Changes in severe indices as simulated by two french coupled global climate models

International audienceExtremes are assessed here in an attempt to validate the two French models in their representation of the second part of the 20th century, using different sources of gridded observational datasets. Models show some ability to simulate extremal behaviour of the climate even if discrepancies are noticeable between models and observations. These may be partly due to the low resolution used for the present study simulations. Extreme indices, calculated using the STARDEX (STAtistical and Regional dynamical Downscaling of EXtremes for European regions) methodology, are investigated in different IPCC (International Panel on Climate Change) scenarios performed by the French community. Investigation of 21st century severe indices simulated in these simulations shows some interesting features. In some parts of the world, extreme temperatures experience a more rapid increase than the mean, suggesting that the Power Density Function (PDF) may not only be shifted toward higher temperatures but also changed in its shape. Extremes of precipitation also experience a change toward more intense precipitation events in winter and longer dry events in summer. Approaching future changes in extreme indices through their relationship to mean annual temperature may be a useful approach in multi-model studies, since it provides a measure of the sensitivity of extremes to warming conditions in these models

Near-infrared integral-field spectra of the planet/brown dwarf companion AB Pic b

Chauvin et al. 2005 imaged a co-moving companion at ~260 AU from the young star AB Pic A. Evolutionary models predictions based on J H K photometry of AB Pic b suggested a mass of ~13 - 14 MJup, placing the object at the deuterium-burning boundary. We used the adaptive-optics-fed integral field spectrograph SINFONI to obtain high quality medium-resolution spectra of AB Pic b (R = 1500-2000) over the 1.1 - 2.5 microns range. Our analysis relies on the comparison of our spectra to young standard templates and to the latest libraries of synthetic spectra developed by the Lyon's Group. AB Pic b is confirmed to be a young early-L dwarf companion. We derive a spectral type L0-L1 and find several features indicative of intermediate gravity atmosphere. A comparison to synthetic spectra yields Teff = 2000+100-300 K and log(g) = 4 +- 0.5 dex. The determination of the derived atmospheric parameters of AB Pic b is limited by a non-perfect match of current atmosphere spectra with our near-infrared observations of AB Pic b. The current treatment of dust settling and missing molecular opacity lines in the atmosphere models could be responsible. By combining the observed photometry, the surface fluxes from atmosphere models and the known distance of the system, we derive new mass, luminosity and radius estimates of AB Pic b. They confirm independently the evolutionary model predictions. We finally review the current methods used to characterize planetary mass companions and discuss them in the perspective of future planet deep imaging surveys.Comment: 8 pages, 8 figure

An accurate distance to 2M1207Ab

In April 2004 the first image was obtained of a planetary mass companion (now known as 2M1207 b) in orbit around a self-luminous object different from our own Sun (the young brown dwarf 2MASSW J1207334-393254, hereafter 2M1207 A). 2M1207 b probably formed via fragmentation and gravitational collapse, offering proof that such a mechanism can form bodies in the planetary mass regime. However, the predicted mass, luminosity, and radius of 2M1207 b depend on its age, distance, and other observables such as effective temperature. To refine our knowledge of the physical properties of 2M1207 b and its nature, we obtained an accurate determination of the distance to the 2M1207 A and b system by measurements of its trigonometric parallax at the milliarcsec level. With the ESO NTT/SUSI2 telescope, in 2006 we began a campaign of photometric and astrometric observations to measure the trigonometric parallax of 2M1207 A. An accurate distance ($52.4\pm 1.1$ pc) to 2M1207A was measured. From distance and proper motions we derived spatial velocities fully compatible with TWA membership. With this new distance estimate, we discuss three scenarios regarding the nature of 2M1207 b: (1) a cool ($1150\pm150$ K) companion of mass $4\pm1$ M$_{\rm{Jup}}$, (2) a warmer ($1600\pm100$ K) and heavier ($8\pm2$ M$_{\rm{Jup}}$) companion occulted by an edge-on circum-secondary disk or (3) a hot protoplanet collision afterglow.Comment: 5 pages, 3 figures, accepted for publication as letter in A&A, 6/11/200

