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Context. Direct imaging of exoplanets is polluted by speckle noise that severely limits the achievable contrast. Angular and spectral differential imaging have been proposed to make use of the temporal and chromatic properties of the speckles. Both modes, associated with extreme adaptive-optics and coronagraphy, are at the core of the new generation of planet imagers SPHERE and GPI. Aims. We aim to illustrate and characterize the impact of the SDI and SDI+ADI (ASDI) data reduction on the detection of giant planets. We also propose an unbiased method to derive the detection limits from SDI/ASDI data. Methods. Observations of AB Dor B and β Pictoris made with VLT/NaCo were used to simulate and quantify the effects of SDI and ASDI. The novel method is compared to the traditional injection of artificial point sources. Results. The SDI reduction process creates a typical radial positive-negative pattern of any point-source. Its characteristics and its self-subtraction depend on the separation, but also on the spectral properties of the object. This work demonstrates that the self-subtraction cannot be reduced to a simple geometric effect. As a consequence, the detection performances of SDI observations cannot be expressed as a contrast in magnitude with the central star without the knowledge of the spectral properties of detectable companions. In addition, the residual noise cannot be converted into contrast and physical characteristics (mass, temperature) by standard calibration of flux losses. The proposed method takes the SDI bias into account to derive detection limits without the cost of massively injecting artificial sources into the data. Finally, the sensitivity of ASDI observations can be measured only with a control parameter on the algorithms that controls the minimum rotation that is necessary to build the reference image. [ABSTRACT FROM AUTHOR]

Perfect fluids are characterized as having the smallest ratio of shear viscosity to entropy density, η/s, consistent with quantum uncertainty and causality. So far, nearly perfect fluids have only been observed in the quark-gluon plasma and in unitary atomic Fermi gases, exotic systems that are amongst the hottest and coldest objects in the known universe, respectively. We use angle resolved photoemission spectroscopy to measure the temperature dependence of an electronic analog of η/s in an optimally doped cuprate high-temperature superconductor, finding it too is a nearly perfect fluid around, and above, its superconducting transition temperature Tc. [ABSTRACT FROM AUTHOR]

HD 95086 is an intermediate-mass debris-disk-bearing star. VLT/NaCo 3.8μm observations revealed it hosts a 5±2MJup companion (HD 95086 b) at ≃56 AU. Follow-up observations at 1.66 and 2.18 μm yielded a null detection, suggesting extremely red colors for the planet and the need for deeper direct-imaging data. In this Letter, we report H- (1.7μm) and K1- (2.05μm) band detections of HD 95086 b from Gemini Planet Imager (GPI) commissioning observations taken by the GPI team. The planet position in both spectral channels is consistent with the NaCo measurements and we confirm it to be comoving. Our photometry yields colors of H-L'= 3.6±1.0 mag and K1-L'=2.4±0.7 mag, consistent with previously reported 5-s upper limits in H and Ks. The photometry of HD 95086 b best matches that of 2M 1207 b and HR 8799 cde. Comparing its spectral energy distribution with the BT-SETTL and LESIA planet atmospheric models yields Teff~600-1500 K and log g~2.1-4.5. Hot-start evolutionary models yield M=5±2 MJup. Warm-start models reproduce the combined absolute fluxes of the object for M=4-14 MJup for a wide range of plausible initial conditions (Sinit=8-13 kB/baryon). The color-magnitude diagram location of HD 95086 b and its estimated Teff and log g suggest that the planet is a peculiar L-T transition object with an enhanced amount of photospheric dust. [ABSTRACT FROM AUTHOR]

