Trigonometric parallaxes of young field L dwarfs

• Published in: Astronomy and astrophysics (Berlin) 2014-08, Vol.568, p.1-32
• Main Authors: Zapatero Osorio, M. R., Béjar, V. J. S., Miles-Páez, P. A., Peña Ramírez, K., Rebolo, R., Pallé, E.
• Format: Article
• Language: English
• Subjects: Age; Approximation; astrometry; Astronomy; Atmospheres; brown dwarfs; Derivation; Gravitation; proper motions; Solar neighborhood; Spectra; Spectroscopy; stars: distances; stars: low-mass
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/201321340

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 spectra