70 Ophiuchi 2?
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ę Torben Krogh & Mogens Winther,
(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)
70 Ophiuchi A and B are orange-red
dwarf stars, like Epsilon Eridani at
left center of meteor. See a 2MASS
Survey image of 70 Ophiuchi AB
from the NASA Star and Exoplanet
This binary system is located about 16.6 light-years away in the northeastern part (18:05:27.29+02:30:00.36, ICRS 2000.0) of Constellation Ophiuchus, the Serpent Holder -- east of Muliphen (Gamma Ophiuchi). Sir William Friedrich Wilhelm Herschel (1738-1822, portrait) observed the system's two known stars in August 1779 but credited Father Christian Mayer (1719-1783) as having recognized its duplicity even earlier. Mayer had claimed to have discovered that many of the more conspicuous stars in the southern heavens were surrounded by smaller stars, of which one such observation was recorded in "Tables d'Aberration et de Mutation" (Mannheim, 1778), but virtually all of which could not be confirmed by contemporaries such as Herschel.
AB Binary Star System
Stars A and B have a relatively wide separation. Previous estimates that AB are separated "on average" by 23.3 AUs (4.560" of a semi-major axis with a parallax of 0.19596, RECONS; 1/2000 table for Gl 702) in a highly eccentric orbit (e= 0.495) that swings between 11.7 and 34.8 AUs and takes 88.3 years to complete (Wulff Dieter Heintz, 1988; Batten and Fletcher, 1991; and D.J. Barlow, 1994) may have been revised slightly. Based on more recent measurements (Dimitri Pourbaix, 2000) found in the new Sixth Catalog of Visual Orbits of Binary Stars, 70 Ophiuchi A and B may be separated on average by a semi-major axis of 23.2 AUs (4.554") in a highly elliptical orbit (e= 0.499) that takes 83.38 years to complete. The distance separating the two stars varies from 11.4 and 34.8 AUs; they are always separated from each other by at least the orbital distance of Saturn in the Solar System. Lastly, the inclination of the orbit is 121.2░ (revised from 120.8░), from the perspective of an observer on Earth.
A main sequence orange-red dwarf (K0-1 Ve), the primary has only about 92 percent of Sol's mass (RECONS), about 89 percent of its diameter (Johnson and Wright, 1983, page 691), 43 percent of its visual luminosity (51 percent with infrared adjustment), and from 30 to 100 percent of Sol's abundance of elements heavier than hydrogen (metallicity), based on its abundance of iron (Caryrel de Strobel et al, 1991; page 302). On the other hand, residual dust left over from the star's infancy has been detected in the binary system, as has been found in the Solar System (Kuchner et al, 1998, in pdf). Based on chromospheric activity and rotational period, Star A (and Star B) are likely to be around 1.1 and 1.9 billion years old (Mamajek et al, 2008). 70 Ophiuchi A has the variable star designation V2391 Ophuichi. Some useful star catalogue numbers include: V2391 Oph, 70 Oph, HR 6752, Gl 702 A, Hip 88601, HD 165341, SAO 123107, LHS 458, ADS 11046 A, and Struve 2272 A.
Estimates provided by the NASA Star and Exoplanet Database -- where the inner edge of Star A's habitable zone could be located at around 0.690 AUs from the star, while the outer edge lies farther out at around 1.366 AUs -- appear to be incorrect for this spectral class K star. Instead, estimates from 40 Omicron Eridani A (another K0-1 star) can applied as a rough proxy, which indicated that the inner edge of Star B's habitable zone could be located around 0.56 AU from the star, while the outer edge lies around 1.10 AUs. Accounting for infrared heating, the distance from 40 Omicron A where an Earth-type planet would be "comfortable" with liquid water is centered around only 0.830 AU -- between the orbital distances of Venus and Earth in the Solar System, where a planet's period would be about 227 days or just under two thirds of an Earth year.
A main sequence orange-red dwarf (K5-6 Ve), star B has only about 70 percent of Sol's mass (RECONS), about 73 percent of its diameter (Johnson and Wright, 1983, page 691), and 8.4 to nine percent of its visual luminosity (16 percent with infrared adjustment). Some useful star catalogue numbers include: Gl 702 B, LHS 459, ADS 11046 B, and Struve 2272 B.
While the NASA Star and Exoplanet Database does not yet have estimates available, 62 Cygni B (another a K5-6 dwarf star) can be used as a rough proxy for 70 Ophiuchi B. According to calculations performed for the NASA Star and Exoplanet Database, the inner edge of Star B's habitable zone could be located at around 0.315 AUs from the star, while the outer edge lies farther out at around 0.617 AUs. Accounting for infrared heating, the distance from 61 Cygni B where an Earth-type planet would be "comfortable" with liquid water is centered around only 0.466 AU -- -- between the orbital distances of Mercury and Venus in the Solar System. At that distance from the star, such a planet would have an orbital period of about 146 days -- just over 40 percent of an Earth year.
70 Ophiuchi Ab or Bb?
Over the past two centuries, many investigators of the visual orbits of this well-known binary system have found evidence for a third body of fairly low mass perturbing the motion of one or other of the two visible components. According to Robert Burnham, Jr., the earliest observers who found that 70 Ophiuchi showed clear deviations in Keplerian motion included: J. H. Madler (1842), W.S. Jacob (1855), T.J.J. See (1896), and 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, Kaj Aage Gunnar Strand (1907-2000) -- who later became scientific director of the U.S. Naval Observatory -- 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 astrometic 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. A more recent search using radial-velocity variations suggests that no objects exceeding four Jupiter-masses orbits orbits within 5.2 AUs of either star (Wittenmyer et al, 2006).
The following star systems are located within 10 ly of 70 Ophiuchi.
|Star System||Spectra &|
|Ross 652 A
|M3.5 Ve |
|Wolf 630 A-C||M2.5 Ve |
|BD-12 4523 AB||M3.0 V |
|Wolf 629 AB||M3.5 V |
|BD-03 4233||M0 V||9.1|
|BD+02 3312||K7 V||9.3|
|Ross 154||M3.5-6 Ve||9.3|
|G 154-44||M4.5 V||9.9|
Up-to-date technical summaries on individual stars can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARICNS for Star A and Star B, the Nearby Star and Exoplanet Database for Stars A and B, and the Research Consortium on Nearby Stars (RECONS) list of the 100 Nearest Star Systems. Additional information may be available at Roger Wilcox's Internet Stellar Database.
One story is that the Ancient Greeks named this constellation after Aesculapius (the first doctor, a son of Apollo and Coronis, and grandfather of Hippocrates, the famous Greek physician). Aesculapius was killed by Zeus at the urging of Hades for threatening to make mankind immortal like the gods by bringing the dead back to life. In admiration of the doctor's skills, however, Zeus raised the doctor and the serpent from which he had first learned the medicinal usefulness of certain herbs into the heavens. Located along the equatorial region of the sky, Ophiuchus is one of the larger constellations. For more information and an illustration of the constellation, go to Christine Kronberg's Ophiuchus. For another illustration, see David Haworth's Ophiuchus.
For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.
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