70 Ophiuchi is a binary star system of magnitude 4.03. It appears in the northeastern part of Ophiuchus, on the east side of a V-shaped asterism called "Poniatowski's Bull."
Components and Orbit
70 Oph is double star with components of magnitude 4.22 and 6.01. They are easily separable with a small telescope, swinging from 2" to 7" apart over a period of 88.4 years. William Herschel observed the system's two known stars in 1779, but credited Christian Mayer as having recognized its duplicity even earlier. Contrast effects make pair quite colorful; the 19th-century Admiral Smythe called them "pale topaz and violet."
At 16.6 light years away, 70 Oph is the 51st closest star system to Earth. The average separation between the stars is 23.3 AU. A fair orbital eccentricity (e=0.495) takes them as close as 11.7 AU and as far as 34.8 AU. Their last close approach was in 1984; their next greatest separation will be in 2028. This remarkable double star is near its closest approach to the Sun, the distance shortening to 15 light years 75,000 years from now.
Properties
The binary consists of two low mass cool yellow-orange class K dwarfs - a rarity, since most naked eye stars are hotter and more luminous than the Sun. The primary (70 Oph A) is a class K0 V star with a temperature of 5290 K, a luminosity 51% of the Sun's, and a radius 0.85 times solar. The respective parameters for the fainter secondary star (70 Oph B) are class K5 Ve, temperature 4250 K, 0.16 solar luminosities, and 0.70 solar radii.
From the orbital period and separation, the stars have a combined mass of 1.60 solar masses; from the location of their center of mass, 70 Oph A contains 0.89 solar masses, and for 70 Oph B contains 0.71 solar masses. Like the Sun, 70 Oph A is magnetically active, and has a surrounding corona of hot gas. Active regions rotating in and out of view reveal a rotation period of 19.7 days. Residual dust left over from the star's infancy has been detected in the binary system.
Perturbations
Over the past two centuries, many investigators of this well-known binary system's orbits have found evidence for a third body perturbing the motion of the two visible components. In 1855, Captain W. S. Jacob of the Madras Observatory of the East India Company claimed that the orbit of the binary showed an anomaly, and it was "highly probable" that there was a "planetary body in connexion with this system". T. J. J. See made a stronger claim for the existence of a dark companion in this system in 1899, but Forest Ray Moulton soon published a paper proving that a three-body system with the specified orbital parameters would be highly unstable. The claims by Jacob and See have both been shown to be erroneous, but Jacob's claim was probably one of the first for an exoplanet based on astrometric evidence.
Others claiming existence for a dark companion included J. H. Madler (1842), E. Doolittle (1897), and T. Lewis (1906). However, those and subsequent observers failed to agree on the orbital period or amplitude of the perturbation, or even on which of the two stars was disturbed in its motion. In 1937, Gunnar Strand found no evidence for a third body, but in 1943, Dirk Reuyl and Erik Holberg found indications of a 17-year perturbation from a body with about 10 times Jupiter's mass, using astrometric plates made at McCormick Observatory between 1914 and 1943. Most recently, however, Batten et al (1984) and Heintz (1988) have found no evidence supporting detectable perturbations.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]