Deep infrared imaging of close companions to austral A- and F-type stars

The search for substellar companions around stars with different masses along the main sequence is critical to understand the different processes leading to the formation of low-mass stars, brown dwarfs, and planets. In particular, the existence of a large population of low-mass stars and brown dwarfs physically bound to early-type main-sequence stars could imply that the massive planets recently imaged at wide separations (10-100 AU) around A-type stars are disc-born objects in the low-mass tail of the binary distribution. Our aim is to characterize the environment of early-type main-sequence stars by detecting brown dwarf or low-mass star companions between 10 and 500 AU. High contrast and high angular resolution near-infrared images of a sample of 38 southern A- and F-type stars have been obtained between 2005 and 2009 with the instruments VLT/NaCo and CFHT/PUEO. Multi-epoch observations were performed to discriminate comoving companions from background contaminants. About 41 companion candidates were imaged around 23 stars. Follow-up observations for 83% of these stars allowed us to identify a large number of background contaminants. We report the detection of 7 low-mass stars with masses between 0.1 and 0.8 Msun in 6 multiple systems: the discovery of a M2 companion around the A5V star HD14943 and the detection of the B component of the F4V star HD41742 quadruple system; we resolve the known companion of the F6.5V star HD49095 as a short-period binary system composed by 2 M/L dwarfs. We also resolve the companions to the astrometric binaries iot Crt (F6.5V) and 26 Oph (F3V), and identify a M3/M4 companion to the F4V star omi Gru, associated with a X-ray source. The global multiplicity fraction measured in our sample of A and F stars is >16%. A parallel velocimetric survey of our stars let us conclude that the imaged companions can impact on the observed radial velocity measurements.Comment: 21 pages, 12 figures, 7 tables. Accepted for publication in Astronomy and Astrophysics. The full version of the preprint including the appendices (24 pages of figures), can be retrieved at http://www-laog.obs.ujf-grenoble.fr/~dehrenre/articles/afsurvey

Search for associations containing young stars (SACY). VI. Is multiplicity universal? Stellar multiplicity in the range 3-1000 au from adaptive-optics observations

Context. Young loose nearby associations are unique samples of close (<150 pc), young (approx 5-100 Myr) pre-main sequence (PMS) stars. A significant number of members of these associations have been identified in the SACY collaboration. We can use the proximity and youth of these members to investigate key ingredients in star formation processes, such as multiplicity. Aims. We present the statistics of identified multiple systems from 113 confirmed SACY members. We derive multiplicity frequencies, mass-ratio, and physical separation distributions in a consistent parameter space, and compare our results to other PMS populations and the field. Methods. We have obtained adaptive-optics assisted near-infrared observations with NACO (ESO/VLT) and IRCAL (Lick Observatory) for at least one epoch of all 113 SACY members. We have identified multiple systems using co-moving proper-motion analysis and using contamination estimates. We have explored ranges in projected separation and mass-ratio of a [3-1000 au], and q [0.1-1], respectively. Results. We have identified 31 multiple systems (28 binaries and 3 triples). We derive a multiplicity frequency (MF) of MF_(3-1000au)=28.4 +4.7, -3.9% and a triple frequency (TF) of TF_(3-1000au)=2.8 +2.5, -0.8% in the separation range of 3-1000 au. We do not find any evidence for an increase in the MF with primary mass. The estimated mass-ratio of our statistical sample (with power-law index gamma=-0.04 +/- 0.14) is consistent with a flat distribution (gamma = 0). Conclusions. We show further similarities (but also hints of discrepancies) between SACY and the Taurus region: flat mass-ratio distributions and statistically similar MF and TF values. We also compared the SACY sample to the field (in the separation range of 19-100 au), finding that the two distributions are indistinguishable, suggesting a similar formation mechanism.Comment: 16 Pages, accepted in A&A 28 May 201

New constraints on the formation and settling of dust in the atmospheres of young M and L dwarfs

We obtained medium-resolution near-infrared spectra of seven young M9.5-L3 dwarfs classified in the optical. We aim to confirm the low surface gravity of the objects in the NIR. We also test whether atmospheric models correctly represent the formation and the settling of dust clouds in the atmosphere of young late-M and L dwarfs. We used ISAAC at VLT to obtain the spectra of the targets. We compared them to those of mature and young BD, and young late-type companions to nearby stars with known ages, in order to identify and study gravity-sensitive features. We computed spectral indices weakly sensitive to the surface gravity to derive near-infrared spectral types. Finally, we found the best fit between each spectrum and synthetic spectra from the BT-Settl 2010 and 2013 models. Using the best fit, we derived the atmospheric parameters of the objects and identify which spectral characteristics the models do not reproduce. We confirmed that our objects are young BD and we found NIR spectral types in agreement with the ones determined at optical wavelengths. The spectrum of the L2-gamma dwarf 2MASSJ2322-6151 reproduces well the spectrum of the planetary mass companion 1RXS J1609-2105b. BT-Settl models fit the spectra and the 1-5 $\mu$m SED of the L0-L3 dwarfs for temperatures between 1600-2000 K. But the models fail to reproduce the shape of the H band, and the NIR slope of some of our targets. This fact, and the best fit solutions found with super-solar metallicity are indicative of a lack of dust, in particular at high altitude, in the cloud models. The modeling of the vertical mixing and of the grain growth will be revised in the next version of the BT-Settl models. These revisions may suppress the remaining non-reproducibilities.Comment: Accepted in A&A, February 6, 201

