115 research outputs found
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
Addressing Systematics in the Traceback Age of the Pictoris Moving Group
We characterize the impact of several sources of systematic errors on the
computation of the traceback age of the Pictoris Moving Group
(PMG). We find that uncorrected gravitational redshift and convective
blueshift bias absolute radial velocity measurements by 0.6
kms, which leads to erroneously younger traceback ages by 2
Myr. Random errors on parallax, proper motion, and radial velocity measurements
lead to an additional bias of 1.5 Myr on traceback ages. Contamination
of astrometric and kinematic data by kinematic outliers and unresolved multiple
systems in the full input sample of 76 members and candidates of PMG
also erroneously lowers traceback ages by 3 Myr. We apply our new
numerical traceback analysis tool to a core sample of 25 carefully vetted
members of PMG using Gaia Data Release 3 (DR3) data products and other
kinematic surveys. Our method yields a corrected age of 20.4 2.5 Myr,
bridging the gap between kinematic ages (1119 Myr) and other age-dating
methods, such as isochrones and lithium depletion boundary (2026 Myr). We
explore several association size metrics that can track the spatial extent of
PMG over time, and we determine that minimizing the variance along the
heliocentric curvilinear coordinate (i.e., toward the Galactic
Center) offers the least random and systematic errors, due to the wider UVW
space velocity dispersion of members of PMG along the U-axis, which
tends to maximize the spatial growth of the association along the
-axis over time
Jadis synchrones, désormais GALS, les architectures de FPGA
Il est de plus en plus difficile de répondre à la demande conflictuelle de circuits plus grands et plus rapides par les avancées seules des technologies des semi-conducteurs. À un certain point, on s'attend à ce que les concepteurs et les fabricants doivent abandonner la méthodologie de conception synchrone traditionnelle pour une méthodologie localement synchrone globalement asynchrone (GALS). De tels changements engendrent plus de contraintes de synchronisation, mais également plus de flexibilité.
En conséquence, une méthodologie pour l'implémentation de composants GALS sur FPGA synchrones traditionnels est d'abord présentée. Les objecfifs sont de définir un ensemble minimal de composants asynchrones de base, de permettre leur implémentation et d'établir les contraintes et les limitations de tels circuits. Les résultats de simulation confirment que des conceptions GALS implémentées à l'aide de ressources du FPGA (tableau de correspondance et bascules) et des outils courants de placement et routage permettent l'implémentation de composants asynchrones tels que la ligne à retard, l'élément C de Muller et l'arbitre. Ces composants peuvent être implémentés dans des FPGA synchrones traditionnels tant que ces conceptions sont soumises à des contraintes appropriées et qu'elles sont ufilisées en fonction des limitations du circuit.
Pour atteindre de meilleures performances, une nouvelle architecture de FPGA compatible avec les dispositifs synchrones existants et qui soufient intrinsèquement les conceptions GALS est présentée. L'objecfif principal est simple : l'architecture proposée doit apparaître inchangée pour les concepfions synchrones, mais doit inclure un ensemble minimal de composants de base pour empêcher la métastabilité lors de communicafions asynchrones. Les résultats de simulation, d'un générateur d'horloge qui peut être arrêté, sont présentés. Tous ces résultats démontrent qu'avec très peu de circuits adaptés, une cellule standard de FPGA peut devenir appropriée pour les méthodologies GALS.