The dressing of quasiparticles in solids is investigated by changes in the electronic structure E(k) driven by femtosecond laser pulses. Employing time- and angle-resolved photoemission on an optimally doped cuprate above Tc, we observe two effects with different characteristic temporal evolutions and, therefore, different microscopic origins. First, a marked change in the effective mass due to the 70-meV kink in E(k) is found to occur during the experiment's 100-fs temporal resolution and is assigned to laser-driven perturbation of an electronic interaction dressing the bare band. Second, a change in kF is explained by effective photodoping due to particle-hole asymmetry and offers opportunities for ultrafast optoelectronic switches based on an optically driven insulator-superconductor transition. [ABSTRACT FROM AUTHOR]

Context. It has long been suggested that circumstellar disks surrounding young stars may be the signposts of planets, and even more so since the recent discoveries of embedded substellar companions. According to models, the planet-disk interaction may create large structures, gaps, rings, or spirals in the disk. In that sense, the Herbig star HD 142527 is particularly compelling, as its massive disk displays intriguing asymmetries that suggest the existence of a dynamical peturber of unknown nature. Aims. Our goal was to obtain deep thermal images of the close circumstellar environment of HD 142527 to re-image the reported close-in structures (cavity, spiral arms) of the disk and to search for stellar and substellar companions that could be connected to their presence. Methods. We obtained high-contrast images with the NaCo adaptive optics system at the Very Large Telescope in L -band. We applied different analysis strategies using both classical PSF-subtraction and angular differential imaging to probe for any extended structures or point-like sources. Results. The circumstellar environment of HD 142527 is revealed at an unprecedented spatial resolution down to the subarcsecond level for the first time at 3.8 μm. Our images reveal important radial and azimuthal asymmetries that invalidate an elliptical shape for the disk. It instead suggests a bright inhomogeneous spiral arm plus various fainter spiral arms. We also confirm an inner cavity down to 30 AU and two important dips at position angles of 0 and 135 deg. The detection performance in angular differential imaging enables exploration of the planetary mass regime for projected physical separations as close as 40 AU. Use of our detection map together with Monte Carlo simulations sets stringent constraints on the presence of planetary mass, brown dwarf or stellar companions as a function of the semi-major axis. They severely limit any presence of massive giant planets with semi-major axis beyond 50 AU, i.e. probably within the large disk’s cavity which extends radially up to 145 AU or even farther outside. [ABSTRACT FROM AUTHOR]

A microscopic scaling relation linking the normal and superconducting states of the cuprates in the presence of a pseudogap is presented using angle-resolved photoemission spectroscopy. This scaling relation, complementary to the bulk universal scaling relation embodied by Homes' law, explicitly connects the momentum-dependent amplitude of the d-wave superconducting order parameter at T ~ 0 to quasiparticle scattering mechanisms operative at T ≿ Tc. The form of the scaling is proposed to be a consequence of the marginal Fermi-liquid phenomenology and the inherently strong dissipation of the normal pseudogap state of the cuprates. [ABSTRACT FROM AUTHOR]

In the pseudogap phase of the copper oxide superconductors, a significant portion of the Fermi surface is still gapped at temperatures above the superconducting transition temperature TC. Instead of a closed Fermi surface, the low-energy electronic excitations appear to form unconnected Fermi arcs separated by gapped regions. It is generally believed that the spectral function is particle–hole symmetric (at low energies) in both regions—with a peak at the Fermi level on the Fermi arcs and a local minimum at the Fermi level in the gapped regions. Here, using high resolution angle-resolved photoemission and new techniques of analysis, we show that on a sizable portion of the Fermi surface, the electronic structure in the immediate vicinity of the Fermi level is not particle–hole symmetric in the pseudogap phase. This is clear evidence that an alternative ground state competes with the superconductivity. The observations are also consistent with the possibility that the Fermi arcs are, in fact, the inner surface of the predicted Fermi pockets. [ABSTRACT FROM AUTHOR]

