612 research outputs found
High resolution imaging of young M-type stars of the solar neighborhood: Probing the existence of companions down to the mass of Jupiter
Context. High contrast imaging is a powerful technique to search for gas
giant planets and brown dwarfs orbiting at separation larger than several AU.
Around solar-type stars, giant planets are expected to form by core accretion
or by gravitational instability, but since core accretion is increasingly
difficult as the primary star becomes lighter, gravitational instability would
be the a probable formation scenario for yet-to-be-found distant giant planets
around a low-mass star. A systematic survey for such planets around M dwarfs
would therefore provide a direct test of the efficiency of gravitational
instability. Aims. We search for gas giant planets orbiting around late-type
stars and brown dwarfs of the solar neighborhood. Methods. We obtained deep
high resolution images of 16 targets with the adaptive optic system of VLT-NACO
in the Lp band, using direct imaging and angular differential imaging. This is
currently the largest and deepest survey for Jupiter-mass planets around
Mdwarfs. We developed and used an integrated reduction and analysis pipeline to
reduce the images and derive our 2D detection limits for each target. The
typical contrast achieved is about 9 magnitudes at 0.5" and 11 magnitudes
beyond 1". For each target we also determine the probability of detecting a
planet of a given mass at a given separation in our images. Results. We derived
accurate detection probabilities for planetary companions, taking into account
orbital projection effects, with in average more than 50% probability to detect
a 3MJup companion at 10AU and a 1.5MJup companion at 20AU, bringing strong
constraints on the existence of Jupiter-mass planets around this sample of
young M-dwarfs.Comment: Accepted for publication in A&
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
Discovery of a probable 4-5 Jupiter-mass exoplanet to HD 95086 by direct-imaging
Direct imaging has just started the inventory of the population of gas giant
planets on wide-orbits around young stars in the solar neighborhood. Following
this approach, we carried out a deep imaging survey in the near-infrared using
VLT/NaCo to search for substellar companions. We report here the discovery in
L' (3.8 microns) images of a probable companion orbiting at 56 AU the young
(10-17 Myr), dusty, and early-type (A8) star HD 95086. This discovery is based
on observations with more than a year-time-lapse. Our first epoch clearly
revealed the source at 10 sigma while our second epoch lacked good observing
conditions hence yielding a 3 sigma detection. Various tests were thus made to
rule out possible artifacts. This recovery is consistent with the signal at the
first epoch but requires cleaner confirmation. Nevertheless, our astrometric
precision suggests the companion to be comoving with the star, with a 3 sigma
confidence level. The planetary nature of the source is reinforced by a
non-detection in Ks-band (2.18 microns) images according to its possible
extremely red Ks - L' color. Conversely, background contamination is rejected
with good confidence level. The luminosity yields a predicted mass of about
4-5MJup (at 10-17 Myr) using "hot-start" evolutionary models, making HD 95086 b
the exoplanet with the lowest mass ever imaged around a star.Comment: accepted for publication to APJ
Deep imaging survey of young, nearby austral stars: VLT/NACO near-infrared Lyot-coronographic observations
Context. High contrast and high angular resolution imaging is the optimal search technique for substellar companions to nearby stars at physical separations larger than typically 10 AU. Two distinct populations of substellar companions, brown dwarfs and planets, can be probed and characterized. As a result, fossile traces of processes of formation and evolution can be revealed by physical and orbital properties, both for individual systems and as an ensemble.
Aims. Since November 2002, we have conducted a large, deep imaging, survey of young, nearby associations of the southern hemisphere. Our goal is detection and characterization of substellar companions with projected separations in the range 10–500 AU. We have observed a sample of 88 stars, primarily G to M dwarfs, younger than 100 Myr, and within 100 pc of Earth.
Methods. The VLT/NACO adaptive optics instrument of the ESO Paranal Observatory was used to explore the faint circumstellar environment between typically 0.1 and 10". Diffraction-limited observations in H and K_s-band combined with Lyot-coronagraphy enabled us to reach primary star-companion brightness ratios as small as 10^(-6). The existence of planetary mass companions could therefore be probed. We used a standardized observing sequence to precisely measure the position and flux of all detected sources relative to their visual primary star. Repeated observations at several epochs enabled us to discriminate comoving companions from background objects.
Results. We report the discovery of 17 new close (0.1–5.0") multiple systems. HIP 108195 AB and C (F1 III-M6), HIP 84642 AB (a~14 AU, K0-M5) and TWA22 AB (a~1.8 AU; M6-M6) are confirmed comoving systems. TWA22 AB is likely to be a rare astrometric calibrator that can be used to test evolutionary model predictions. Among our complete sample, a total of 65 targets were observed with deep coronagraphic imaging. About 240 faint companion candidates were detected around 36 stars. Follow-up observations with VLT or HST for 83% of these stars enabled us to identify a large fraction of background contaminants. Our latest results that pertain to the substellar companions to GSC 08047-00232, AB Pic and 2M1207 (confirmed during this survey and published earlier), are reviewed. Finally, a statistical analysis of our complete set of coronagraphic detection limits enables us to place constraints on the physical and orbital properties of giant planets between typically 20 and 150 AU
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
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 role of cluster evolution in disrupting planetary systems and disks: the Kozai mechanism
We examine the effects of dynamical evolution in clusters on planetary
systems or protoplanetary disks orbiting the components of binary stars. In
particular, we look for evidence that the companions of host stars of planetary
systems or disks could have their inclination angles raised from zero to
between the threshold angles (39.23 degrees and 140.77 degrees) that can induce
the Kozai mechanism. We find that up to 20 per cent of binary systems have
their inclination angles increased to within the threshold range. Given that
half of all extrasolar planets could be in binary systems, we suggest that up
to 10 per cent of extrasolar planets could be affected by this mechanism.Comment: 5 pages, 4 figures, accepted for publication in MNRAS. References
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