DISCOVERY OF A FAINT COMPANION TO ALCOR USING MMT/AO 5 {mu}m IMAGING
- University of Rochester, Department of Physics and Astronomy, Rochester, NY 14627-0171 (United States)
- Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 (United States)
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 ({approx}0.3 M {sub sun}) active star which is responsible for Alcor's X-ray emission detected by ROSAT (L {sub X} {approx_equal} 10{sup 28.3} erg s{sup -1}). Alcor is a nuclear member of the Ursa Major star cluster (UMa; d {approx_equal} 25 pc, age {approx_equal} 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{sup -1}, which is comparable to the predicted velocity amplitude induced by the newly discovered companion ({approx}1 km s{sup -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 ({chi}{sup 2} 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)
- OSTI ID:
- 21301278
- Journal Information:
- Astronomical Journal (New York, N.Y. Online), Vol. 139, Issue 3; Other Information: DOI: 10.1088/0004-6256/139/3/919; Country of input: International Atomic Energy Agency (IAEA); ISSN 1538-3881
- Country of Publication:
- United States
- Language:
- English
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