59 research outputs found
Gaia and VLT astrometry of faint stars: Precision of Gaia DR1 positions and updated VLT parallaxes of ultracool dwarfs
We compared positions of the Gaia first data release (DR1) secondary data set
at its faint limit with CCD positions of stars in 20 fields observed with the
VLT/FORS2 camera. The FORS2 position uncertainties are smaller than one
milli-arcsecond (mas) and allowed us to perform an independent verification of
the DR1 astrometric precision. In the fields that we observed with FORS2, we
projected the Gaia DR1 positions into the CCD plane, performed a polynomial fit
between the two sets of matching stars, and carried out statistical analyses of
the residuals in positions. The residual RMS roughly matches the expectations
given by the Gaia DR1 uncertainties, where we identified three regimes in terms
of Gaia DR1 precision: for G = 17-20 stars we found that the formal DR1
position uncertainties of stars with DR1 precisions in the range of 0.5-5 mas
are underestimated by 63 +/- 5\%, whereas the DR1 uncertainties of stars in the
range 7-10 mas are overestimated by a factor of two. For the best-measured and
generally brighter G = 16-18 stars with DR1 positional uncertainties of <0.5
mas, we detected 0.44 +/- 0.13 mas excess noise in the residual RMS, whose
origin can be in both FORS2 and Gaia DR1. By adopting Gaia DR1 as the absolute
reference frame we refined the pixel scale determination of FORS2, leading to
minor updates to the parallaxes of 20 ultracool dwarfs that we published
previously. We also updated the FORS2 absolute parallax of the Luhman 16 binary
brown dwarf system to 501.42 +/- 0.11 masComment: 7 pages, 4 figures, 2 tables, accepted for publication in A&A on
August 1, 201
Astrometric search for a planet around VB 10
We observed VB 10 in August and September 2009 using the FORS2 camera of the
VLT with the aim of measuring its astrometric motion and of probing the
presence of the announced planet VB 10b. We used the published STEPS
astrometric positions of VB 10 over a time-span of 9 years, which allowed us to
compare the expected motion of VB 10 due to parallax and proper motion with the
observed motion and to compute precise deviations. The achieved single-epoch
precisions of our observations are about 0.1 mas and the data showed no
significant residual trend, while the presence of the planet should have
induced an apparent proper motion larger than 10 mas/yr. Subtraction of the
predicted orbital motion from the observed data produces a large trend in
position residuals of VB 10. We estimated the probability that this trend is
caused by random noise. Taking all the uncertainties into account and using
Monte-Carlo resampling of the data, we are able to reject the existence of VB
10b with the announced mass of 6.4 M_J with the false alarm probability of only
0.0005. A 3.2 M_J planet is also rejected with a false alarm probability of
0.023.Comment: 6 pages, 6 figures, 2 tables, accepted for publication in A&
Precision multi-epoch astrometry with VLT cameras FORS1/2
We investigate the astrometric performance of the FORS1 and FORS2 cameras of
the VLT at long time scales with emphasis on systematic errors which normally
prevent attainning a precision better than 1mas. The study is based on multi-
epoch time series of observations of a single sky region imaged with a time
spacing of 2-6 years at FORS1 and 1-5 months at FORS2. We performed a detailed
analysis of a random error of positions that was shown to be dominated by the
uncertainty of the star photocenter determination. The component of the random
error corresponding to image motion was found to be caused primarily by optical
aberrations and variations of atmospheric PSF size but not by the effect of
atmospheric image motion. Comparison of observed and model annual/monthly epoch
average positions yielded estimates of systematic errors for which temporal
properties and distribution in the CCD plane are given. At frame center, the
systematic component is about 25 mu-as. Systematic errors are shown to be
caused mainly by a combined effect of the image asymmetry and seeing variations
which therefore should be strongly limited to avoid generating random and
systematic errors. For a series of 30 images, we demonstrated presicion of
about 50 mu-as stable on daily, monthly, and annual time scales. Relative
proper motion and trigonometric parallaxes of stars in the center of the test
field were derived with a precision of 20 mu-as/yr and 40 mu-as for 17-19 mag
stars.Comment: 16 pages, 16 figures, 4 tables, accepted in A&A; typos and language
corrections; version sent to the printe
Filtration of atmospheric noise in narrow-field astrometry with very large telescopes
This paper presents a non-classic approach to narrow field astrometry that
offers a significant improvement over conventional techniques due to enhanced
reduction of atmospheric image motion. The method is based on two key elements:
apodization of the entrance pupil and the enhanced virtual symmetry of
reference stars. Symmetrization is implemented by setting special weights to
each reference star. Thus a reference field itself forms a virtual net filter
that effectively attenuates the image motion spectrum. Atmospheric positional
error was found to follow a power dependency ~ R^{K \mu /2} D^{-K/2+1/3} on
angular field size R and aperture D; here K is some optional even integer
2<=K<=sqrt{8N+1}-1 limited by a number N of reference stars, and \mu <= 1 is a
term dependent on K and the magnitude and sky star distribution in the field.
