218 research outputs found
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
Discovery of the Widest Very Low Mass Binary
We report the discovery of a very low mass binary system (primary mass <0.1
Msol) with a projected separation of ~5100 AU, more than twice that of the
widest previously known system. A spectrum covering the 1-2.5 microns
wavelength interval at R ~1700 is presented for each component. Analysis of the
spectra indicates spectral types of M6.5V and M8V, and the photometric distance
of the system is ~62 pc. Given that previous studies have established that no
more than 1% of very low mass binary systems have orbits larger than 20 AU, the
existence of such a wide system has a bearing on very low mass star formation
and evolution models.Comment: accepted ApJL, 4 page
Discovery of the brightest T dwarf in the northern hemisphere
We report the discovery of a bright (H=12.77) brown dwarf designated SIMP
J013656.5+093347. The discovery was made as part of a near-infrared proper
motion survey, SIMP (Sondage Infrarouge de Mouvement Propre), which uses proper
motion and near-infrared/optical photometry to identify brown dwarf candidates.
A low resolution (lambda/dlambda~40) spectrum of this brown dwarf covering the
0.88-2.35 microns wavelength interval is presented. Analysis of the spectrum
indicates a spectral type of T2.5+/-0.5. A photometric distance of 6.4+/-0.3 pc
is estimated assuming it is a single object. Current observations rule out a
binary of mass ratio ~1 and separation >5 AU. SIMP 0136 is the brightest T
dwarf in the northern hemisphere and is surpassed only by Eps Indi Bab over the
whole sky. It is thus an excellent candidate for detailed studies and should
become a benchmark object for the early-T spectral class.Comment: 4 pages, 3 figures, To be published in November 1, 2006 issue of
ApJL. Following IAU recommendation, the survey acronym (IBIS) was changed to
SIM
WEIRD: Wide-orbit Exoplanet search with InfraRed Direct imaging
We report results from the Wide-orbit Exoplanet search with InfraRed Direct
imaging (WEIRD), a survey designed to search for Jupiter-like companions on
very wide orbits (1000 to 5000 AU) around young stars (120 Myr) that are
known members of moving groups in the solar neighborhood (70 pc). Sharing
the same age, distance, and metallicity as their host while being on large
enough orbits to be studied as "isolated" objects make such companions prime
targets for spectroscopic observations and valuable benchmark objects for
exoplanet atmosphere models. The search strategy is based on deep imaging in
multiple bands across the near-infrared domain. For all 177 objects of our
sample, , , [3.6] and [4.5] images were obtained with
CFHT/MegaCam, GEMINI/GMOS, CFHT/WIRCam, GEMINI/Flamingos-2, and /IRAC.
Using this set of 4 images per target, we searched for sources with red
and colors, typically reaching good completeness
down to 2Mjup companions, while going down to 1Mjup for some targets, at
separations of AU. The search yielded 4 candidate companions with
the expected colors, but they were all rejected through follow-up proper motion
observations. Our results constrain the occurrence of 1-13 Mjup planetary-mass
companions on orbits with a semi-major axis between 1000 and 5000 AU at less
than 0.03, with a 95\% confidence level.Comment: 55 pages, 16 figures, accepted to A
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
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