DISCOVERY OF A FAINT COMPANION TO ALCOR USING MMT/AO 5 μm IMAGING

By Eric Mamajek, M. Kenworthy, P. Hinz, M. Meyer. 2009

212 citations recorded by the source corpus.

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 M☉) active star which is responsible for Alcor's X-ray emission detected by ROSAT (LX ≃ 1028.3 erg s−1). 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−1, which is comparable to the predicted velocity amplitude induced by the newly discovered companion (∼1 km s−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 ± 0.19 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 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 < 40 pc).

Subfields: Astronomy and Astrophysics, Instrumentation

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In Astronomy and Astrophysics

1. B. Macintosh
2. C. Marois
3. R. Doyon
4. B. Bowler
5. J. Graham
6. J. Patience
7. A. Schneider
8. J. Bulger

In Instrumentation

1. B. Macintosh
2. C. Marois
3. J. Graham
4. J. Bulger
5. R. Doyon
6. J. Patience
7. Inseok Song
8. N. Madhusudhan

Citations in the corpus

• The Volume-limited A-Star (VAST) survey – I. Companions and the unexpected X-ray detection of B6–A7 stars (2011, background): “Recent discoveries of low-mass companions to Alcor (Mamajek et al. 2010; Zimmerman et al. 2010) and ζ Virginis (Hinkley et al. 2010) have both noted that the unexplained X-ray emission from the primary can be explained by the lower mass companion, and demonstrated how X-ray emission from A-type star”