A brown dwarf companion to the intermediate-mass star HR6037

In the course of an imaging survey we have detected a visual companion to the intermediate-mass star HR 6037. In this letter, we present two epoch observations of the binary with NACO/VLT, and near-IR spectroscopy of the secondary with ISAAC/VLT. The NACO observations allow us to confirm HR 6037B as a co-moving companion. Its J and H band ISAAC spectra suggest the object has an spectral type of M9+-1, with a surface gravity intermediate between that of 10 Myr dwarfs and field dwarfs with identical spectral type. The comparison of its Ks-band photometry with evolutionary tracks allows us to derive a mass, effective temperature, and surface gravity of 62+-20 MJup, Teff = 2330+-200 K, and log g = 5.1+-0.2, respectively. The small mass ratio of the binary, -0.03, and its long orbital period, -5000 yr, makes HR 6037 a rare and uncommon binary system.Comment: (5 pages, 4 figures, accepted for publication in A&A Letters

Direct exoplanet detection and characterization using the ANDROMEDA method: Performance on VLT/NaCo data

Context. The direct detection of exoplanets with high-contrast imaging requires advanced data processing methods to disentangle potential planetary signals from bright quasi-static speckles. Among them, angular differential imaging (ADI) permits potential planetary signals with a known rotation rate to be separated from instrumental speckles that are either statics or slowly variable. The method presented in this paper, called ANDROMEDA for ANgular Differential OptiMal Exoplanet Detection Algorithm is based on a maximum likelihood approach to ADI and is used to estimate the position and the flux of any point source present in the field of view. Aims. In order to optimize and experimentally validate this previously proposed method, we applied ANDROMEDA to real VLT/NaCo data. In addition to its pure detection capability, we investigated the possibility of defining simple and efficient criteria for automatic point source extraction able to support the processing of large surveys. Methods. To assess the performance of the method, we applied ANDROMEDA on VLT/NaCo data of TYC-8979-1683-1 which is surrounded by numerous bright stars and on which we added synthetic planets of known position and flux in the field. In order to accommodate the real data properties, it was necessary to develop additional pre-processing and post-processing steps to the initially proposed algorithm. We then investigated its skill in the challenging case of a well-known target, $\beta$ Pictoris, whose companion is close to the detection limit and we compared our results to those obtained by another method based on principal component analysis (PCA). Results. Application on VLT/NaCo data demonstrates the ability of ANDROMEDA to automatically detect and characterize point sources present in the image field. We end up with a robust method bringing consistent results with a sensitivity similar to the recently published algorithms, with only two parameters to be fine tuned. Moreover, the companion flux estimates are not biased by the algorithm parameters and do not require a posteriori corrections. Conclusions. ANDROMEDA is an attractive alternative to current standard image processing methods that can be readily applied to on-sky data

A probable giant planet imaged in the Beta Pictoris disk

Since the discovery of its dusty disk in 1984, Beta Pictoris has become the prototype of young early-type planetary systems, and there are now various indications that a massive Jovian planet is orbiting the star at ~ 10 AU. However, no planets have been detected around this star so far. Our goal was to investigate the close environment of Beta Pic, searching for planetary companion(s). Deep adaptive-optics L'-band images of Beta Pic were recorded using the NaCo instrument at the Very Large Telescope. A faint point-like signal is detected at a projected distance of ~ 8 AU from the star, within the North-East side of the dust disk. Various tests were made to rule out with a good confidence level possible instrumental or atmospheric artifacts. The probability of a foreground or background contaminant is extremely low, based in addition on the analysis of previous deep Hubble Space Telescope images. The object L'=11.2 apparent magnitude would indicate a typical temperature of ~1500 K and a mass of ~ 8 Jovian masses. If confirmed, it could explain the main morphological and dynamical peculiarities of the Beta Pic system. The present detection is unique among A-stars by the proximity of the resolved planet to its parent star. Its closeness and location inside the Beta Pic disk suggest a formation process by core accretion or disk instabilities rather than a binary-like formation process.Comment: 5 pages, 3 figures, 1 table. A&A Letters, in pres