659 research outputs found
First radius measurements of very low mass stars with the VLTI
e present 4 very low mass stars radii measured with the VLTI using the 2.2
microns VINCI test instrument. The observations were carried out during the
commissioning of the 104-meter-baseline with two 8-meter-telescopes. We measure
angular diameters of 0.7-1.5 mas with accuracies of 0.04-0.11 mas, and for
spectral type ranging from M0V to M5.5V. We determine an empirical mass-radius
relation for M dwarfs based on all available radius measurements. The observed
relation agrees well with theoretical models at the present accuracy level,
with possible discrepancy around 0.5-0.8 Msolar that needs to be confirmed. In
the near future, dozens of M dwarfs radii will be measured with 0.1-1%
accuracy, with the VLTI, thanks to the improvements expected from the near
infrared instrument AMBER. This will bring strong observational constraints on
both atmosphere and interior physics.Comment: Accepted for publication in Astronomy and Astrophysics Letters, 4
pages, 3 figure
Analytical determination of orbital elements using Fourier analysis. I. The radial velocity case
We describe an analytical method for computing the orbital parameters of a
planet from the periodogram of a radial velocity signal. The method is very
efficient and provides a good approximation of the orbital parameters. The
accuracy is mainly limited by the accuracy of the computation of the Fourier
decomposition of the signal which is sensitive to sampling and noise. Our
method is complementary with more accurate (and more expensive in computer
time) numerical algorithms (e.g. Levenberg-Marquardt, Markov chain Monte Carlo,
genetic algorithms). Indeed, the analytical approximation can be used as an
initial condition to accelerate the convergence of these numerical methods. Our
method can be applied iteratively to search for multiple planets in the same
system.Comment: accepted to A&
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
Analytical determination of orbital elements using Fourier analysis. II. Gaia astrometry and its combination with radial velocities
The ESA global astrometry space mission Gaia has been monitoring the position
of a billion stars since 2014. The analysis of such a massive dataset is
challenging in terms of the data processing involved. In particular, the blind
detection and characterization of single or multiple companions to stars
(planets, brown dwarfs, or stars) using Gaia astrometry requires highly
efficient algorithms. In this article, we present a set of analytical methods
to detect and characterize companions in scanning space astrometric time series
as well as via a combination of astrometric and radial velocity time series. We
propose a general linear periodogram framework and we derive analytical
formulas for the false alarm probability (FAP) of periodogram peaks. Once a
significant peak has been identified, we provide analytical estimates of all
the orbital elements of the companion based on the Fourier decomposition of the
signal. The periodogram, FAP, and orbital elements estimates can be computed
for the astrometric and radial velocity time series separately or in tandem.
These methods are complementary with more accurate and more computationally
intensive numerical algorithms (e.g., least-squares minimization, Markov chain
Monte Carlo, genetic algorithms). In particular, our analytical approximations
can be used as an initial condition to accelerate the convergence of numerical
algorithms. Our formalism has been partially implemented in the Gaia exoplanet
pipeline for the third Gaia data release. Since the Gaia astrometric time
series are not yet publicly available, we illustrate our methods on the basis
of Hipparcos data, together with on-ground CORALIE radial velocities, for three
targets known to host a companion: HD 223636 (HIP 117622), HD 17289 (HIP
12726), and HD 3277 (HIP 2790).Comment: Accepted in A&
DE082349 is a juvenile binary brown dwarf at 20.7 pc
Astrometric monitoring of the nearby early-L dwarf DE082349 has revealed a
low-mass companion in a 248-day orbit that was announced in an earlier work.
Here, we present new astrometric and spectroscopic observations that allow us
to characterise the system in detail. The optical spectrum shows LiI-absorption
indicative of a young age and/or substellar mass for the primary component. The
near-infrared spectrum is best reproduced by a binary system of brown dwarfs
with spectral types of L1.5 L5.5 and effective temperatures of
K and K. To conform with the photocentric orbit size measured with
astrometry and the current understanding of substellar evolution, the system
must have an age in the 80--500 Myr range. Evolutionary models predict
component masses in the ranges of and
with a mass ratio of .
Multi-epoch radial velocity measurements unambiguously establish the
three-dimensional orbit of the system and allow us to investigate its kinematic
properties. DE082349 emerges as a rare example of a nearby brown dwarf
binary with orbit, component properties, and age that are characterised well.
It is a juvenile resident of the solar neighbourhood, but does not appear to
belong to a known young association or moving group.Comment: 9 pages, 11 figures. Accepted for publication in A&
The HARPS search for southern extrasolar planets XXV. Results from the metal-poor sample
Searching for extrasolar planets around stars of different metallicity may
provide strong constraints to the models of planet formation and evolution. In
this paper we present the overall results of a HARPS (a high-precision
spectrograph mostly dedicated to deriving precise radial velocities) program to
search for planets orbiting a sample of 104 metal-poor stars (selected [Fe/H]
below -0.5). Radial velocity time series of each star are presented and
searched for signals using several statistical diagnostics. Stars with detected
signals are presented, including 3 attributed to the presence of previously
announced giant planets orbiting the stars HD171028, HD181720, and HD190984.
Several binary stars and at least one case of a coherent signal caused by
activity-related phenomena are presented. One very promising new, possible
giant planet orbiting the star HD107094 is discussed, and the results are
analyzed in light of the metallicity-giant planet correlation. We conclude that
the frequency of giant planets orbiting metal-poor stars may be higher than
previously thought, probably reflecting the higher precision of the HARPS
survey. In the metallicity domain of our sample, we also find evidence that the
frequency of planets is a steeply rising function of the stellar metal content,
as found for higher metallicity stars.Comment: Accepted for publication in A&
Planets and Stellar Activity: Hide and Seek in the CoRoT-7 system
Since the discovery of the transiting super-Earth CoRoT-7b, several
investigations have yielded different results for the number and masses of
planets present in the system, mainly owing to the star's high level of
activity. We re-observed CoRoT-7 in January 2012 with both HARPS and CoRoT, so
that we now have the benefit of simultaneous radial-velocity and photometric
data. This allows us to use the off-transit variations in the star's light
curve to estimate the radial-velocity variations induced by the suppression of
convective blueshift and the flux blocked by starspots. To account for
activity-related effects in the radial-velocities which do not have a
photometric signature, we also include an additional activity term in the
radial-velocity model, which we treat as a Gaussian process with the same
covariance properties (and hence the same frequency structure) as the light
curve. Our model was incorporated into a Monte Carlo Markov Chain in order to
make a precise determination of the orbits of CoRoT-7b and CoRoT-7c. We measure
the masses of planets b and c to be 4.73 +/- 0.95 Mearth and 13.56 +/- 1.08
Mearth, respectively. The density of CoRoT-7b is (6.61 +/- 1.72)(Rp/1.58
Rearth)^(-3) g.cm^(-3), which is compatible with a rocky composition. We search
for evidence of an additional planet d, identified by previous authors with a
period close to 9 days. We are not able to confirm the existence of a planet
with this orbital period, which is close to the second harmonic of the stellar
rotation at around 7.9 days. Using Bayesian model selection we find that a
model with two planets plus activity-induced variations is most favoured.Comment: Accepted 2014 July 2. Received 2014 June 30; in original form 2013
May 30 (17 pages, 9 figures
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