Edasich / Iota Draconis

© James B. Kaler, UIUC
(used with permission)


Photo from Stars, Planet Project,
and Iota Draconis.

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 UCSD or exoplanets.org press release and graphics, more details below). See an animation of the orbit of this substellar object around Edasich, with a table of basic orbital and physical characteristics.

The Star

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).

NASA Observatorium



Edasich is redder than
Pollux but almost as
orange as Arcturus.



See a discussion of
the "main sequence"
as part of stellar
evolution and death.

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.

NASA Observatorium


See a discussion of
"core helium burning"
as part of stellar
evolution and death.

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.)

H. Bond (STSci), R. Ciardullo (PSU), WFPC2, HST, NASA -- larger image
(White dwarfs are remnant stellar cores that have cast off their outer gas
layers, like planetary nebula NGC 2440.)

Planet or Brown Dwarf "b"

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).

NASA, JPL (Planet Quest)



With Earth-Sol
comparison image.



Edasich "b" is either a giant
planet or a brown dwarf.

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.)

Hunt for Substellar Objects

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.

© American Scientist



Artwork by Linda Huff
(for Martin et al, 1997)
used with permission.

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.