Zeta Herculis (Rutilicus / Ruticulus) 3?

The visual duplicity of the system was discovered by Sir William (Friedrich Wilhelm) Herschel (1738-1822) as early as July 18, 1782 (Thomas J.J. See, 1895). Over the next two centuries, however, astronomers measured unexpected perturbations in the orbits of the binary system indicative of an unseen body now believed to be orbiting Star A only -- more below. (See an animation of the binary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.) Once designated the title member of the Zeta Herculis stellar moving group, some astronomers have proposed that the system be removed from that group of stars (i.e., because it is too metal rich) and that its associated group of stars be re-designated as the Zeta Reticuli "kinematic" or stellar moving group (del Peloso et al, 2000; and Olin J. Eggen, 1971 and 1958).

Star A is a yellowish subgiant star of spectral and luminosity type F9 to G0 IV, but it has been classed as orange as G2. This star has around as much as 1.45 (+/- 0.01) times Sol's mass (Morel et al, 2001; Chmielewski et al, 1995; Peter van de Kamp, 1938, ; and Y.C. Chiang, 1928), 1.7 to 2.6 times Sol's diameter (Fracassini et al, 1994; and Johnson and Wright, 1983: page 687), and 6.6 times its visual luminosity and perhaps around 8.1 times its bolometric luminosity (Morel et al, 2001; and Chmielewski et al, 1995). The star is probably closer to 1.1 (rather than 1.8) times as enriched as Sol with elements heavier than hydrogen ("metallicity") based on its abundance of iron (Morel et al, 2001); and Cayrel de Strobel et al, 1991, page 300; and Helfer el al, 1963). According to Professor James Kaler's summary of Zeta Herculis, massive Star A probably has halted core hydrogen fusion, although it may only be around 3.4 to 4.4 billion years old (Morel et al, 2001; and Chmielewski et al, 1995). Given its high estimated mass, Star A may have spent much of its main sequence life as a late A- to early F-type dwarf before evolving into a yellowish subgiant. It has been designated NSV 7915 as a New Suspected Variable star. Useful star catalogue numbers for the star include: Zet Her A, 40 Her, HR 6212, Gl 635 A, HD 150680, HIP 81693, BD+31 2884, SAO 65485, LHS 3234, LTT 14952, LFT 1299, Struve 2084 A, and ADS 10157 A.

According to the Sixth Catalog of Orbits of Visual Binary Stars, Star A and its visual stellar companion B have an average separation of 14.4 AUs (a = 1.33") in a highly elliptical orbit (e~ 0.46) with a period of 34.45 years that had the widest separation in 1956 and 1990 and is inclined by 131 degrees from the perspective of Earth (Staffan Söderhjelm, 1999; Wulff-Dieter Heintz, 1994; Sarah L. Lippincott, 1981; and Paul Baize, 1949). In addition, based on observations of orbital pecularities made over two centuries, Star A has been long suspected of having a unseen companion (W. Doberck, 1897: "due to a most curious and persistent constancy of error"), which may be as small as a brown dwarf mass of as little as 0.05 Solar-mass (Morel et al, 2001; Scarfe et al, 1983; D.W. McCarthy, Jr., 1983; Paul Baize, 1976; Louis Berman, 1941; George C. Comstock, 1917; T. Lewis, 1900; and W. Doberck, 1897).

Star B is a orange-red main sequence dwarf star of spectral and luminosity type G7-K0 V (Morel et al, 2001; and Struve and Radcliffe, 1954). This star has around 98 +/- 2 percent of Sol's mass (Morel et al, 2001), a diameter around 86 to 88 percent of Sol's (Fracassini et al, 1994; and Johnson and Wright, 1983: page 687), and 62 percent of its visual luminosity and 77 percent of its bolometric luminosity (Morel et al, 2001; and Chmielewski et al, 1995). Useful star catalogue numbers for the star include: Zet Her B, Gl 635 B, BD+31 2884 B, Struve 2084 B, and ADS 10157 B.

The orbital distance from Zeta Herculis B where an Earth-type planet would be "comfortable" with liquid water is centered around 0.88 AU -- between the orbital distances of Venus and Earth in the Solar System. It possible that such an orbit would be stable even with the closest approach of Star A at 7.8 AUs. At that distance from the star, such a planet would have an orbital period close to 303 days. (See an animation of the binary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)