A test of pre-main-sequence evolutionary models across the stellar/substellar boundary based on spectra of the young quadruple GG Tauri

We present spatially separated optical spectra of the components of the young hierarchical quadruple GG Tau. Spectra of GG Tau Aa and Ab (separation 0 ".25 similar to 35 AU) were obtained with the Faint Object Spectrograph on board the Hubble Space Telescope. Spectra of GG Tau Ba and Bb (separation 1 ".48 similar to 207 AU) were obtained with both the HIRES and the LRIS spectrographs on the W. M. Keck telescopes. The components of this minicluster, which span a wide range in spectral type (K7-M7), are used to test both evolutionary models and the temperature scale for very young, low-mass stars under the assumption of coeval formation. Of the evolutionary models tested, those of Baraffe et al. yield the most consistent ages when combined with a temperature scale intermediate between that of dwarfs and giants. The version of the Baraffe et al. models computed with a mixing length nearly twice the pressure scale height is of particular interest, as it predicts masses for GG Tau Aa and Ab that are in agreement with their dynamical mass estimate. Using this evolutionary model and a coeval (at 1.5 Myr) temperature scale, we find that the coldest component of the GG Tau system, GG Tau Bb, is substellar with a mass of 0.044 +/- 0.006 M.. This brown dwarf companion is especially intriguing as it shows signatures of accretion, although this accretion is not likely to alter its mass significantly. GG Tau Bb is currently the lowest mass, spectroscopically confirmed companion to a T Tauri star, and is one of the coldest, lowest mass T Tauri objects in the Taurus-Auriga star-forming region.