Braking the Gas in the beta Pictoris Disk

(Abridged) The main sequence star beta Pictoris hosts the best studied circumstellar disk to date. Nonetheless, a long-standing puzzle has been around since the detection of metallic gas in the disk: radiation pressure from the star should blow the gas away, yet the observed motion is consistent with Keplerian rotation. In this work we search for braking mechanisms that can resolve this discrepancy. We find that all species affected by radiation force are heavily ionized and dynamically coupled into a single fluid by Coulomb collisions, reducing the radiation force on species feeling the strongest acceleration. For a gas of solar composition, the resulting total radiation force still exceeds gravity, while a gas of enhanced carbon abundance could be self-braking. We also explore two other braking agents: collisions with dust grains and neutral gas. Grains surrounding beta Pic are photoelectrically charged to a positive electrostatic potential. If a significant fraction of the grains are carbonaceous (10% in the midplane and larger at higher altitudes), ions can be slowed down to satisfy the observed velocity constraints. For neutral gas to brake the coupled ion fluid, we find the minimum required mass to be $\approx$ 0.03 M_\earth, consistent with observed upper limits of the hydrogen column density, and substantially reduced relative to previous estimates. Our results favor a scenario in which metallic gas is generated by grain evaporation in the disk, perhaps during grain-grain collisions. We exclude a primordial origin for the gas, but cannot rule out the possibility of its production by falling evaporating bodies near the star. We discuss the implications of this work for observations of gas in other debris disks.Comment: 19 pages, 12 figures, emulateapj. Accepted for publication in Ap

First NACO observations of the Brown Dwarf LHS 2397aB

Observations of the standard late type M8 star LHS 2397aA were obtained at the ESO-VLT 8m telescope ``Yepun'' using the NAOS/CONICA Adaptive Optics facility. The observations were taken during the NACO commissioning, and the infrared standard star LHS 2397aA was observed in the H, and Ks broad band filters. In both bands the brown dwarf companion LHS2397aB was detected. Using a program recently developed (Bouy et al., 2003) for the detection of stellar binaries we calculated the principal astrometric parameters (angular binary separation and position angle P.A.) and the photometry of LHS 2397aA and LHS 2397aB. Our study largely confirms previous results obtained with the AO-Hokupa'a facility at Gemini-North (Freed et al., 2003); however a few discrepancies are observed.Comment: 5 page

Sparse aperture masking at the VLT II. Detection limits for the eight debris disks stars β\beta Pic, AU Mic, 49 Cet, η\eta Tel, Fomalhaut, g Lup, HD181327 and HR8799

Context. The formation of planetary systems is a common, yet complex mechanism. Numerous stars have been identified to possess a debris disk, a proto-planetary disk or a planetary system. The understanding of such formation process requires the study of debris disks. These targets are substantial and particularly suitable for optical and infrared observations. Sparse Aperture masking (SAM) is a high angular resolution technique strongly contributing to probe the region from 30 to 200 mas around the stars. This area is usually unreachable with classical imaging, and the technique also remains highly competitive compared to vortex coronagraphy. Aims. We aim to study debris disks with aperture masking to probe the close environment of the stars. Our goal is either to find low mass companions, or to set detection limits. Methods. We observed eight stars presenting debris disks ( $\beta$ Pictoris, AU Microscopii, 49 Ceti, $\eta$ Telescopii, Fomalhaut, g Lupi, HD181327 and HR8799) with SAM technique on the NaCo instrument at the VLT. Results. No close companions were detected using closure phase information under 0.5 of separation from the parent stars. We obtained magnitude detection limits that we converted to Jupiter masses detection limits using theoretical isochrones from evolutionary models. Conclusions. We derived upper mass limits on the presence of companions in the area of few times the diffraction limit of the telescope around each target star.Comment: 7 pages, All magnitude detection limits maps are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5

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&

Pressure-induced enhancement of superconductivity and superconducting-superconducting transition in CaC_6\_6

We measured the electrical resistivity, $\varrho(T)$, of superconducting CaC$\_6$ at ambient and high pressure up to 16 GPa. For $P \leq$8 GPa, we found a large increase of $T\_c$ with pressure from 11.5 up to 15.1 K. At 8 GPa, $T\_c$ drops and levels off at 5 K above 10 GPa. Correspondingly, the residual $\varrho$ increases by $\approx$ 200 times and the $\varrho(T)$ behavior becomes flat. The recovery of the pristine behavior after depressurization is suggestive of a phase transition at 8 GPa between two superconducting phases with good and bad metallic properties, the latter with a lower $T\_c$ and more static disorder

