71 Tauri: Hyades Enigma Resolved?

Abstract

71 Tauri (HD 28052; F0 IV-V) is an enigmatic object for two reasons: (1) it is the second brightest X-ray source in the Hyades, yet early F stars as a rule are not strong coronal emitters; and (2) it lies a magnitude above the cluster main sequence, but radial velocity studies and speckle imaging suggest that it is single. Recently, long-slit ultraviolet spectra of the star, obtained with the Space Telescope Imaging Spectrograph (STIS), serendipitously have revealed the presence of a stellar companion at a distance of 0.1" directly south of the primary. The companion is seen only in its far-UV chromospheric emission lines. The nature of this object cannot be determined from our STIS spectra alone, but its high emission levels are most readily explained if it is a close binary of coronally active dG/dK stars. The presence of the secondary can account for the striking X-ray properties of 71 Tau but not its unusual location in the cluster color-magnitude diagram. It is conceivable that the primary itself is a close double of nearly equal stars, making 71 Tau a possible quadruple system. The alternative-that 71 Tau is ~150 Myr older than other members of the Hyades, approaching the end of core hydrogen burning for a 2 M_solar star-would challenge the presumed synchrony of star formation in the cluster.