Discovery of a Faint Companion to Alcor Using MMT/AO 5 μm Imaging

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We report the detection of a faint stellar companion to the famous nearby A5V star Alcor (80 UMa). The companion has M-band (λ = 4.8 μm) magnitude 8.8 and projected separation 1".11 (28 AU) from Alcor. The companion is most likely a low-mass (0.3) active star which is responsible for Alcor's X-ray emission detected by ROSAT (L X ≃ 10²⁸.³ erg/s). Alcor is a nuclear member of the Ursa Major star cluster (UMa; d ≃ 25 pc, age ≃ 0.5 Gyr), and has been occasionally mentioned as a possible distant (709") companion of the stellar quadruple Mizar (ζ UMa). Comparing the revised Hipparcos proper motion for Alcor with the mean motion for other UMa nuclear members shows that Alcor has a peculiar velocity of 1.1 km/s, which is comparable to the predicted velocity amplitude induced by the newly-discovered companion (1 km/s). Using a precise dynamical parallax for Mizar and the revised Hipparcos parallax for Alcor, we find that Mizar and Alcor are physically separated by 0.36 ± 0.19 pc (74 ± 39 kAU; minimum 18 kAU), and their velocity vectors are marginally consistent (χ² probability 6%). Given their close proximity and concordant motions we suggest that the Mizar quadruple and the Alcor binary be together considered the 2nd closest stellar sextuplet. The addition of Mizar-Alcor to the census of stellar multiples with six or more components effectively doubles the local density of such systems within the local volume (d < 40 pc).