The young, tight, and low-mass binary TWA22AB: a new calibrator for evolutionary models?*
Orbit, spectral types, and temperature
Laboratoire d'Astrophysique de Grenoble, BP 53, 38041 Grenoble Cedex 9, France e-mail: firstname.lastname@example.org
2 ESO, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
3 Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão, 1226 - Cidade Universitária, 05508-900 São Paulo - SP, Brazil
4 Observatoire Aquitain des Sciences de l'Univers, CNRS-UMR 5804, BP 89, 33270 Floirac, France
5 Department of Physics and Astronomy, University of Georgia, Athens, GA 30602, USA
Accepted: 17 May 2009
Context. Tight binaries discovered in young, nearby associations are ideal targets for providing dynamical mass measurements to test the physics of evolutionary models at young ages and very low masses.
Aims. We report the binarity of TWA22 for the first time. We aim at monitoring the orbit of this young and tight system to determine its total dynamical mass using an accurate distance determination. We also intend to characterize the physical properties (luminosity, effective temperature, and surface gravity) of each component based on near-infrared photometric and spectroscopic observations.
Methods. We used the adaptive-optics assisted imager NACO to resolve the components, to monitor the complete orbit and to obtain the relative near-infrared photometry of TWA22 AB. The adaptive-optics assisted integral field spectrometer SINFONI was also used to obtain medium-resolution (=1500-2000) spectra in JHK bands. Comparison with empirical and synthetic librairies were necessary for deriving the spectral type, the effective temperature, and the surface gravity for each component of the system.
Results. Based on an accurate trigonometric distance (17.5 ± 0.2 pc) determination, we infer a total dynamical mass of 220 ± 21 for the system. From the complete set of spectra, we find an effective temperature K for TWA22 A and K for TWA22 B and surface gravities between 4.0 and 5.5 dex. From our photometry and an M6 ± 1 spectral type for both components, we find luminosities of log(L/) = -2.11 ± 0.13 dex and log(L/) = -2.30 ± 0.16 dex for TWA22 A and B, respectively. By comparing these parameters with evolutionary models, we question the age and the multiplicity of this system. We also discuss a possible underestimation of the mass predicted by evolutionary models for young stars close to the substellar boundary.
Key words: stars: fundamental parameters / stars: low-mass, brown dwarfs / binaries: close / stars: formation / instrumentation: adaptive optics / instrumentation: spectrographs
© ESO, 2009