Xi Boötis 2

Xi Boötis A

This star is a yellow-orange main sequence dwarf star of spectral and luminosity type G8 Ve. It may have 90 to 94 percent (+/- 20 percent) of Sol's mass (RECONS; and Daniel M. Popper, 1980, page 36), 89 percent of its diameter (Johnson and Wright, 1983, page 681), and about 49 percent of its luminosity. It may be somewhat less enriched as Sol with elements heavier than hydrogen ("metallicity") based on the ionization balance of iron and a comparison between observed and computed profiles for the extremely strong ionized calcium (Ruck and Smith, 1995) -- metallicity measurements based on iron alone range from 55 to 155 percent of Sol's (Cayrel de Strobel et al, 1991, page 26). While strong chromospheric activity suggest that the star may be as young as 60 million years (see HD 131156 A in Table 2 from Don Barry, 1988), the lack of an easily observable dust disk -- as is found at Epsilon Eridani -- suggests that the stars may be well over a billion years old. Xi Bootis A classified as a BY Draconis type, rotating variable star whose visual magnitude varies from 4.52 to 4.67 over 10.137 days. Useful star catalogue numbers for the Xi Boötis A include: Xi or Chi or Ksi Boo, 37 Boo, HR 5544, Gl 566 A, Hip 72659, HD 131156, BD+19 2870, SAO 101250, Struve 1888 A, and ADS 9413 A.

Previous orbital calculations (see ADS 9413 from Table II in Roland Wielen, 1962) have been slightly revised. According to new measurements (Staffan Soderhjelm, 1999) found in the new Sixth Catalog of Visual Orbits of Binary Stars, star A and B are separated by an "average distance" of about 33.6 AUs (semi-major axis of 4.94" with a HIPPARCOS parallax of 0.14710 +/- 0.00080"). They move in an elliptical orbit (e= 0.51) that takes about 151.6 years to complete, travelling as close as 16.5 AUs and as far away as 50.7 AUs. Their orbit is inclined about 139° from the perspective of an observer on Earth. (See an animation of the orbits of Stars A and B and their potentially habitable zones, with a table of basic orbital and physical characteristics.)

© Torben Krogh & Mogens Winther,

(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)


Xi Boötis B is an orange-red dwarf star,
like Epsilon Eridani at left center of
meteor

Xi Boötis B

This star is a orange-red main sequence dwarf star of spectral and luminosity type K4-5 Ve. This star may have 67 to 76 percent of Sol's mass (RECONS), 71 percent of its diameter (Johnson and Wright, 1983, page 681), and about 6.1 percent of its luminosity. Useful star catalogue numbers for the star include Gl 566 B, Struve 1888 B, and ADS 9413 B.

Xi Boötis C?

In 1943, Kaj Aage Gunnar Strand (1907-2000; obit) -- who later became scientific director of the U.S. Naval Observatory) -- announced his detection of small systematic variations in the orbital motion of the binary system based on photographic observations taken between 1939 and 1942 at Sproul Observatory. These astrometric perturbations suggested that there was a third, unseen body orbiting one of the visible stars with a period of 2.2 years and a computed mass of about one-tenth the mass of Sol. Despite the relative proximity of this system to Earth, however, a star with such a low mass would probably be too faint to be detected through visual observation because of glare from the brighter stars. In 1988, Campbell et al also reported finding small velocity variations that could be the result of a substellar object of one to nine times the mass of Jupiter that may orbit Xi Boötis B. The detected velocity variations were too small to be cause by a body orbiting star A, which is more massive (see HR 5544 on page 919).

Hunt for Substellar Companions

Since at least one of the stars of Xi Boötis is fairly similar to our Sun, some speculate whether the system might contain planets that harbor life. The failure, thus far, to find large substellar objects like brown dwarfs or a Jupiter- or Saturn-class planet in a "torch" orbit (closer than the Mercury to Sun distance) around Xi Boötis A -- with even the highly sensitive radial-velocity methods of Geoffrey W. Marcy and R. Paul Butler -- bodes well for the possibility of Earth-type terrestrial planets around this star (Cumming et al, 1999). The distance from star A where an Earth-type planet would be "comfortable" with liquid water is centered around 0.70 AU -- about the orbital distance of Venus in the Solar System, with an orbital period of about 221 days, or around six-tenths of an Earth year. For star B, the liquid water zone is centered aroud 0.25 AU -- closer than the orbital distance of Mercury, with an orbital period of just 54 days. Astronomers would find it very difficult to detect an Earth-type planet around either of these stars using present methods.