Edasich / Iota Draconis
|Home | Stars | Orbits | Habitability | Life ||
Dupree, Ronald Gilliland,
NASA and ESA
(Larger ultraviolet image -- more).
A highly evolved, orange-red giant star, Edasich is
still much smaller than the red supergiant Betelgeuse,
at left. (See a Digitized Sky Survey image of Edasich
at John Whatmough's web site, "Extrasolar Visions.")
Edasich, or Iota Draconis, is a third magnitude star located around 100 light-years (ly) from Sol. In late 2001, the California Planet Research Team estimated that it is about 98 ly or 30 parsecs away (exoplanets.org). They revised an earlier HIPPARCOS parallax estimate of l02 ly made in the 1990s that had an error margin (Plx= 31.92 +/- 0.51 mas) just big enough to suggest that the star may actually lie about 100 ly away, in agreement with Earth-based parallax measurements computed before 1978 as reported by Robert Burnham, Jr. (1931-93). Burnham also noted that the annual proper motion of this star is only 0.01" and that its radial velocity is 6.5 miles (about 10.5 km) per second in approach. Edasich has a ninth-magnitude, visual companion located about four arc-minutes away. (Note - While Edasich is not included in the Bright Star Map available on-line, it is available on the "150ly-h.zip" map file for the PC version of ChView that contains stars known to be located within 150 ly of Sol.)
The proper name for Iota Draconis is derived from the Arabic "Al Dhiba" or "Al Dhih" for Male Hyena, from which comes Eldsich or, more more commonly by the end of the 19th Century, Ed Asich. In the venerable Star Names: Their Lore and Meaning, Richard Hinckley Allen noted that the star was also known in China as "Tso Choo" for the Left Pivot. The star lies in the southeastern part (15:24:55.8+58:57:57.8, ICRS 2000.0) of Constellation Draco (see chart and photo), the Dragon -- west of Theta Draconis, southeast of Thuban (Alpha Draconis), and south of the Big Dipper (or Plough). Edasich was Earth's North Celestial Pole Star around 4,420 BCE.
On January 8, 2002, astronomers announced the accidental
discovery of a giant planet around this star (see
press release and
more details below). See an animation of the
orbit of this substellar
object around Edasich, with a table of basic orbital
and physical characteristics.
Edasich is a orange-red giant star of spectral and luminosity type K2 III. The star may have only 105 percent of Sol's mass (exoplanets.org) but 13 times its diameter. It has almost 40 times Sol's visual luminosity based on the Johnson Vmag value of 3.29 estimated HIPPARCOS observations during the 1990s, but Robert Burnham, Jr. (1931-93) noted that it had as much as 45 times Sol's luminosity in an estimate made before 1978 (apparently not based on his reported magnitude of 3.30) -- although a dimmer magnitude of 3.5 had been found earlier (Paul W. Merrill, 1922). According to the California Planet Research Team, Edasich may be slightly more (107 percent) enriched than Sol with elements heavier than hydrogen ("metallicity") based on its abundance of iron exoplanets.org, although an earlier reference suggests that it is at least twice as enriched (Cayrel de Strobel et al, 1991, pp. 298 -- and 294 to compare Epsilon Virginis with Sol).
It is notable that Edasich has an estimated mass near that of Sol but is more evolved. Although the star may have shed some of its mass already after progressing in the giant branch farther than, say, Pollux, it may be significantly older than Sol's 4.6 billion years. The star has probably shut down hydrogen fusion in its helium-rich core. Edasich is a New Suspected Variable star designated as NSV 7077. Useful catalogue numbers and designations for the star include: Iot Dra, 12 Dra, HR 5744*, Hip 75458, HD 137759, BD+59 1654, SAO 29520, FK5 571, and ADS 9688.
As a star that has evolved out of the "main sequence," Edasich has is shifting from the fusion of hydrogen to helium at its core to the fusion of helium to carbon and oxygen, with trace activity of other nuclear processes. This helium-burning, orange-red giant stage is relatively brief, lasting tens to hundreds of million years (e.g., lasting around 700 million years for a star of one Solar mass).
Eventually, the star will lose much of its current mass, from an intensified stellar wind that eventually puffs out its outer gas envelopes of hydrogen and helium (and lesser amounts of higher elements such as carbon and oxygen) into interstellar space as a planetary nebula. The result will be a planet-sized, white dwarf core that gradually cools and fades in brightness from the shutdown of thermonuclear fusion. (Nearby white dwarfs include solitary Van Maanen's Star and the dim companions of Sirius, Procyon, and 40 (Omicron2) Eridani.)