As compared to conventional techniques for which K=2, the improvement in
accuracy increases by some orders. Limitations to astrometric performance of
monopupil large ground-based telescopes are estimated. The total atmospheric
and photon noise for at a 10 m telescope at good 0.4" seeing was found to be,
depending on sky star density, 10 to 60 microarcsec per 10 min exposure in R
band. For a 100 m telescope and FWHM=0.1" (low-order adaptive optics
corrections) the potential accuracy is 0.2 to 2 microarcsec.Comment: 18 pages, 17 figure
Astrometric detection of exoplanets from the ground
Astrometry is a powerful technique to study the populations of extrasolar
planets around nearby stars. It gives access to a unique parameter space and is
therefore required for obtaining a comprehensive picture of the properties,
abundances, and architectures of exoplanetary systems. In this review, we
discuss the scientific potential, present the available techniques and
instruments, and highlight a few results of astrometric planet searches, with
an emphasis on observations from the ground. In particular, we discuss
astrometric observations with the Very Large Telescope (VLT) Interferometer and
a programme employing optical imaging with a VLT camera, both aimed at the
astrometric detection of exoplanets. Finally, we set these efforts into the
context of Gaia, ESA's astrometry mission scheduled for launch in 2013, and
present an outlook on the future of astrometric exoplanet detection from the
ground.Comment: 9 pages, 3 figures. Invited contribution to the SPIE conference
"Techniques and Instrumentation for Detection of Exoplanets VI" held in San
Diego, CA, August 25-29, 201
High-precision astrometry on the VLT/FORS1 at time scales of few days
We investigate the accuracy of astrometric measurements with the VLT/FORS1
camera and consider potential applications. The study is based on two-epoch
(2000 and 2002/2003) frame series of observations of a selected Galactic Bulge
sky region that were obtained with FORS1 during four consecutive nights each.
Reductions were carried out with a novel technique that eliminates atmospheric
image motion and does not require a distinction between targets and reference
objects. The positional astrometric precision was found to be limited only by
the accuracy of the determination of the star photocentre, which is typically
200-300 microarcsec per single measurement for bright unsaturated stars
B=18-19. Several statistical tests have shown that at time-scales of 1-4 nights
the residual noise in measured positions is essentially a white noise with no
systematic instrumental signature and no significant deviation from a Gaussian
distribution. Some evidence of a good astrometric quality of the VLT for frames
separated by two years has also been found. Our data show that the VLT with
FORS1/2 cameras can be effectively used for astrometric observations of
planetary microlensing events and other applications where a high accuracy is
required, that is expected to reach 30-40 microarcsec for a series of 50 frames
(one hours with R filter).Comment: 11 pages, 9 figures, accepted for publication in A&
Astrometric planet search around southern ultracool dwarfs II: Astrometric reduction methods and a deep astrometric catalogue
We describe the astrometric reduction of images obtained with the FORS2/VLT
camera in the framework of an astrometric planet search around 20
M/L-transition dwarfs. We present the correction of systematic errors, the
achieved astrometric performance, and a new astrometric catalogue containing
the faint reference stars in 20 fields located close to the Galactic plane. We
detected three types of systematic errors in the FORS2 astrometry: the relative
motion of the camera's two CCD chips, errors that are correlated in space, and
an error contribution of yet unexplained origin. The relative CCD motion has
probably a thermal origin and usually is 0.001-0.010 px (~0.1-1 mas), but
