Physics

The PICASSO experiment at SNOLAB reports new results for spin-dependent WIMP interactions on $^{19}$F using the superheated droplet technique. A new generation of detectors and new features which enable background discrimination via the rejection of non-particle induced events are described. First results are presented for a subset of two detectors with target masses of $^{19}$F of 65 g and 69 g respectively and a total exposure of 13.75 $\pm$ 0.48 kgd. No dark matter signal was found and for WIMP masses around 24 GeV/c$^2$ new limits have been obtained on the spin-dependent cross section on $^{19}$F of $\sigma_F$ = 13.9 pb (90% C.L.) which can be converted into cross section limits on protons and neutrons of $\sigma_p$ = 0.16 pb and $\sigma_n$ = 2.60 pb respectively (90% C.L). The obtained limits on protons restrict recent interpretations of the DAMA/LIBRA annual modulations in terms of spin-dependent interactions.

La matière sombre est un mystère dans le domaine de l’astrophysique depuis déjà plusieurs
années. De nombreuses observations montrent que jusqu’à 85 % de la masse gravitationnelle
totale de l’univers serait composée de cette matière de nature inconnue.
Une théorie expliquant cette masse manquante considérerait les WIMPs (Weakly Interacting
Massive Particles), particules stables, non chargées, prédites par des extensions
du modèle standard, comme candidats.
Le projet PICASSO (Projet d’Identification des CAndidats Supersymétriques à la
matière Sombre) est une expérience qui tente de détecter directement leWIMP. Le projet
utilise des détecteurs à gouttelettes de fréon (C4F10) surchauffées. La collision entre un
WIMP et le noyau de fluor crée un recul nucléaire qui cause à son tour une transition de
phase de la gouttelette liquide à une bulle gazeuse. Le bruit de ce phénomène est alors
capté par des senseurs piézoélectriques montés sur les parois des détecteurs.
Le WIMP n’est cependant pas la seule particule pouvant causer une telle transition
de phase. D’autres particules environnantes peuvent former des bulles, telles les particules
a où même des rayons g . Le système d’acquisition de données (DAQ) est aussi
en proie à du bruit électronique qui peut être enregistré, ainsi que sensible à du bruit
acoustique extérieur au détecteur. Finalement, des fractures dans le polymère qui tient
les gouttelettes en place peut également causer des transitions de phase spontanées.
Il faut donc minimiser l’impact de tous ces différents bruit de fond. La pureté du
matériel utilisé dans la fabrication des détecteurs devient alors très importante. On fait
aussi appel à des méthodes qui impliquent l’utilisation de variables de discrimination
développées dans le but d’améliorer les limites d’exclusion de détection du WIMP.
Dark matter has been a mystery for astrophysicists for years now. Numerous observations
have shown that up to 85 % of the gravitationmass of the universe is made of this
unknown type of matter. One of the theories explaining this missing mass problem considers
WIMPs (Weakly Interacting Massive Particles), neutral stable particles predicted
by extensions of the standard model, as possible candidates.
The PICASSO experiment (Project In Canada to Search for Supersymetric Objects)
tries to detect this particle directly. The technique uses superheated droplet detectors,
with freon (C4F10) as the active medium. When a WIMP hits the fluorine nucleus, it
creates a nuclear recoil, which in turn triggers a phase transition from a liquid droplet
to a gaseous bubble. The acoustic noise of this event is then recorded by piezoelectric
transducers mounted on the walls of the detector.
There are however other particles than the WIMPs that can trigger this phase transition.
a particles, or even g rays can create bubbles. The Data Acquisition System (DAQ)
is also subject to electronic noise that can be picked up, and to acoustic noise coming
from an exterior source. Fractures in the polymer holding the droplets in place can also
trigger spontaneous phase transitions.
There is therefore a need to minimize the impact of these background noises. The
level of purity of the ingredients used in detector fabrication then becomes very important.
Digital processing methods are also used to develop discrimination variables that
improve the limits of detection of the WIMP.

We present spatially resolved photometric and spectroscopic observations of two wide brown dwarf binaries uncovered by the SIMP near-infrared proper motion survey. The first pair (SIMP J1619275+031350AB) has a separation of 0.691" (15.2 AU) and components T2.5+T4.0, at the cooler end of the ill-understood J-band brightening. The system is unusual in that the earlier-type primary is bluer in J-Ks than the later-type secondary, whereas the reverse is expected for binaries in the late-L to T dwarf range. This remarkable color reversal can possibly be explained by very different cloud properties between the two components. The second pair (SIMP J1501530-013506AB) consists of an L4.5+L5.5 (separation 0.96", 30-47 AU) with a surprisingly large flux ratio (Delta J =1.79 mag) considering the similar spectral types of its components. The large flux ratio could be explained if the primary is itself an equal-luminosity binary, which would make it one of the first known triple brown dwarf systems. Adaptive optics observations could not confirm this hypothesis, but it remains a likely one, which may be verified by high-resolution near-infrared spectroscopy. These two systems add to the handful of known brown dwarf binaries amenable to resolved spectroscopy without the aid of adaptive optics and constitute prime targets to test brown dwarf atmosphere models.

A modified bayesian analysis to search for young brown dwarfs.