On January 8, 2002, a team of astronomers (including Sabine Frink, David S. Mitchell, Andreas Quirrenbach, Debra A. Fischer, Geoffrey W. Marcy, and R. Paul Butler) announced the discovery of a giant planet companion "b" to Edasich, at the 199th Meeting of the American Astronomical Society in Washington, DC (see UCSD or exoplanets.org press release, and graphics). The discovery came by accident when the astronomers made Doppler-shift measurements of Edasich while analyzing astrometric reference stars for NASA's Space Interferometry Mission (SIM), using the 0.6 m (24 inch) Coudé Auxiliary Telescope at the University of California's Lick Observatory, on Mount Hamilton in Califonia. Edasich was previously suspected of being a binary star, unresolved by speckle interferometry (Harold A. McAlister, 1978; see note 31 on page 292).
The object has at least 8.6 times the mass of Jupiter, with a similar diameter (about 11 times that of Earth). However, because the method of detection relies on radial velocities using the Doppler technique only determines the object's minimum mass, companion b may actually be a brown dwarf. Further work to determine the total mass of this object is planned when NASA launches SIM in 2009.
Planet b appears to move around Edasich with an average separation of 1.34 AUs with a period of almost 551 days, or one and a half years. Its orbit is highly eccentric (e= 0.71). In fact, it would have been extremely difficult to detect this substellar object around a highly evolved star like Edasich, because giant stars often pulsate and produce radial velocity patterns similar to substellar companions. In the case of Edasich, however, the high orbital eccentricity of its companion made its motion distinguishable from stellar pulsation as the cause of the observed velocity variations. The presence of the planet has been confirmed by continued monitoring (Quirrenbach et al, 2011). (See an animation of the orbit of this substellar object around Edasich, with a table of basic orbital and physical characteristics.)
Given the highly eccentric orbit of Edasich's companion b, it is highly unlikely that any planet could have formed in the star's water zone before it left the main sequence. Even if such an orbit were possible, any Earth-type planets that orbited Edasich during its youth would by now have been burnt to a cinder, and possibly fallen into the star from frictional drag with the giant star's gaseous envelope. Astronomers would find it very difficult to detect an Earth-sized planet around this star using present methods.
Brown Dwarfs or Planets?
When brown dwarfs were just a theoretical concern, astronomers differentiated those hypothetical objects from planets by how they were formed. If a substellar object was formed the way a star does, from a collapsing cloud of interstellar gas and dust, then it would be called a brown dwarf. If it was formed by gradually accumulating gas and dust inside a star's circumstellar disk, however, it was called a planet. Once the first brown dwarf candidates were actually found, however, astronomers realized that it was actually quite difficult to definitely rule on the validity of competing hypotheses about how a substellar object was actually formed without having been there. This problem is particularly difficult to resolve in the case of stellar companions, objects that orbit a star -- or two.
University of California at Berkeley astronomer Ben R. Oppenheimer, who helped to discover a nearby brown dwarf, Gliese 229 b, is part of a growing group that would like to define a brown dwarf as an substellar object with the mass of 13 to 80 (or so) Jupiters. While these objects cannot fuse "ordinary" hydrogen (a single proton nucleus) like stars, they have enough mass to briefly fuse deuterium (hydrogen with a proton-neutron nucleus). Therefore, stellar companions with less than 13 Jupiter masses would be defined as planets.
Other prominent astronomers, such as San Francisco State University astronomer Geoffrey W. Marcy who also has helped to discover many extrasolar planets, note that there may in fact be many different physical processes that lead to the formation of planets. Similarly, there may also be many different processes that lead to the creation of brown dwarfs, and some of these may also lead to planets. Hence, more observational data may be needed before astronomers can determine how to make justifiable distinctions in the classification of such substellar objects.
The following table includes all star systems known to be located within 10 light-years (ly), plus more bright stars within 10 to 20 ly, of Edasich.
|Star System||Spectra &|
|Hip 78184||M0 V||8.6|
|* plus bright stars *||. . .|
|BD+61 1574||G5 V||12|
|CL Draconis AB||F0 IV |
|HR 5436||F4 IV||14|
|HR 5451||F6-8 V||14|
|BD+66 855||F5 V||16|
|BD+64 1017 AB||F8-G0 V |
|Aldhibain 3?||G8 III? |
Try Professor Jim Kaler's Stars site for other information about Iota Draconis at the University of Illinois' Department of Astronomy. The late John Whatmough created illustrated web pages on this system at Extrasolar Visions.
Up-to-date technical data on these stars may be found from: the HIPPARCOS Catalogue using the VizieR Search Service mirrored from the Centre de Données astronomiques de Strasbourg (CDS); NASA's ADS Abstract Service for the Astrophysics Data System; and the SIMBAD Astronomical Database mirrored from CDS, which may require an account to access.
Constellation Draco is associated with the dragon slain by Cadmus, the brother of Europa. It is a large and elongated constellation of the northern hemisphere and is one of the few constellations which really resemble the object they were named after. For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Draco. For another illustration, see David Haworth's Draco.
For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.
© 1998-2011 Sol Company. All Rights Reserved.