Superconductors are characterized by an energy gap that represents the energy needed to break the pairs of electrons (Cooper pairs) apart. At temperatures considerably above those associated with superconductivity, the high-transition-temperature copper oxides have an additional ‘pseudogap’. It has been unclear whether this represents preformed pairs of electrons that have not achieved the coherence necessary for superconductivity, or whether it reflects some alternative ground state that competes with superconductivity. Paired electrons should display particle–hole symmetry with respect to the Fermi level (the energy of the highest occupied level in the electronic system), but competing states need not show such symmetry. Here we report a photoemission study of the underdoped copper oxide Bi2Sr2CaCu2O8+δ that shows the opening of a symmetric gap only in the anti-nodal region, contrary to the expectation that pairing would take place in the nodal region. It is therefore evident that the pseudogap does reflect the formation of preformed pairs of electrons and that the pairing occurs only in well-defined directions of the underlying lattice. [ABSTRACT FROM AUTHOR]

The corrosion inhibitor Hostacor KS1 is used in polyethylene glycol (PEG) solutions to treat waterlogged iron--wood composite artifacts, but the manufacturer has now replaced this product by Hostacor IT. An electrochemical study compares the behaviour of Hostacor KS1 and Hostacor IT in various Concentrations and grades of PEG solution. The corrosion potential was measured versus time, and cathodic and anodic polarization experiments were conducted for polished archaeological wrought iron (without corrosion products) in PEG solutions with and without inhibitor(s). The results reveal that both Hostacor KS1 and Hostacor IT spontaneously passivate iron in PEG solutions, by acting as anodic inhibitors that react with dissolved oxygen in the solution, and so prevent the corrosion of iron. The similarity of the electrochemical test results indicates that Hostacor IT is a suitable replacement for Hostacor KS1. [ABSTRACT FROM AUTHOR]

Although the study of X-ray binaries has led to major breakthroughs in high-energy astrophysics, their circumbinary environment at scales of ∼100–10,000 au has not been thoroughly investigated. In this paper, we undertake a novel and exploratory study by employing direct and high-contrast imaging techniques on a sample of X-ray binaries, using adaptive optics and the vortex coronagraph on Keck/NIRC2. High-contrast imaging opens up the possibility to search for exoplanets, brown dwarfs, circumbinary companion stars, and protoplanetary disks in these extreme systems. Here we present the first near-infrared high-contrast images of 13 high-mass X-ray binaries located within ∼2–3 kpc. The key results of this campaign involve the discovery of several candidate circumbinary companions ranging from substellar (brown dwarf) to stellar masses. By conducting an analysis based on Galactic population models, we discriminate sources that are likely background/foreground stars and isolate those that have a high probability (≳60%–99%) of being gravitationally bound to the X-ray binary. This paper seeks to establish a preliminary catalog for future analyses of proper motion and subsequent observations. With our preliminary results, we calculate the first estimate of the companion frequency and the multiplicity frequency for X-ray binaries: ≈0.6 and 1.8 ± 0.9, respectively, considering only the sources that are most likely bound to the X-ray binary. In addition to extending our comprehension of how brown dwarfs and stars can form and survive in such extreme systems, our study opens a new window to our understanding of the formation of X-ray binaries. [ABSTRACT FROM AUTHOR]

