626 research outputs found
Orbital Stability of Planets in Binary Systems: A New Look at Old Results
About half of all known stellar systems with Sun-like stars consist of two or
more stars, significantly affecting the orbital stability of any planet in
these systems. This observational evidence has prompted a large array of
theoretical research, including the derivation of mathematically stringent
criteria for the orbital stability of planets in stellar binary systems, valid
for the "coplanar circular restricted three-body problem". In the following, we
use these criteria to explore the validity of results from previous theoretical
studies.Comment: 3 pages, 1 figure; submitted to: Exoplanets: Detection, Formation and
Dynamics, IAU Symposium 249, eds. Y.-S. Sun, S. Ferraz-Mello, and J.-L. Zhou
(Cambridge: Cambridge University Press
High precision radial velocities with GIANO spectra
Radial velocities (RV) measured from near-infrared (NIR) spectra are a
potentially excellent tool to search for extrasolar planets around cool or
active stars. High resolution infrared (IR) spectrographs now available are
reaching the high precision of visible instruments, with a constant improvement
over time. GIANO is an infrared echelle spectrograph at the Telescopio
Nazionale Galileo (TNG) and it is a powerful tool to provide high resolution
spectra for accurate RV measurements of exoplanets and for chemical and
dynamical studies of stellar or extragalactic objects. No other high spectral
resolution IR instrument has GIANO's capability to cover the entire NIR
wavelength range (0.95-2.45 micron) in a single exposure. In this paper we
describe the ensemble of procedures that we have developed to measure high
precision RVs on GIANO spectra acquired during the Science Verification (SV)
run, using the telluric lines as wavelength reference. We used the Cross
Correlation Function (CCF) method to determine the velocity for both the star
and the telluric lines. For this purpose, we constructed two suitable digital
masks that include about 2000 stellar lines, and a similar number of telluric
lines. The method is applied to various targets with different spectral type,
from K2V to M8 stars. We reached different precisions mainly depending on the H
-magnitudes: for H ~ 5 we obtain an rms scatter of ~ 10 m s-1, while for H ~ 9
the standard deviation increases to ~ 50 - 80 m s-1. The corresponding
theoretical error expectations are ~4 m s-1 and 30 m s-1, respectively. Finally
we provide the RVs measured with our procedure for the targets observed during
GIANO Science Verification.Comment: 26 pages, 15 figures, 6 table
Nonlinear effects in 4D-Var
The ability of a data assimilation system to deal effectively
with nonlinearities arising from the prognostic model or the relationship
between the control variables and the available observations has received a
lot of attention in theoretical studies based on very simplified test models.
Less work has been done to quantify the importance of nonlinearities in
operational, state-of-the-art global data assimilation systems. In this paper
we analyse the nonlinear effects present in ECMWF 4D-Var and evaluate the
ability of the incremental formulation to solve the nonlinear assimilation
problem in a realistic NWP environment. We find that nonlinearities have
increased over the years due to a combination of increased model resolution
and the ever-growing importance of observations that are nonlinearly related
to the state. Incremental 4D-Var is well suited for dealing with these
nonlinear effects, but at the cost of increasing the number of outer loop
relinearisations. We then discuss strategies for accommodating the increasing
number of sequential outer loops in the tight schedules of operational global
NWP.</p
Continuous data assimilation for global numerical weather prediction
A new configuration of the European Centre for Medium-Range Weather Forecasts (ECMWF) incremental 4D-Var data assimilation (DA) system is introduced which builds upon the quasi-continuous DA concept proposed in the mid-1990s. Rather than working with a fixed set of observations, the new 4D-Var configuration exploits the near-continuous stream of incoming observations by introducing recently arrived observations at each outer loop iteration of the assimilation. This allows the analysis to benefit from more recent observations. Additionally, by decoupling the start time of the DA calculations from the observational data cut-off time, real-time forecasting applications can benefit from more expensive analysis configurations that previously could not have been considered. In this work we present results of a systematic comparison of the performance of a Continuous DA system against that of two more traditional baseline 4D-Var configurations. We show that the quality of the analysis produced by the new, more continuous configuration is comparable to that of a conventional baseline that has access to all of the observations in each of the outer loops, which is a configuration not feasible in real-time operational numerical weather prediction. For real-time forecasting applications, the Continuous DA framework allows configurations which clearly outperform the best available affordable non-continuous configuration. Continuous DA became operational at ECMWF in June 2019 and led to significant 2 to 3% reductions in medium-range forecast root mean square errors, which is roughly equivalent to 2-3 hr of additional predictive skill.Peer reviewe
AF Lep b: the lowest mass planet detected coupling astrometric and direct imaging data
Aims. Using the direct imaging technique we searched for low mass companions
around the star AF Lep that presents a significant proper motion anomaly (PMa)
signal obtained from the comparison of Hipparcos and Gaia eDR3 catalogs.