In this talk, I will present the latest modifications we brought to our Bayesian method in the goal of searching for young, low-mass objects in nearby, young associations. With this new method, we can assess membership probabilities for objects with only their sky position and proper motion, as well as predicting their suspected radial velocity and distance. We have applied this method to the whole sky using the 2MASS and WISE surveys, and started a spectroscopic follow-up of the most promising candidates to search for signs of youth in their optical and/or NIR spectra. We will present here our first results, showing that we can get a fraction of young objects as high as 75% if we carefully choose which candidates to follow. We also find that this specific method has the potential to unveil objects whose masses span the whole range from low-mass stars to planetary-mass objects. I will also briefly talk about a Montecarlo simulation that sheds light on the expected recovery rate and contamination fraction of our method.

Using the CFBDSIR wide field survey for brown dwarfs, we identified CFBDSIRJ214947.2-040308.9, a late T dwarf with atypically red J-Ks colour. We obtained an X-Shooter spectra, with signal detectable from 0.8 to 2.3 micron, which confirmed a T7 spectral type with an enhanced Ks-band flux indicative of a potentially low-gravity, young, object. The comparison of our near infrared spectrum with atmosphere models, for solar metallicity, shows that CFBDSIRJ214947.2-040308.9 is probably a 650-750 K, log g=3.75-4.0 substellar object. Using evolution models, this translates into a planetary mass object, with an age in the 20-200 Myr range. An independent Bayesian analysis from proper motion measurements results in a 87% probability that this free-floating planet is a member of the 50-120 Myr old AB Doradus moving group, which strengthens the spectroscopic youth diagnosis. By combining our atmospheric characterisation with the age and metallicity constraints arising from the probable membership to the AB Doradus moving group, we find that CFBDSIRJ214947.2-040308.9 is probably a 4-7 Jupiter masses free-floating planet with an effective temperature of ~700K and a log g of ~4.0, typical of the late T-type exoplanets that are targeted by direct imaging. We stress that this object could be used as a benchmark for understanding the physics of the similar T-type exoplanets that will be discovered by the upcoming high contrast imagers.

"Context. Though only a handful of extrasolar planets have been discovered via direct-imaging, each of these discoveries had a tremendous impact on our understanding of planetary formation, stellar formation, and cool atmosphere physics.
Aims: Since many of these newly imaged giant planets orbit massive A or even B stars, we investigate whether giant planets could be found orbiting low-mass stars at large separations.
Methods: We have been conducting an adaptive optic imaging survey to search for planetary-mass companions of young M dwarfs in the solar neigbourhood, in order to probe different initial conditions of planetary formation.
Results: We report here the direct-imaging discovery of 2MASS J01033563-5515561(AB)b, a 12-14 MJup companion at a projected separation of 84 AU from a pair of young late-M stars, with which it shares proper motion. We also detected a Keplerian-compatible orbital motion.
Conclusions: This young L-type object at the planet/brown dwarf mass boundary is the first ever imaged around a binary system at a separation compatible with formation in a disc."

A modified bayesian analysis to search for young brown dwarfs

In this talk, I will present the latest modifications we brought to our Bayesian method in the goal of searching for young, low-mass objects in nearby, young associations. With this new method, we can assess membership probabilities for objects with only their sky position and proper motion, as well as predicting their suspected radial velocity and distance. We have applied this method to the whole sky using the 2MASS and WISE surveys, and started a spectroscopic follow-up of the most promising candidates to search for signs of youth in their optical and/or NIR spectra. We will present here our first results, showing that we can get a fraction of young objects as high as 75% if we carefully choose which candidates to follow. We also find that this specific method has the potential to unveil objects whose masses span the whole range from low-mass stars to planetary-mass objects.

Identifying Low-Mass Candidate Members of Nearby, Young Moving Groups with Bayesian Inference

Using a novel Bayesian Inference method, applied to a sample of red sources with pho- tometry and proper motion based on a correlation of 2MASS with WISE, we have identified more than 450 highly probable > M5 members of several young moving groups such as TW Hydrae, b Pictoris (BPMG) and Tucana-Horologium (THA). We have already experimented with this approach and successfully discovered and confirmed new members of BPMG and THA with spectral types in the range K5 - M5 (Malo et al., 2013). We have used SpeX at IRTF, OSIRIS at SOAR, SIMON at CTIO 1.5m to obtain near-IR spectra of 90 objects up to date. Since the shapes of the H and K-band continuum and the equivalent widths of atomic lines such as K I and Na I are sensitive to surface gravity, we can use those to identify most promising candidates with signs of youth. For 112 of the brightest candidates, we have also obtained op- tical spectra using GMOS at Gemini, which also permits us to measure the equivalent width of the gravity-sensitive Na I doublet, as well as Ha emission. We propose to present 1) Modifications and new photometric tests in the bayesian inference used in this project, 2) Several new candidates with a clearly triangular H-band continuum and/or lower than normal Na I doublet equivalent widths, 3) A new very low mass possible companion to a candidate member to the BPMG, 6) A new 12-14 Mjup companion to a M5 + M5 binary candidate member to the THA, discovered from NACO imaging (Delorme et al., 2013), 7) An assessment of the reliability of the predicted statistical distances in our analysis through existing parallax measurements and 8) a search for large-separation exoplanets around our young candidates.

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 both radial velocity and parallax are 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 NIR colors for their spectral type. Based on our analysis, we identify 24 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.

We have been conducting an adaptive optic imaging survey to search for planetary-mass companions of young M dwarfs in the solar neigbourhood, in order to probe different initial conditions of planetary formation. We report here the direct-imaging dis-covery of 2MASS J01033563-5515561(AB)b, a 12-14 M Jup companion at a projected sep-aration of 84 AU from a pair of young late-M stars, with which it shares proper motion. This young L-type object at the planet/brown dwarf mass boundary is the first ever imaged around a binary system at a separation compatible with formation in a disc.