Introduction L’hypothermie peropératoire lors des coeliochirurgies est due 1) à l’exposition à l’ambiance de la salle d’opération alors que l’anesthésie obère la thermorégulation et 2) à l’insufflation péritonéale de CO 2 froid et sec. Ce travail expérimental compare l’efficacité de 2 méthodes de prévention de l’hypothermie et de leur combinaison : 1) le réchauffement externe par couverture à air chaud pulsé et 2) l’humidification et le réchauffement du CO 2 insufflé. Matériel et méthodes Après autorisation institutionnelle, 16 expérimentations ont été réalisées sur 4 porcs Large white durant 4 sessions espacées de 8 jours. Lors de chaque session, les 4 sujets ont été étudiés, selon une séquence randomisée par carré latin, sous anesthésie générale (AG), dans l’une des 4 conditions suivantes : 1) AG : AG seule sans insufflation ; 2) couverture chauffante (CC) : couverture à air chaud (38 °C) forcé placée sous l’animal (Bair Hugger © , 3 M, USA) et insufflation péritonéale par du CO 2 froid et sec ; 3) conditionnement (CO) du CO 2 insufflé par réchauffement et humidification (37 °C, Humigard © Fisher&Paykel, Nouvelle Zélande) sans réchauffement externe ; 4) association du conditionnement du CO 2 insufflé et du réchauffement externe par couverture chauffante (CO + CC). l’insufflation était maintenue durant 4 heures à raison de 120 L/h, par compensation d’une fuite et maintien d’une pression péritonéale de 10 mmHg. La température centrale est enregistrée toutes les 15 min à partir d’une sonde oesophagienne (Physiogard SM 785 © Odam, France). L’évolution de la température centrale dans chaque condition expérimentale a été modélisée en utilisant un modèle linéaire mixte. Pour la variable temps, des variables nominales ou des pentes linéaires ont été utilisées. Pour les corrections liées aux comparaisons multiples des tests simultanés et un risque d’erreur alpha global de 5 % ont été utilisés. Résultats Les variations de température centrale dans les différents groupes sont représentées sur la Fig. 1 . En tenant compte de toute la durée expérimentale, la variation de température centrale par rapport à l’état de base n’a pas été significative dans le groupe CO + CC ( p = 0,38) alors qu’on observe une diminution significative de la température centrale dans les groupes AG ( p < 0,0001), CO ( p < 0,0002) et CC ( p = 0,0134). En prenant en compte le temps comme variable catégorielle, la différence entre les températures centrales entre les groupes AG et CO + CC est statistiquement significative après 210 min d’insufflation ( p = 0,045) ( Fig. 1 ). Discussion Dans les groupes CC et CO, la température centrale ne diffère pas par rapport au groupe AG mettant en évidence l’efficacité de chacune de ces 2 techniques pour contrebalancer la déperdition thermique liée à l’insufflation péritonéale. Le groupe CO + CC est le seul à montrer une température centrale supérieure de manière statistiquement significative vs le groupe AG seule à partir de 210 min. L’association des deux techniques serait donc au minimum additive pour lutter contre la déperdition thermique pour les interventions longues (plus de 3 h) et à même de contrebalancer à la fois les pertes liées à l’insufflation et celles liées à l’anesth [ABSTRACT FROM AUTHOR]

