A new algorithm for point spread function subtraction in high-contrast imaging: a demonstration with angular differential imaging

Direct imaging of exoplanets is limited by bright quasi-static speckles in the point spread function (PSF) of the central star. This limitation can be reduced by subtraction of reference PSF images. We have developed an algorithm to construct an optimized reference PSF image from a set of reference images. This image is built as a linear combination of the reference images available and the coefficients of the combination are optimized inside multiple subsections of the image independently to minimize the residual noise within each subsection. The algorithm developed can be used with many high-contrast imaging observing strategies relying on PSF subtraction, such as angular differential imaging (ADI), roll subtraction, spectral differential imaging, reference star observations, etc. The performance of the algorithm is demonstrated for ADI data. It is shown that for this type of data the new algorithm provides a gain in sensitivity by up to a factor 3 at small separation over the algorithm used in Marois et al. (2006).Comment: 7 pages, 11 figures, to appear in May 10, 2007 issue of Ap

BANYAN. II. Very Low Mass and Substellar Candidate Members to Nearby, Young Kinematic Groups With Previously Known Signs of Youth

We present Bayesian Analysis for Nearby Young AssociatioNs II (BANYAN II), a modified Bayesian analysis for assessing the membership of later-than-M5 objects to any of several Nearby Young Associations (NYAs). In addition to using kinematic information (from sky position and proper motion), this analysis exploits 2MASS-WISE color-magnitude diagrams in which old and young objects follow distinct sequences. As an improvement over our earlier work, the spatial and kinematic distributions for each association are now modelled as ellipsoids whose axes need not be aligned with the Galactic coordinate axes, and we use prior probabilities matching the expected populations of the NYAs considered versus field stars. We present an extensive contamination analysis to characterize the performance of our new method. We find that Bayesian probabilities are generally representative of contamination rates, except when a parallax measurement is considered. In this case contamination rates become significantly smaller and hence Bayesian probabilities for NYA memberships are pessimistic. We apply this new algorithm to a sample of 158 objects from the literature that are either known to display spectroscopic signs of youth or have unusually red near-infrared colors for their spectral type. Based on our analysis, we identify 25 objects as new highly probable candidates to NYAs, including a new M7.5 bona fide member to Tucana-Horologium, making it the latest-type member. In addition, we reveal that a known L2{\gamma} dwarf is co-moving with a bright M5 dwarf, and we show for the first time that two of the currently known ultra red L dwarfs are strong candidates to the AB Doradus moving group. Several objects identified here as highly probable members to NYAs could be free-floating planetary-mass objects if their membership is confirmed.Comment: 35 pages, 10 figures; accepted for publication in The Astrophysical Journal (in press); Several typographic correction

On the Age of the Widest Very Low Mass Binary

We have recently identified the widest very low-mass binary (2M0126AB), consisting of an M6.5V and an M8V dwarf with a separation of ~5100 AU, which is twice as large as that of the second widest known system and an order of magnitude larger than those of all other previously known wide very low-mass binaries. If this binary belongs to the field population, its constituents would have masses of ~0.09 Msun, at the lower end of the stellar regime. However, in the discovery paper we pointed out that its proper motion and position in the sky are both consistent with being a member of the young (30 Myr) Tucana/Horologium association, raising the possibility that the binary is a pair of ~0.02 Msun brown dwarfs. We obtained optical spectroscopy at the Gemini South Observatory in order to constrain the age of the pair and clarify its nature. The absence of lithium absorption at 671 nm, modest Halpha emission, and the strength of the gravity-sensitive Na doublet at 818 nm all point toward an age of at least 200 Myr, ruling out the possibility that the binary is a member of Tucana/Horologium. We further estimate that the binary is younger than 2 Gyr based on its expected lifetime in the galactic disk.Comment: accepted for publication in The Astrophysical Journa

Photometric Variability of the T2.5 Brown Dwarf SIMP J013656.5+093347; Evidence for Evolving Weather Patterns

We report the discovery of a photometric variability in the bright T2.5 brown dwarf SIMP J013656.5+093347. Continuous J-band photometry has been obtained for several hours on four different nights. The light curves show a periodic modulation with a period of ~2.4 hours, a peak-to-peak amplitude of ~50 mmag and significant night-to-night evolution. We suggest that the light curve modulation is due to the brown dwarf's rotation and that the longer term variations come from surface features evolution and/or differential rotation. We obtained complementary observations over a single night in the J and Ks bands; the object displays correlated photometric variability in both bands, albeit with smaller Ks-band amplitude. The ratio of the Ks and J variability amplitudes puts strong constraints on the physical mechanisms at play. Based on theoretical models of brown dwarf atmospheres, our results suggest that the atmosphere of SIMP0136 is comprised of both grain-free and colder (by ~100 K) grain-bearing cloudy regions. This discovery, and its interpretation, provide a natural explanation of the so-called J-band brightening.Comment: Accepted for publication in ApJ. Modification in the replacement : One author missing in the astro-ph metadat

El temple romà de Vic. Estat de conservació, diagnosi de patologies i propostes d'intervenció

Els estudis organolèptics duts a terme al temple romà de Vic han permès la determinació dels factors d’alteració que afecten l’estabilitat de la pedra emprada en la construcció d’aquest monument, causant-hi una sèrie de degradacions i patologies. Aquestes alteracions s’han produït principalment per l’exposició del temple als factors meteorològics, alteracions que han estat accelerades els darrers anys per l’aparició de contaminació ambiental en l’atmosfera que l’envolt

Oloron-Sainte-Marie – 36-37 rue des Oustalots

La commune d’Oloron-Sainte-Marie est connue depuis plusieurs années maintenant pour ses nombreuses découvertes archéologiques. Et plus particulièrement la partie haute de la terrasse rissienne où se développe le quartier actuel de Sainte-Marie, ancien évêché, et où se situe une grande partie de l’occupation gallo-romaine d’Iluro (l’Oloron Sainte-Marie) antique. Dans le cadre d’un partenariat avec l’association Trait d’Union et la ville d’Oloron Sainte-Marie, plusieurs missions en relations av..