Abstract
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.
Highlights
Nearby Young Associations (NYAs) provide a unique means of studying the formation processes and physical properties of stars and brown dwarfs (BDs) at ages ranging from 8 Myr to 120 Myr
Our analysis suggests that this object could be an unresolved binary
We have presented several modifications to the Bayesian inference method introduced by Malo et al (2013) in order to assess the probabilities that late-type objects are members to several NYAs
Summary
Nearby Young Associations (NYAs) provide a unique means of studying the formation processes and physical properties of stars and brown dwarfs (BDs) at ages ranging from 8 Myr to 120 Myr. It would allow us to study the low-mass end of the IMF in different environments while providing a unique test bench for evolutionary models at young ages, in addition to providing a sample of age-calibrated young systems in the solar neighborhood The latter is interesting for the dynamic field of exoplanet imaging: low-mass stars (LMSs) or BDs are intrinsically fainter than their more massive equivalents, and young planets are hotter ( brighter) than older ones because of the thermal energy stored during their initial contraction. NYAs represent interesting test benches for planetary formation theories, since 10 and 30 Myr respectively correspond to the formation timescales of giant and terrestrial planets (Song et al 2003)
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