Introduction L’utilisation du vidéolaryngoscope GlideScope ® (GS, Verathon, États-Unis) nécessite l’utilisation d’un mandrin rigide GlideRite ® qui permet d’épouser la courbure de la lame. Toutefois, des complications liées à l’utilisation de ce mandrin à type de traumatisme pharyngé sont rapportées. Leur incidence est évaluée à 1 % dans une série récente de 2000 intubations [1] . Le mandrin RPIS (Rapid Positionning Intubation Stylet, Airway Management Enterprises, États-Unis) possède une extrémité souple, qui limiterait le risque de lésions pharyngées lors des intubation avec le GS. Cette extrémité souple est orientable vers le haut et le bas, ce qui pourrait faciliter le cathéterisme de la glotte. Notre objectif est d’évaluer la courbe d’apprentissage de l’utilisation du mandrin RPIS et de comparer le temps nécessaire pour intuber sous GlideScope ® avec le RPIS ou avec le mandrin GlideRite ® . Matériel et méthodes Les manipulations se déroulent sur un mannequin d’intubation difficile (KarlStorz). Les intubations sont réalisées sous GS par des opérateurs expérimentés à l’utilisation du GS mais naïfs au RPIS. Le temps d’intubation est défini entre la prise en main de la sonde d’intubation adaptée sur le mandrin choisi, et la confirmation du bon positionnement endotrachéal de la sonde par un observateur. Le temps de référence (Tref) d’intubation de chaque opérateur est déterminé lors de l’utilisation du mandrin rigide GlideRite ® . Puis, 5 intubations sont réalisées successivement, à l’aide du mandrin RPIS. Le temps d’intubation est mesuré pour chaque tentative (T1 à T5) et le meilleur temps de chaque opérateur est colligé (T min RPIS). Une analyse statistique de non infériorité a été réalisée. La courbe d’apprentissage de l’utilisation du RPIS est évaluée par la progression du temps d’intubation moyenné de l’ensemble des opérateurs à chaque tentative. La satisfaction des opérateurs est évaluée à l’aide d’une échelle numérique allant de 0 à 10. Résultats Trente opérateurs ont participé à l’étude. Les temps moyens d’intubation à chaque tentative sont représentés dans la figure. Le Tref moyen est de 11 ± 3,2 sec (m ± SD). Le temps moyen de la 5ème tentative avec le RPIS est 13 ± 5,8 sec. La moyenne des meilleurs temps avec le RPIS est de 10 ± 3,1 sec. Aucune différence entre ces moyennes n’est significative. La note moyenne de satisfaction pour le RPIS est de 7 ( Fig. 1 ). Discussion L’intubation sous GS avec le mandrin RPIS n’est pas plus longue qu’en utilisant le mandrin GlideRite ® . Le mandrin RPIS est facile d’utilisation, avec une courbe d’apprentissage rapide. Après 5 manipulations, les temps d’intubation avec le mandrin GlideRite ® ou avec le mandrin RPIS sont statistiquement et cliniquement équivalents. L’apport de l’extrémité souple du mandrin RPIS semble intéressant pour limiter le risque de lésion pharyngée. L’extrémité orientable permet un guidage précis de la sonde d’intubation dans la trachée, notamment lors d’une intubation difficile. Le mandrin RPIS permet d’allier les avantages d’une intubation sous GlideScope, sans les risques du mandrin métallique rigide. Ces résultats sont à confirmer par des études cliniques de plus grands effectifs. [ABSTRACT FROM AUTHOR]

High-resolution angle-resolved photoelectron spectroscopy is used to examine the electronic band structure of FeTe0.5Se0.5 near the Brillouin zone center. A consistent separation of the α1 and α2 bands is observed with little kz dependence of the α1 band. First-principles calculations for bulk and thin films demonstrate that the antiferromagnetic coupling between the Fe atoms and hybridization-induced spin-orbit effects lifts the degeneracy of the Fe dxz and dyz orbitals at the zone center leading to orbital ordering. These experimental and computational results provide a natural microscopic basis for the nematicity observed in the Fe-based superconductors. [ABSTRACT FROM AUTHOR]

Context. Spectral differential imaging (SDI) is part of the observing strategy of current and future high-contrast imaging instruments. It aims to reduce the stellar speckles that prevent the detection of cool planets by using in/out methane-band images. It attenuates the signature of off-axis companions to the star, such as angular differential imaging (ADI). However, this attenuation depends on the spectral properties of the low-mass companions we are searching for. The implications of this particularity on estimating the detection limits have been poorly explored so far. Aims. We perform an imaging survey to search for cool (Teff< 1000–1300 K) giant planets at separations as close as 5–10 AU. We also aim to assess the sensitivity limits in SDI data taking the photometric bias into account. This will lead to a better view of the SDI performance. Methods. We observed a selected sample of 16 stars (age <200 Myr, distance <25 pc) with the phase-mask coronagraph, SDI, and ADI modes of VLT/NaCo. Results. We do not detect any companions. As for the estimation of the sensitivity limits, we argue that the SDI residual noise cannot be converted into mass limits because it represents a differential flux, unlike what is done for single-band images, in which fluxes are measured. This results in degeneracies for the mass limits, which may be removed with the use of single-band constraints. We instead employ a method of directly determining the mass limits and compare the results from a combined processing SDI-ADI (ASDI) and ADI. The SDI flux ratio of a planet is the critical parameter for the ASDI performance at close-in separations (≲1′′). The survey is sensitive to cool giant planets beyond 10 AU for 65% and 30 AU for 100% of the sample. Conclusions. For close-in separations, the optimal regime for SDI corresponds to SDI flux ratios higher than ~2. According to the BT-Settl model, this translates into Teff ≲ 800 K, which is significantly lower than the methane condensation temperature (~1300 K). The methods described here can be applied to the data interpretation of SPHERE. In particular, we expect better performance with the dual-band imager IRDIS, thanks to more suitable filter characteristics and better image quality. [ABSTRACT FROM AUTHOR]

