Discovery of a Faint Companion to Alcor Using MMT/AO 5 $\mu$m Imaging
  We report the detection of a faint stellar companion to the famous nearby A5V
star Alcor (80 UMa). The companion has M-band ($\lambda$ = 4.8 $\mu$m)
magnitude 8.8 and projected separation 1".11 (28 AU) from Alcor. The companion
is most likely a low-mass ($\sim$0.3 \msun) active star which is responsible
for Alcor's X-ray emission detected by ROSAT (L$_{\rm X}$ $\simeq$ 10$^{28.3}$
erg/s). Alcor is a nuclear member of the Ursa Major star cluster (UMa; d
$\simeq$ 25 pc, age $\simeq$ 0.5 Gyr), and has been occasionally mentioned as a
possible distant (709") companion of the stellar quadruple Mizar ($\zeta$ 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 ($\sim$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 $\pm$ 0.19 pc (74 $\pm$ 39 kAU; minimum
18 kAU), and their velocity vectors are marginally consistent ($\chi^2$
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).
