发现一颗孤立诞生的宽双星褐矮星
Discovery of a Wide Binary Brown Dwarf Born in Isolation
摘要：
在使用斯皮策太空望远镜对金牛座恒星形成区中具有圆盘的恒星进行调查期间，我们在巴纳德215暗云中发现了一对年轻的棕矮星FU Tau A和B。它们的投影角距为5.7”，相当于金牛座距离的800 AU。为了评估这两个对象的性质，我们已经获得了它们的光谱，并构建了它们的光谱能量分布。这两个来源都是年轻的（~ 100万年），根据他们的阿尔法发射，重力敏感的光谱特征，和中红外过量发射。FU Tau A的自行提供了它属于金牛座的额外证据。根据Chabrier和Baraffe的演化模型，我们分别测量了FU Tau A和B的M7.25和M9.25的光谱类型，其对应于~0.05和~0.015 M \cdot的质量。相对于通过FU Tau B的等时线，以及相对于金牛座光谱类型附近的其他成员，FU Tau A显着过亮，这可能表明它是一个无法解析的双星。基于金牛座产生两颗不相关的棕矮星的概率很低（~ 3x 10 ^-4），预测的距离为a </- 6”，FU Tau A和B很可能是一个双星系统的组成部分。巴纳德215只包含另一颗年轻恒星，位于金牛座的偏远地区，使FU Tau A和B成为第一颗被光谱证实的褐矮星，它们是孤立形成的，而不是在星团或聚集体中形成的。由于它们是孤立诞生的，并且是一个弱束缚的双星，与恒星的动力学相互作用在它们的形成中不可能发挥作用，因此对褐矮星的诞生来说不是必不可少的。勘误：表1中FU Tau B的K波段幅度不正确，应为13.33。表3和图5中FU Tau B的热光度是不正确的，因为这个错误和一个单独的算术错误。光度的正确值是0.0039 Lsun。在图5中的原始赫兹-罗素图中，FU Tau A和B显示出不同的等时年龄，这对于双星系统的成员来说是出乎意料的。在图5的修正版本中，这种差异减小了，因为这两个天体现在都在100万年的等时线之上。考虑到在如此年轻的年龄，模型等时线存在很大的不确定性，图5中FU Tau A和B的位置可能与共演化大致一致。
英文摘要：
During a survey for stars with disks in the Taurus star-forming region using the 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 angular separation of 5.7", corresponding to 800 AU at the distance of Taurus. To assess the nature of these two objects, we have obtained spectra of them and have constructed their spectral energy distributions. Both sources are young (~1 Myr) according to their Halpha emission, gravity-sensitive spectral features, and mid-IR excess emission. The proper motion of FU Tau A provides additional evidence of its membership in Taurus. We measure spectral types of M7.25 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 and Baraffe. FU Tau A is significantly overluminous relative to an isochrone passing through FU Tau B and relative to other members of Taurus near its spectral type, which may indicate that it is an unresolved binary. FU Tau A and B 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 a projected separation of a </- 6". Barnard 215 contains only one other young star and is in a remote area of Taurus, making FU Tau A and B the first spectroscopically-confirmed brown dwarfs discovered forming in isolation rather than in a stellar cluster or aggregate. Because they were born in isolation and comprise a weakly bound binary, dynamical interactions with stars could not have played a role in their formation, and thus are not essential for the birth of brown dwarfs. ERRATUM: The K-band magnitude for FU Tau B in Table 1 is incorrect and should be 13.33. The bolometric luminosity of FU Tau B in Table 3 and Figure 5 is incorrect because of that mistake and a separate arithmetic error. The correct value of the luminosity is 0.0039 Lsun. FU Tau A and B exhibited different isochronal ages in the original Hertzsprung-Russell diagram in Figure 5, which was unexpected for members of a binary system. This discrepancy is reduced in the corrected version of Figure 5 since both objects are now above the isochrone for 1 Myr. Given the large uncertainties in model isochrones at such young ages, the positions of FU Tau A and B in Figure 5 could be roughly consistent with coevality.
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