Methods. We observed AF Lep in two epochs with VLT/SPHERE using its subsystems
IFS and IRDIS in the near-infrared (NIR) covering wavelengths ranging from the
Y to the K spectral bands (between 0.95 and 2.3 {\mu}m). The data were then
reduced using the high-contrast imaging techniques angular differential imaging
(ADI) and spectral differential imaging (SDI) to be able to retrieve the signal
from low mass companions of the star. Results. A faint companion was retrieved
at a separation of ~0.335" from the star and with a position angle of ~70.5 deg
in the first epoch and with a similar position in the second epoch. This
corresponds to a projected separation of ~9 au. The extracted photometry
allowed us to estimate for the companion a mass between 2 and 5 MJup. This mass
is in good agreement with what is expected for the dynamic mass of the
companion deduced using astrometric measures (5.2-5.5 MJup). This is the first
companion with a mass well below the deuterium burning limit discovered
coupling direct imaging with PMa measures. Orbit fitting done using the orvara
tool allowed to further confirm the companion mass and to define its main
orbital parameters.Comment: 10 pages, 8 Figures, accepted for publication on A&
The structural connectome constrains fast brain dynamics
Brain activity during rest displays complex, rapidly evolving patterns in space and time. Structural connections comprising the human connectome are hypothesized to impose constraints on the dynamics of this activity. Here, we use magnetoencephalography (MEG) to quantify the extent to which fast neural dynamics in the human brain are constrained by structural connections inferred from diffusion MRI tractography. We characterize the spatio-temporal unfolding of whole-brain activity at the millisecond scale from source-reconstructed MEG data, estimating the probability that any two brain regions will significantly deviate from baseline activity in consecutive time epochs. We find that the structural connectome relates to, and likely affects, the rapid spreading of neuronal avalanches, evidenced by a significant association between these transition probabilities and structural connectivity strengths (r = 0.37, p<0.0001). This finding opens new avenues to study the relationship between brain structure and neural dynamics
The VLT/NaCo large program to probe the occurrence of exoplanets and brown dwarfs at wide orbits: II- Survey description, results and performances
In anticipation of the VLT/SPHERE planet imager guaranteed time programs, we
have conducted a preparatory survey of 86 stars between 2009 and 2013 in order
to identify new faint comoving companions to ultimately carry out a
comprehensive analysis of the occurence of giant planets and brown dwarf
companions at wide (10-2000 AU) orbits around young, solar-type stars. We used
NaCo at VLT to explore the occurrence rate of giant planets and brown dwarfs
between typically 0.1 and 8''. Diffraction-limited observations in H-band
combined with angular differential imaging enabled us to reach primary
star-companion brightness ratios as small as 10-6 at 1.5''. 12 systems were
resolved as new binaries, including the discovery of a new white dwarf
companion to the star HD8049. Around 34 stars, at least one companion candidate
was detected in the observed field of view. More than 400 faint sources were
detected, 90% of them in 4 crowded fields. With the exception of HD8049B, we
did not identify any new comoving companions. The survey also led to spatially
resolved images of the thin debris disk around HD\,61005 that have been
published earlier. Finally, considering the survey detection limits, we derive
a preliminary upper limit on the frequency of giant planets for semi-major axes
of [10,2000] AU: typically less than 15% between 100 and 500 AU, and less than
10% between 50 and 500 AU for exoplanets more massive than 5 MJup and 10 MJup
respectively, considering a uniform input distribution and with a confidence
level of 95%.Comment: 19 pages, 8 figures, 12 Tables, accepted to A&
Stellar companions and Jupiter-like planets in young associations
Recently, combining high-contrast imaging and space astrometry we found that
Jupiter-like (JL) planets are frequent in the beta Pic moving group (BPMG)
around those stars where their orbit can be stable, prompting further analysis
and discussion. We broaden our previous analysis to other young nearby
associations to determine the frequency, mass, and separation of companions in
general and JL in particular and their dependencies on the mass and age of the
associations. We collected available data about companions including those
revealed by visual observations, eclipses, spectroscopy, and astrometry. We
determined search completeness and found that it is very high for stellar
companions, while completeness corrections are still large for JL companions.
Once these corrections are included, we found a high frequency of companions,
both stellar (>0.52+/-0.03) and JL (0.57+/-0.11). The two populations are
separated by a gap that corresponds to the brown dwarf desert. Within the
population of massive companions, we found trends in frequency, separation, and
mass ratios with stellar mass. Planetary companions pile up in the region just
outside the ice line and we found them to be frequent once completeness was
considered. The frequency of JL planets decreases with the overall mass and
possibly the age of the association. We tentatively identify the two
populations as due to disk fragmentation and core accretion, respectively. The
distributions of stellar companions with a semi-major axis <1000 au is well
reproduced by a simple model of formation by disk fragmentation. The observed
trends with stellar mass can be explained by a shorter but much more intense
phase of accretion onto the disk of massive stars and by a more steady and
prolonged accretion on solar-type stars. Possible explanations for the trends
in the population of JL planets with association mass and age are briefly
discussed.Comment: Accepted by Astronomy and Astrophysics, 52 pages, 23 figure
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