The phase diagram associated with high-Tc superconductors is complicated by an array of different ground states. The parent material represents an antiferromagnetic insulator but with doping superconductivity becomes possible with transition temperatures previously thought unattainable. The underdoped region of the phase diagram is dominated by the so-called pseudogap phenomena, whereby in the normal state the system mimics superconductivity in its spectral response but does not show the complete loss of resistivity associated with the superconducting state. An understanding of this regime presents one of the great challenges for the field. In the present study we revisit the structure of the phase diagram as determined in photoemission studies. By careful analysis of the role of nanoscale inhomogeneities in the overdoped region, we are able to more carefully separate out the gaps due to the pseudogap phenomena from the gaps due to the superconducting transition. Within a mean-field description, we are thus able to link the magnitude of the doping-dependent pseudogap directly to the Heisenberg exchange interaction term, J∑sisj, contained in the t-J model. This approach provides a clear indication that the pseudogap is associated with spin singlet formation. [ABSTRACT FROM AUTHOR]

Context. Atmospheric magnetic fields in stars with convective envelopes heat stellar chromospheres, and thus increase the observed flux in the Ca ii H and K doublet. Starting with the historical Mount Wilson monitoring program, these two spectral lines have been widely used to trace stellar magnetic activity, and as a proxy for rotation period (Prot) and consequently for stellar age. Monitoring stellar activity has also become essential in filtering out false-positives due to magnetic activity in extra-solar planet surveys. The Ca ii emission is traditionally quantified through the R'HK-index, which compares the chromospheric flux in the doublet to the overall bolometric flux of the star. Much work has been done to characterize this index for FGK-dwarfs, but M dwarfs - the most numerous stars of the Galaxy - were left out of these analyses and no calibration of their Ca ii H and K emission to an R'HK exists to date. Aims. We set out to characterize the magnetic activity of the low- and very-low-mass stars by providing a calibration of the R'HK-index that extends to the realm of M dwarfs, and by evaluating the relationship between R'HK and the rotation period. Methods. We calibrated the bolometric and photospheric factors for M dwarfs to properly transform the S -index (which compares the flux in the Ca ii H and K lines to a close spectral continuum) into the R'HK. We monitored magnetic activity through the Ca ii H and K emission lines in the HARPS M dwarf sample. Results. The R'HK index, like the fractional X-ray luminosity LX/Lbol, shows a saturated correlation with rotation, with saturation setting in around a ten days rotation period. Above that period, slower rotators show weaker Ca ii activity, as expected. Under that period, the R'HK index saturates to approximately 10-4. Stellar mass modulates the Ca ii activity, with R'HK showing a constant basal activity above 0.6 M๏ and then decreasing with mass between 0.6 M๏ and the fully-convective limit of 0.35 M๏. Short-term variability of the activity correlates with its mean level and stars with higher R'HK indexes show larger R'HK variability, as previously observed for earlier spectral types. [ABSTRACT FROM AUTHOR]