sometimes amounts to 0.02-0.05 px (3-6 mas). This instability and
space-correlated errors are detected and mitigated using reference stars. The
third component of unknown origin has an amplitude of 0.03-0.14 mas and is
independent of the observing conditions. We find that a consecutive sequence of
32 images of a well-exposed star over 40 min at 0.6" seeing results in a median
r.m.s. of the epoch residuals of 0.126 mas. Overall, the epoch residuals are
distributed according to a normal law with a chi2~1. We compiled a catalogue of
12000 stars with I-band magnitudes of 16-22 located in 20 fields, each covering
~2x2'. It contains I-band magnitudes, ICRF positions with 40-70 mas precision,
and relative proper motions and absolute trigonometric parallaxes with a
precision of 0.1 mas/yr and 0.1 mas at the bright end, respectively.Comment: 17 pages, 19 figures, 4 tables, accepted for publication in A&A on
March 14, 201
Astrometric planet search around southern ultracool dwarfs III. Discovery of a brown dwarf in a 3-year orbit around DE0630-18
Using astrometric measurements obtained with the FORS2/VLT camera, we are
searching for low-mass companions around 20 nearby ultracool dwarfs. With a
single-measurement precision of 0.1 milli-arcseconds, our survey is sensitive
to a wide range of companion masses from planetary companions to binary
systems. Here, we report the discovery and orbit characterisation of a new
ultracool binary at a distance of 19.5 pc from Earth that is composed of the
M8.5-dwarf primary DE0630-18 and a substellar companion. The nearly edge-on
orbit is moderately eccentric (e=0.23) with an orbital period of 1120 d, which
corresponds to a relative separation in semimajor axis of approximately 1.1 AU.
We obtained a high-resolution optical spectrum with UVES/VLT and measured the
system's heliocentric radial velocity. The spectrum does not exhibit lithium
absorption at 670.8 nm, indicating that the system is not extremely young. A
preliminary estimate of the binary's physical parameters tells us that it is
composed of a primary at the stellar-substellar limit and a massive brown-dwarf
companion. DE0630-18 is a new very low-mass binary system with a
well-characterised orbit.Comment: 4 pages, 7 figures. Accepted for publication in A&
Astrometric orbit of a low-mass companion to an ultracool dwarf
Little is known about the existence of extrasolar planets around ultracool
dwarfs. Furthermore, binary stars with Sun-like primaries and very low-mass
binaries composed of ultracool dwarfs show differences in the distributions of
mass ratio and orbital separation that can be indicative of distinct formation
mechanisms. Using FORS2/VLT optical imaging for high precision astrometry we
are searching for planets and substellar objects around ultracool dwarfs to
investigate their multiplicity properties for very low companion masses. Here
we report astrometric measurements with an accuracy of two tenths of a
milli-arcsecond over two years that reveal orbital motion of the nearby L1.5
dwarf DENIS-P J082303.1-491201 located at 20.77 +/- 0.08 pc caused by an unseen
companion that revolves about its host on an eccentric orbit in 246.4 +/- 1.4
days. We estimate the L1.5 dwarf to have 7.5 +/- 0.7 % of the Sun's mass that
implies a companion mass of 28 +/- 2 Jupiter masses. This new system has the
smallest mass ratio (0.36 +/- 0.02) of known very low-mass binaries with
characterised orbits. With this discovery we demonstrate 200 micro-arcsecond
astrometry over an arc-minute field and over several years that is sufficient
to discover sub-Jupiter mass planets around ultracool dwarfs. We also show that
the achieved parallax accuracy of < 0.4 % makes it possible to remove distance
as a dominant source of uncertainty in the modelling of ultracool dwarfs.Comment: 9 pages, 8 figures, accepted for publication in Astronomy and
Astrophysics. The reduced astrometry data will be made publically available
through the CD
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