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We present spatially separated optical spectra of the components of the younghierarchical quadruple GG Tau. Spectra of GG Tau Aa and Ab (separation 0".25 ~35 AU) were obtained with the Faint Object Spectrograph aboard the Hubble SpaceTelescope. Spectra of GG Tau Ba and Bb (separation 1".48 ~ 207 AU) wereobtained with both the HIRES and the LRIS spectrographs on the W. M. Kecktelescopes. The components of this mini-cluster, which span a wide range inspectral type (K7 - M7), are used to test both evolutionary models and thetemperature scale for very young, low mass stars under the assumption of coevalformation. Of the evolutionary models tested, those of Baraffe et al. (1998,A&A, 337, 403) yield the most consistent ages when combined with a temperaturescale intermediate between that of dwarfs and giants. The version of theBaraffe et al. models computed with a mixing length nearly twice the pressurescale height is of particular interest as it predicts masses for GG Tau Aa andAb that are in agreement with their dynamical mass estimate.  Using this evolutionary model and a coeval (at 1.5 Myrs) temperature scale,we find that the coldest component of the GG Tau system, GG Tau Bb, issubstellar with a mass of 0.044 +/- 0.006 Msun. This brown dwarf companion isespecially intriguing as it shows signatures of accretion, although thisaccretion is not likely to alter its mass significantly. GG Tau Bb is currentlythe lowest mass, spectroscopically confirmed companion to a T Tauri star, andis one of the coldest, lowest mass T Tauri objects in the Taurus-Auriga starforming region.Comment: 25 pages, 6 figures, accepted for publication in The Astrophysical  Journa

A Test of Pre–Main‐Sequence Evolutionary Models across the Stellar/Substellar Boundary Based on Spectra of the Young Quadruple GG Tauri