We report the discovery of a low-mass companion to the nearby (d = 47 pc) F7V star HD 984. The companion is detected 0.19 arcsec away from its host star in the L band with the Apodized Phase Plate on NaCo/Very Large Telescope and was recovered by L-band noncoronagraphic imaging data taken a few days later. We confirm the companion is comoving with the star with SINFONI integral field spectrograph H + K data. We present the first published data obtained with SINFONI in pupil-tracking mode. HD 984 has been argued to be a kinematic member of the 30 Myr-old Columba group, and its HR diagram position is not altogether inconsistent with being a zero-age main sequence star of this age. By consolidating different age indicators, including isochronal age, coronal X-ray emission, and stellar rotation, we independently estimate a main-sequence age of 115 ± 85 Myr (95 per cent CL) which does not rely on this kinematic association. The mass of directly imaged companions are usually inferred from theoretical evolutionary tracks, which are highly dependent on the age of the star. Based on the age extrema, we demonstrate that with our photometric data alone, the companion's mass is highly uncertain: between 33 and 96 MJup (0.03-0.09 M) using the COND evolutionary models. We compare the companion's SINFONI spectrum with field dwarf spectra to break this degeneracy. Based on the slope and shape of the spectrum in the H band, we conclude that the companion is an M6.0 ± 0.5 dwarf. The age of the system is not further constrained by the companion, as M dwarfs are poorly fit on low-mass evolutionary tracks. This discovery emphasizes the importance of obtaining a spectrum to spectral type companions around F-stars. [ABSTRACT FROM AUTHOR]

Directly detecting thermal emission from young extrasolar planets allows measurement of their atmospheric compositions and luminosities, which are influenced by their formation mechanisms. Using the Gemini Planet Imager, we discovered a planet orbiting the ~20-million-year-old star 51 Eridani at a projected separation of 13 astronomical units. Near-infrared observations show a spectrum with strong methane and water-vapor absorption. Modeling of the spectra and photometry yields a luminosity (normalized by the luminosity of the Sun) of 1.6 to 4.0 x 10-6 and an effective temperature of 600 to 750 kelvin. For this age and luminosity, "hot-start" formation models indicate a mass twice that of Jupiter. This planet also has a sufficiently low luminosity to be consistent with the "cold-start" core-accretion process that may have formed Jupiter. [ABSTRACT FROM AUTHOR]

Context. Stars in young nearby associations are the only targets that allow giant planet searches in the near future, at all separations by coupling indirect techniques, such as radial velocity (RV) and deep imaging. These stars are first priority targets for the forthcoming planet imagers on eight- to ten-metre class telescopes. Young stars rotate more rapidly and are more active than their older counterparts. Both effects can limit the ability to detect planets using RV. Aims. We wish to explore the planet detection abilities of a representative sample of stars in close and young associations with RV data and to explore the complementarity between this technique and direct imaging. Methods. We observed 26 such targets with spectral types from A to K and ages from 8 to 300 Myr with HARPS. We computed the detection limits with two methods, in particular, a method we have recently developed that takes the frequency distribution of the RV variations into account. We also attempted to improve the detection limits in a few cases by correcting for the stellar activity. Results. Our A-type stars RV show high-frequency variations due to pulsations, while our F-K stars clearly show activity with more or less complex patterns. For F-K stars, the RV jitter and v sin i rapidly decrease with star age. The data allow us to search for planets with periods typically ranging from 1 day to 100 days, and up to more than 500 days in a few cases. Within the present detection limits, no planet was found in our sample. For the bulk of our F-K stars, the detection limits fall to sub-Jupiter masses. We show that these limits can be significantly improved by correcting even partially for stellar activity, down to a few Neptune masses for the least active stars. The detection limits on A-type stars can be significantly improved, down to a few Jupiter-mass planets, provided an appropriate observing strategy. We finally show the tremendous potential of coupling RV and adaptive-optics deep imaging results. Conclusions. The RV technique allows the detection of planets lighter than Jupiter, reaching a few Neptune masses around young stars aged typically 30 Myr or more. Detection limits increase at younger ages, but (sub-)Jupiter mass planets are still detectable. In the next few years, using complementary techniques will allow a full exploration of the Jupiter-mass planets' content of many of these stars. [ABSTRACT FROM AUTHOR]