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Abstract: During a survey for stars with disks in the Taurus star-forming region usingthe Spitzer Space Telescope, we have discovered a pair of young brown dwarfs,FU Tau A and B, in the Barnard 215 dark cloud. They have a projected angularseparation of 5.7-, corresponding to 800 AU at the distance of Taurus. Toassess the nature of these two objects, we have obtained spectra of them andhave constructed their spectral energy distributions. Both sources are young~1 Myr according to their Halpha emission, gravity-sensitive spectralfeatures, and mid-IR excess emission. The proper motion of FU Tau A providesadditional evidence of its membership in Taurus. We measure spectral types ofM7.25 and M9.25 for FU Tau A and B, respectively, which correspond to masses of~0.05 and ~0.015 M\cdot according to the evolutionary models of Chabrier andBaraffe. FU Tau A is significantly overluminous relative to an isochronepassing through FU Tau B and relative to other members of Taurus near itsspectral type, which may indicate that it is an unresolved binary. FU Tau A andB are likely to be components of a binary system based on the low probability~3x10^-4 that Taurus would produce two unrelated brown dwarfs with aprojected separation of a <- 6-. Barnard 215 contains only one other youngstar and is in a remote area of Taurus, making FU Tau A and B the firstspectroscopically-confirmed brown dwarfs discovered forming in isolation ratherthan in a stellar cluster or aggregate. Because they were born in isolation andcomprise a weakly bound binary, dynamical interactions with stars could nothave played a role in their formation, and thus are not essential for the birthof brown dwarfs. ERRATUM: The K-band magnitude for FU Tau B in Table 1 isincorrect and should be 13.33. The bolometric luminosity of FU Tau B in Table 3and Figure 5 is incorrect because of that mistake and a separate arithmeticerror. The correct value of the luminosity is 0.0039 Lsun. FU Tau A and Bexhibited different isochronal ages in the original Hertzsprung-Russell diagramin Figure 5, which was unexpected for members of a binary system. Thisdiscrepancy is reduced in the corrected version of Figure 5 since both objectsare now above the isochrone for 1 Myr. Given the large uncertainties in modelisochrones at such young ages, the positions of FU Tau A and B in Figure 5could be roughly consistent with coevality.

Author: K. L. Luhman, E. E. Mamajek, P. R. Allen, A. A. Muench, D. P. Finkbeiner


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