the CEMGA java
package (CTI Effects Models for Gaia).Comment: Accepted by MNRAS on 13 February 2011. 15 pages, 7 figures and 5
table
Photometric colors of late-type giants: theory versus observations
To assess the current status in the theoretical modeling of the spectral
properties of late-type giants, we provide a comparison of synthetic
photometric colors of late-type giants (calculated with PHOENIX, MARCS and
ATLAS model atmospheres) with observations, at [M/H]=0.0 and -2.0. Overall,
there is a good agreement between observed and synthetic colors, and synthetic
colors and published Teff-color relations, both at [M/H]=0.0 and -2.0.
Deviations from the observed trends in Teff-color planes are generally within
\pm 150K (or less) in the effective temperature range Teff=3500-4800K.
Synthetic colors calculated with different stellar atmosphere models typically
agree to ~100K, within a large range of effective temperatures and gravities.
Some discrepancies are seen in the Teff-(B-V) plane below Teff~3800K at
[M/H]=0.0, due to difficulties in reproducing the 'turn-off' to the bluer
colors which is seen in the observed data at Teff~3600K. Note that at
[M/H]=-2.0 effective temperatures given by the scale of Alonso et al. (1999)
are generally lower than those resulting from other Teff-color relations based
both on observed and synthetic colors.Comment: 2 pages, 1 figure. Proceedings of the IAU Symposium 232 "The
Scientific Requirements for Extremely Large Telescopes", eds. P. Whitelock,
B. Leibundgut, and M. Dennefel
The impact of CCD radiation damage on Gaia astrometry: I. Image location estimation in the presence of radiation damage
The Gaia mission has been designed to perform absolute astrometric
measurements with unprecedented accuracy; the end-of-mission parallax standard
error is required to be 30 micro-arcseconds for a G2V type star of magnitude
15. These requirements set a stringent constraint on the accuracy of the
estimation of the location of the stellar image on the CCD for each
observation: e.g., 0.3 milli-arseconds (mas) or 0.005 pixels for the same V=15
G2V star. However the Gaia CCDs will suffer from charge transfer inefficiency
(CTI) caused by radiation damage that will degrade the stellar image quality
and may degrade the astrometric performance of Gaia if not properly addressed.
For the first time at this level of detail, the potential impact of radiation
damage on the performance of Gaia is investigated. In this first paper we focus
on the evaluation of the CTI impact on the image location accuracy. We show
that CTI decreases the stellar image signal-to-noise ratio and irreversibly
degrades the image location estimation precision. As a consequence the location
estimation standard errors increase by up to 6% for a radiation damage level
equivalent to the end-of-mission. In addition the CTI-induced image distortion
introduces a systematic bias in the image location estimation (up to 0.05
pixels or 3 mas in the Gaia operating conditions). We present a novel approach
to CTI mitigation that enables, without correction of the raw data, the
unbiased estimation of the image location and flux from damaged observations.
Its implementation reduces the maximum measured location bias for the faintest
magnitude to 0.005 pixels (~4e-4 pixels at magnitude 15). In a second paper we
will investigate how the CTI effects affect the final astrometric accuracy of
Gaia by propagating residual errors through the astrometric solution.Comment: 23 figures, 6 tables, accepted for publication in the Monthly Notices
of the Royal Astronomical Society the 4th of October 201
Space-borne global astrometric surveys: the hunt for extra-solar planets
The proposed global astrometry mission {\it GAIA}, recently recommended
within the context of ESA's Horizon 2000 Plus long-term scientific program,
appears capable of surveying the solar neighborhood within 200 pc for
the astrometric signatures of planets around stars down to the magnitude limit
of =17 mag, which includes late M dwarfs at 100 pc. Realistic end-to-end
simulations of the GAIA global astrometric measurements have yielded first
quantitative estimates of the sensitivity to planetary perturbations and of the
ability to measure their orbital parameters. Single Jupiter-mass planets around
normal solar-type stars appear detectable up to 150 pc (12 mag) with
probabilities 50 per cent for orbital periods between 2.5 and
8 years, and their orbital parameters measured with better than 30 per
cent accuracy to about 100 pc. Jupiter-like objects (same mass and period as
our giant planet) are found with similar probabilities up to 100 pc.These first
experiments indicate that the {\it GAIA} results would constitute an important
addition to those which will come from the other ongoing and planned
planet-search programs. These data combined would provide a formidable testing