Un circuit de masquage des aléas temporels est finalement présenté pour masquer la métastabilité et les problèmes de synchronisafion. Le but est de définir un circuit capable de mettre, physiquement, en application les contraintes qui masquent les sources de métastabilité de façon à ce que la synchronisafion paraisse transparente. Les résultats de simulation confirment qu'un tel circuit peut masquer totalement toutes les sources de métastabilité sans dégradafion des performances, mais avec une latence apparentée au temps nécessaire à la stabilisation d'une bascule de mémoire
The Coolest Isolated Brown Dwarf Candidate Member of TWA
We present two new late-type brown dwarf candidate members of the TW Hydrae
association (TWA) : 2MASS J12074836-3900043 and 2MASS J12474428-3816464, which
were found as part of the BANYAN all-sky survey (BASS) for brown dwarf members
to nearby young associations. We obtained near-infrared (NIR) spectroscopy for
both objects (NIR spectral types are respectively L1 and M9), as well as
optical spectroscopy for J1207-3900 (optical spectral type is L0{\gamma}), and
show that both display clear signs of low-gravity, and thus youth. We use the
BANYAN II Bayesian inference tool to show that both objects are candidate
members to TWA with a very low probability of being field contaminants,
although the kinematics of J1247-3816 seem slightly at odds with that of other
TWA members. J1207-3900 is currently the latest-type and the only isolated
L-type candidate member of TWA. Measuring the distance and radial velocity of
both objects is still required to claim them as bona fide members. Such
late-type objects are predicted to have masses down to 11-15 MJup at the age of
TWA, which makes them compelling targets to study atmospheric properties in a
regime similar to that of currently known imaged extrasolar planets.Comment: 8 pages, 4 figures, accepted for publication in the ApJ Letter
Characterization of Low-mass, Wide-separation Substellar Companions to Stars in Upper Scorpius: Near-infrared Photometry and Spectroscopy
We present new 0.9-2.45 m spectroscopy (), and , ,
, , photometry, obtained at Gemini North, of three low-mass
brown dwarf companions on wide orbits around young stars of the Upper Scorpius
OB association: HIP 78530 B, [PGZ2001] J161031.9-191305 B, and GSC 06214-00210
B. We use these data to assess the companions' spectral type, temperature,
surface gravity and mass, as well as the ability of the BT-Settl and
Drift-Phoenix atmosphere models to reproduce the spectral features of young
substellar objects. For completeness, we also analyze the archival spectroscopy
and photometry of the Upper Scorpius planetary mass companion 1RXS
J160929.1-210524 b. Based on a comparison with model spectra we find that the
companions, in the above order, have effective temperatures of 2700, 2500, 2300
and 1700 K. These temperatures are consistent with our inferred spectral types,
respectively M7 , M9 , M9 , and L4 . From
bolometric luminosities estimated from atmosphere model spectra adjusted to our
photometry, and using evolution models at 5-10 Myr, we estimate masses of
21-25, 28-70, 14-17 and 7-12 , respectively. J1610-1913 B appears
significantly over-luminous for its inferred temperature, which explains its
higher mass estimate. Synthetic spectra based on the BT-Settl and Drift-Phoenix
atmosphere models generally offer a good fit to our observed spectra, although
our analysis has highlighted a few problems. For example, the best fits in the
individual near-infrared bands occur at different model temperatures. Also,
temperature estimates based on a comparison of the broadband magnitudes and
colors of the companions to synthetic magnitudes from the models are
systematically lower than the temperature estimates based on a comparison with
synthetic spectra.Comment: 16 pages, 8 figures, published in the Astrophysical Journa
BANYAN. IV. Fundamental parameters of low-mass star candidates in nearby young stellar kinematic groups - Isochronal Age determination using Magnetic evolutionary models
Based on high resolution optical spectra obtained with ESPaDOnS at CFHT, we
determine fundamental parameters (\Teff, R, \Lbol, \logg\ and metallicity) for
59 candidate members of nearby young kinematic groups. The candidates were
identified through the BANYAN Bayesian inference method of \citet{2013malo},
which takes into account the position, proper motion, magnitude, color, radial
velocity and parallax (when available) to establish a membership probability.
The derived parameters are compared to Dartmouth Magnetic evolutionary models
and to field stars with the goal to constrain the age of our candidates. We
find that, in general, low-mass stars in our sample are more luminous and have
inflated radii compared to older stars, a trend expected for pre-main sequence
stars. The Dartmouth Magnetic evolutionary models show a good fit to
observations of field K and M stars assuming a magnetic field strength of a few
kG, as typically observed for cool stars. Using the low-mass members of
Pictoris moving group, we have re-examined the age inconsistency problem
between Lithium Depletion age and isochronal age (Hertzspring-Russell diagram).
We find that the inclusion of the magnetic field in evolutionary models
increase the isochronal age estimates for the K5V-M5V stars. Using these models
and field strengths, we derive an average isochronal age between 15 and 28 Myr
and we confirm a clear Lithium Depletion Boundary from which an age of
263~Myr is derived, consistent with previous age estimates based on this
method.Comment: Accepted for publication in Ap
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