Orbital characterization of the \beta Pictoris b giant planet

In June 2010, we confirmed the existence of a giant planet in the disk of the young star Beta Pictoris, located between 8 AU and 15 AU from the star. This young planet offers the rare opportunity to monitor a large fraction of the orbit using the imaging technique over a reasonably short timescale. Using the NAOS-CONICA adaptive-optics instrument (NACO) at the Very Large Telescope (VLT), we obtained repeated follow-up images of the Bpic system in the Ks and L' filters at four new epochs in 2010 and 2011. Complementing these data with previous measurements, we conduct a homogeneous analysis, which covers more than eight yrs, to accurately monitor the Bpic b position relative to the star. On the basis of the evolution of the planet's relative position with time, we derive the best-fit orbital solutions for our measurements. More reliable results are found with a Markov-chain Monte Carlo approach. The solutions favor a low-eccentricity orbit e < 0.17, with semi-major axis in the range 8--9 AU corresponding to orbital periods of 17--21 yrs. Our solutions favor a highly inclined solution with a peak around i=88.5+-1.7 deg, and a longitude of ascending node tightly constrained at Omega = -147.5+-1.5 deg. These results indicate that the orbital plane of the planet is likely to be above the midplane of the main disk, and compatible with the warp component of the disk being tilted between 3.5 deg and 4.0 deg. This suggests that the planet plays a key role in the origin of the inner warped-disk morphology of the Bpic disk. Finally, these orbital parameters are consistent with the hypothesis that the planet is responsible for the transit-like event observed in November 1981, and also linked to the cometary activity observed in the Bpic system.Comment: 10 pages, 12 figures, accepted to A&

Diagnosing Latent Tuberculosis Infection in the HIV Era

Tuberculin skin testing (TST) and Interferon-gamma (IFNγ)release assays (IGRAs) are presently the only available assays for the detection of Mycobacterium tuberculosis infected individuals. IGRAs might progressively replace TST, as numerous published reports establish their higher specificity and similar sensitivity when tested in BCG vaccinated, immunocompetent individuals or in populations who may have been in contact with atypical mycobacteria. However, few published reports have commented on their role in TB diagnosis in immunocompromised individuals (HIV, immunosuppressive therapy, cancer…). It is the purpose of this report to review IGRAs published studies in HIV individuals in endemic and non endemic area for tuberculosis (TB). IGRAs were tested in the presence or absence of active TB but correlated to duration of exposure. In newly diagnosed active TB, IGRAs demonstrated a similar sensitivity to TST. In TB non infected individuals, TST and IGRAs also gave similar values when categorization of individuals was correlated to the risk of infection. A higher number of positive IGRAs was observed in individuals from TB endemic areas, in similar proportions to immunocompetent individuals. Comparison between the two IGRAs: QuantiFERON-TB Gold® (QF-TB, Cellestis, Australia) and T-SPOT-TB® (Oxford Immunotec, UK), and against TST, in the same HIV population demonstrates a higher sensitivity of T-SPOT-TB and TST than QF-TB. Indeterminate results, which correspond to the absence of a positive T-cell IFNγ response towards phytohemaglutinin (PHA), is a key point when comparing both IGRAs. This PHA control is indicative of the level of immunosuppression observed in the tested individual. QF-TB seems to present, in HIV populations, more indeterminate results than T-SPOT-TB. The calibration and/or concentration of PBMC on nitrocellulose membrane for the T-SPOT-TB, as compared to a whole blood assay, might explain this difference, with less indeterminate results with the T-SPOT-TB assay. Neither assay is able to differentiate active TB from latent TB infection (LTBI). Several laboratories have tried new antigenic epitopes to solve this issue. It is of importance that these studies need to be repeated on a larger scale by others to validate their results. Two blood assays might add information characterising the evolution from LTBI to active TB: either by losing protective immunity, as demonstrated by the whole blood killing assay, or by evaluating the kinetics of the antibodies synthesized against M. tuberculosis specific antigens. In conclusion, longitudinal studies are still needed to validate IGRAs and other assays and to define their respective predictive values

A survey of young, nearby, and dusty stars to understand the formation of wide-orbit giant planets

Direct imaging has confirmed the existence of substellar companions on wide orbits. To understand the formation and evolution mechanisms of these companions, the full population properties must be characterized. We aim at detecting giant planet and/or brown dwarf companions around young, nearby, and dusty stars. Our goal is also to provide statistics on the population of giant planets at wide-orbits and discuss planet formation models. We report a deep survey of 59 stars, members of young stellar associations. The observations were conducted with VLT/NaCo at L'-band (3.8 micron). We used angular differential imaging to reach optimal detection performance. A statistical analysis of about 60 % of the young and southern A-F stars closer than 65 pc allows us to derive the fraction of giant planets on wide orbits. We use gravitational instability models and planet population synthesis models following the core-accretion scenario to discuss the occurrence of these companions. We resolve and characterize new visual binaries and do not detect any new substellar companion. The survey's median detection performance reaches contrasts of 10 mag at 0.5as and 11.5 mag at 1as. We find the occurrence of planets to be between 10.8-24.8 % at 68 % confidence level assuming a uniform distribution of planets in the interval 1-13 Mj and 1-1000 AU. Considering the predictions of formation models, we set important constraints on the occurrence of massive planets and brown dwarf companions that would have formed by GI. We show that this mechanism favors the formation of rather massive clump (Mclump > 30 Mj) at wide (a > 40 AU) orbits which might evolve dynamically and/or fragment. For the population of close-in giant planets that would have formed by CA, our survey marginally explore physical separations (<20 AU) and cannot constrain this population