Bibcode
Zapatero Osorio, M. R.; Béjar, V. J. S.; Miles-Páez, P. A.; Peña Ramírez, K.; Rebolo, R.; Pallé, E.
Bibliographical reference
Astronomy and Astrophysics, Volume 568, id.A6, 32 pp.
Advertised on:
8
2014
Journal
Citations
51
Refereed citations
50
Description
Aims: We aim to determine the trigonometric parallaxes and proper
motions of a sample of ten field L0-L5 dwarfs with spectroscopic
evidence for low-gravity atmospheres. The ten sources were located in
color-absolute magnitude diagrams and in the Hertzsprung-Russell (HR)
diagram for age and mass derivations and were compared with field and
star cluster dwarfs of related spectral classification and with
state-of-the-art solar-metallicity evolutionary models. Methods:
We obtained J and Ks imaging data using 2-4 m class
telescopes with a typical cadence of one image per month between 2010
January and 2012 December, in which the data cover a time baseline of
nearly three years. We also obtained low resolution optical spectra (R ~
300, 500-1100 nm) using the 10 m Gran Telescopio de Canarias to assess
the presence of lithium absorption in four targets and confirm their
young age. The derived parallaxes and proper motions were combined with
data from the literature to determine Teff, luminosity, and
space velocities. All this information along with the lithium
observations was used to assess the ages and masses of the sample. The
astrometric curves were also examined for periodic perturbations
indicative of unseen companions. Results: Trigonometric
parallaxes and proper motions were derived to typical accuracies of a
milliarcsecond (mas) and ±10 mas yr-1, respectively.
All ten L dwarfs have large motions (μ ≥ 70 mas yr-1)
and are located at distances between 9 and 47 pc. They lie above and on
the sequence of field dwarfs in the diagrams of absolute J and
Ks magnitude versus spectral type and luminosity versus
effective temperature, which implies ages similar to or smaller than
those typical of the field. In the HR diagram, 2MASS J00332386-1521309
(L4), 2MASS J00452143+1634446 (L2), 2MASS J03552337+1133437 (L5), 2MASS
J05012406-0010452 (L4), G 196-3B (L3), 2MASS J17260007+1538190 (L3), and
2MASS J22081363+2921215 (L3) occupy locations that are compatible with
the most likely ages in the interval ≈10-500 Myr if they are single
objects. All of these dwarfs (except for 2MASS J00332386-1521309) show
strong lithium absorption at 670.8 nm, thus confirming the young ages
and masses ranging from ≈11 through ≈45 MJup for this
subsample. The detection of atomic lithium in the atmosphere of 2MASS
J00452143+1634446 (L2) is reported for the first time. The lack of
lithium in 2MASS J00332386-1521309 (L4) is not compatible with its
position in the HR diagram, suggesting a spectral type earlier than L4.
The remaining three dwarfs, 2MASS J02411151-0326587 (L0), 2MASS
J10224821+5825453 (L1), and 2MASS J15525906+2948485 (L0) have locations
in the HR diagram indicative of older ages and higher masses consistent
with the observed lithium depletion previously published. The dynamical
studies based on space velocities derived from our parallaxes and proper
motions fully support the aforementioned results for 2MASS
J00452143+1634446, 2MASS J03552337+1133437, G 196-3B, 2MASS
J10224821+5825453, and 2MASS J15525906+2948485. We did not find evidence
for the presence of astrometric companions with minimum detectable
masses that are typically ≥25 MJup and face-on, circular
orbits with periods between 60-90 d and 3 yr around eight targets. Conclusions: The astrometric and spectroscopic data indicate that
about 60-70% of the field L-type dwarfs in our sample with evidence for
low-gravity atmospheres are indeed young-to-intermediate-age brown
dwarfs of the solar neighborhood with expected ages and masses in the
intervals ≈10-500 Myr and ≈11-45 MJup. The peaked-shape
of the H-band spectra of L dwarfs, a signpost of youth, appears to be
present up to ages of 120-500 Myr and intermediate-to-high gravities.
Tables 3, 5, Figs. 4, 5, 7, 8, 10, and 11 are available in electronic
form at http://www.aanda.org
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