Discovery of a faint companion to alcor using MMT/AO 5 μm imaging

Eric E. Mamajek, Matthew A. Kenworthy, Philip M. Hinz, Michael R. Meyer

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21 Scopus citations


We report the detection of a faint stellar companion to the famous nearby A5V star Alcor (80UMa). The companion has M-band (λ = 4.8μm) magnitude 8.8 and projected separation 111 (28 AU) from Alcor. The companion is most likely a low-mass (∼0.3M ) active star which is responsible for Alcor's X-ray emission detected by ROSAT (L X ≃ 10 28.3ergs-1). Alcor is a nuclear member of the Ursa Major star cluster (UMa; d ≃ 25pc, age ≃ 0.5Gyr), and has been occasionally mentioned as a possible distant (709″) companion of the stellar quadruple Mizar (ζUMa). Comparing the revised Hipparcos proper motion for Alcor with the mean motion for otherUMa nuclear members shows that Alcor has a peculiar velocity of 1.1kms-1, which is comparable to the predicted velocity amplitude induced by the newly discovered companion (∼1kms-1). 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 ± 0.19pc (74 39kAU; minimum 18kAU), 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 second 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 < 40pc).

Original languageEnglish (US)
Pages (from-to)919-925
Number of pages7
JournalAstronomical Journal
Issue number3
StatePublished - 2010


  • Binaries: close
  • Binaries: general
  • Binaries: visual
  • Open clusters and associations: individual (Ursa Major)
  • Stars: individual (Alcor, Mizar)

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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