ground on which to confront theories of planetary formation and evolution.Comment: 13 pages, 10 figures, uses mn.sty, accepted by MNRA
Fourteen New Companions from the Keck & Lick Radial Velocity Survey Including Five Brown Dwarf Candidates
We present radial velocities for 14 stars on the California & Carnegie Planet
Search target list that reveal new companions. One star, HD 167665, was fit
with a definitive Keplerian orbit leading to a minimum mass for the companion
of 50.3 Mjup at a separation from its host of ~5.5 AU. Incomplete or limited
phase coverage for the remaining 13 stars prevents us from assigning to them
unique orbital parameters. Instead, we fit their radial velocities with
Keplerian orbits across a grid of fixed values for Msini and period, P, and use
the resulting reduced chi-square surface to place constraints on Msini, P, and
semimajor axis, a. This technique allowed us to restrict Msini below the brown
dwarf -- stellar mass boundary for an additional 4 companions (HD 150554, HD
8765, HD 72780, HD 74014). If the combined 5 companions are confirmed as brown
dwarfs, these results would comprise the first major catch of such objects from
our survey beyond ~3 AU.Comment: 29 pages, 14 figures, accepted to Ap
GAIA: Composition, Formation and Evolution of the Galaxy
The GAIA astrometric mission has recently been approved as one of the next
two `cornerstones' of ESA's science programme, with a launch date target of not
later than mid-2012. GAIA will provide positional and radial velocity
measurements with the accuracies needed to produce a stereoscopic and kinematic
census of about one billion stars throughout our Galaxy (and into the Local
Group), amounting to about 1 per cent of the Galactic stellar population.
GAIA's main scientific goal is to clarify the origin and history of our Galaxy,
from a quantitative census of the stellar populations. It will advance
questions such as when the stars in our Galaxy formed, when and how it was
assembled, and its distribution of dark matter. The survey aims for
completeness to V=20 mag, with accuracies of about 10 microarcsec at 15 mag.
Combined with astrophysical information for each star, provided by on-board
multi-colour photometry and (limited) spectroscopy, these data will have the
precision necessary to quantify the early formation, and subsequent dynamical,
chemical and star formation evolution of our Galaxy. Additional products
include detection and orbital classification of tens of thousands of
extra-Solar planetary systems, and a comprehensive survey of some 10^5-10^6
minor bodies in our Solar System, through galaxies in the nearby Universe, to
some 500,000 distant quasars. It will provide a number of stringent new tests
of general relativity and cosmology. The complete satellite system was
evaluated as part of a detailed technology study, including a detailed payload
design, corresponding accuracy assesments, and results from a prototype data
reduction development.Comment: Accepted by A&A: 25 pages, 8 figure
The spectroscopic orbit of Capella revisited
Context. Capella is among the few binary stars with two evolved giant
components. The hotter component is a chromospherically active star within the
Hertzsprung gap, while the cooler star is possibly helium-core burning. Aims.
The known inclination of the orbital plane from astrometry in combination with
precise radial velocities will allow very accurate masses to be determined for
the individual Capella stars. This will constrain their evolutionary stage and
possibly the role of the active star's magnetic field on the dynamical
evolution of the binary system. Methods. We obtained a total of 438
high-resolution \'echelle spectra during the years 2007-2010 and used the
measured velocities to recompute the orbital elements. Our double-lined orbital
solution yields average residuals of 64 m/s for the cool component and 297 m/s
for the more rapidly rotating hotter component. Results. The semi-amplitude of
the cool component is smaller by 0.045 km/s than the orbit determination of
Torres et al. from data taken during 1996-1999 but more precise by a factor of
5.5, while for the hotter component it is larger by 0.580 km/s and more precise
by a factor of 3.6. This corresponds to masses of 2.573\pm0.009 M_sun and
2.488\pm0.008 M_sun for the cool and hot component, respectively. Their
relative errors of 0.34% and 0.30% are about half of the values given in Torres
et al. for a combined literature- data solution but with absolute values
different by 4% and 2% for the two components, respectively. The mass ratio of
the system is therefore q = M_A/M_B = 0.9673 \pm 0.0020. Conclusions. Our orbit
is the most precise and also likely to be the most accurate ever obtained for
Capella
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