On the age of the TW Hydrae association and 2M1207334-393254

Abstract

We have estimated the age of the young moving group TW Hydrae Association, a cohort of a few dozen stars and brown dwarfs located near the Sun which share the same kinematic properties and, presumably, the same origin and age. The chronology has been determined by analyzing different properties (magnitudes, colors, activity, lithium) of its members and comparing them with several well-known star forming regions and open clusters, as well as theoretical models. In addition, by using medium-resolution optical spectra of two M8 members of the association (2M1139 and 2M1207 -an accreting brown dwarf with a planetary mass companion), we have derived spectral types and measured H(alpha) and lithium equivalent widths. We have also estimated their effective temperature and gravity, which were used to produce an independent age estimation for these two brown dwarfs. We have also collected spectra of 2M1315, a candidate member with a L5 spectral type and measured its H(alpha) equivalent width. Our age estimate for the association, 10 Myr (in the range 3-20 Myr), agrees with previous values cited in the literature. In the case of the two brown dwarfs, we have derived an age of 15 Myr (in the range 5-30 Myr), which also agree with our estimate for the whole group. We compared our results with recent articles published on the same subject using other techniques, and discuss the limits of the age-dating techniques.