40 (Omicron2) Eridani 3 - Keid
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(Photo from Double Stars,
used with permission)
40 Eridani is a triple star system,
made of an orange-red K dwarf and
a tighter binary of red and white
dwarfs. See a 2MASS Survey image
of 40 Eridani ABC from the NASA
Star and Exoplanet Database.)
Also known as Keid, this triple star system is located less than 16.5 light-years (ly) away in the northernmost part (04:15:16.32-07:39:10.34, ICRS 2000.0) of Constellation Eridanus, the River -- northeast of Zaurak, Gamma Eridani. It is visible in the night sky. Because Omicron1 Eridani was named "The Egg" (Al Baid, now Beid) by the Arabs for its position near the nest of the Ostrich (Theta Eridani 2, which is located much further south), Omicron2 Eridani nearby to the southeast was called "The Egg-shells" (or Al Kaid, now "Keid"). Although Omicron2 (40) Eridani A has become one of the top 100 target stars for NASA's planned Terrestrial Planet Finder (TPF), this NASA has been delayed infinitely.
That Star A was not simply a single star was discovered in 1783 by Sir William Friedrich Wilhelm Herschel (1738-1822, portrait), who was born Friedrich Wilhelm Herschel and who subsequently discovered the planet Uranus in 1781 -- which led to his appointment in 1782 as private astronomer to the King of England. Star A's companion itself was discovered to be a binary pair BC in 1851 by Otto Wilhelm von Struve (1819-1905), who succeeded his father (Friedrich Georg Wilhelm) as director of Russia's Pulkova Observatory, made the first accurate determination of the constant of precession, and discovered some 500 binary stars. Finally, Star B became the first "white dwarf" to be discovered in 1910 when astronomers -- Henry Norris Russell (1877-1957), Edward Charles Pickering (1846-1919), and Williamina ["Mina"] Paton Stevens Fleming (1857-1911) -- realized that this dim star was of spectral type A rather than M (Ken Croswell, 1995, page 55).
A-BC Star System
Star A and the binary pair BC have a wide separation of about 418 AUs and an orbital period of some 8,000 years (Wulff Dieter Heintz, 1974). The B and C pair of stars have an "average" separation of about 35 AUs (of a semi-major axis) in a highly elliptical orbit (e= 0.410) that lasts about 252 years and swings between 21 and 49 AUs, and their orbital inclination is around 108.9 degrees from the Earth's line of sight (Heintz, 1974).
|A-BC Mass Center||0.0||...||...||...||...||...||...||...||...|
|Inner H.Z. Edge A?||0.556||0.439||0||?||...||...||...||...||...|
|Outer H.Z. Edge A?||1.103||1.228||0||?||...||...||...||...||...|
|BC Mass Center||234||~8,000||?||?||...||...||...||...||...|
|Inner H.Z. Edge C?||0.06||0.031||0||108.9||...||...||...||...||...|
|Outer H.Z. Edge C?||0.11||0.083||0||108.9||...||...||...||...||...|
This main sequence, orange-red dwarf (K1 Ve) may have about 89 percent of Sol's mass (RECONS), about 85 percent of its diameter (Johnson and Wright, 1983, page 655), and 36 percent of its luminosity. The star appears to be 46 percent to 102 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Cayrel de Strobel et al, 1991, page 282). It appears to be around 5.2 +/-1.2 billion years old (Ballouz et al, 2010). The system shows a radial velocity of about 25 miles per second (mps) -- about 40 km per second or kps -- in recession, but the true space velocity is about 62 mps (or 100 kps). Some alternative useful star catalogue numbers for the star are: Omi2 Eri A, 40 Eri A, HR 1325, Gl 166 A, Hip 19849, HD 26965, BD-07 780, SAO 131063, LHS 23, and LTT 1907.
© Torben Krogh & Mogens Winther,
(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)
Star A is an orange-red dwarf
star, like Epsilon Eridani
at left center of meteor.
According to calculations performed for the NASA Star and Exoplanet Database, the inner edge of Star A's habitable zone could be located at around 0.556 AUs from the star, while the outer edge lies farther out at around 1.103 AUs. Near the inner H.Z. edge at 0.6 AU from Star A -- between the orbital distance of Mercury and Venus in the Solar System -- a planet would have an orbital period of almost 203 days, or more than half an Earth year.
Star B is a white dwarf, stellar remnant of spectral and luminosity type DA4. It has a mass estimated from about 50.1 percent of Sol's (Provencal et al, 1998), only two percent of its diameter, and 33/10,000th of its brightness. It appears to be about 5.0 +/-1 billion years old, in cooling a "cooling time" of around 100 million years since it left the main sequence as a white dwarf (Ballouz et al, 2010). Some alternative useful star catalogue numbers for the star are: Omi2 Eri B, 40 Eri B, Gl 166 B, HD 26976, BD-07 781, G 160-60, LHS 24, ADS 3093, W 33, and Struve 518.
H. Bond (STSci), R. Ciardullo (PSU), WFPC2, HST, NASA
40 Eridani B is a young white dwarf (a remnant stellar core which enriched its closer companion, Eridani C,
with elements heavier than hydrogen when it cast off its outer gas layers) like planetary nebula NGC 2440.
Any habitable planets around Stars B would have been "fried" through heat and hard radiation long ago when Star B was a giant star and puffed out its outer layers to reveal its remnant stellar core as a white dwarf. For Star B, the current water zone is centered around 0.06 AU which would require an orbital period of about 7.8 days.
A very dim red dwarf (M4.5 Ve) with only about 19.5 percent of Sol's mass (RECONS), 28 percent of its diameter (Johnson and Wright, 1983, page 655), and 7/10,000th of its visual luminosity. It appears to be around 5.2 +/-1.2 billion years old (Ballouz et al, 2010). Star C is also a UV Ceti type, flare star, which has the variable star designation of DY Eridani. Some alternative useful star catalogue numbers for the star are: DY Eri, Omi2 C, 40 Eri C, Gl 166 C, LTT 1909, and LHS 25.
NASA -- larger image
40 Eridani C is a dim red dwarf star, like
Gliese 623 A (M2.5V) and B (M5.8Ve) at lower right.
With a spectral type of M4, Ross 128 can be used as a rough proxy for GJ 1214 (M4.5). According to calculations performed for the NASA Star and Exoplanet Database, the distance from Ross 128 where an Earth-type rocky planet may have liquid water on its surface has been estimated to be between 0.06 and 0.11 AU -- well within the orbital distance of Mercury in the Solar System. In that distance range from the star, such a planet would have a "year" of only 13 to 34 days. For Star C, its regular flares would sterilize a planet in its water zone.
Hunt for Substellar Companions
Using a highly sensitive, radial velocity method, the Lick Planet Search has not detected a brown dwarf or large Jupiter or Saturn class planet in the triple star system thus far (Cumming et al, 1999). A subsequent study ruled out the presence of planets larger than four Jupiter-masses within 5.2 AUs of the stars (Wittenmyer et al, 2006). As astronomers would find it very difficult to detect planets around all three stars using present methods, they had been hoping to use NASA's proposed Terrestrial Planet Finder (TPF) and the ESA's Darwin planned groups of observatories to search for a rocky inner planet in the so-called "habitable zone" (HZ) around 40 Eridani A. As currently planned, the TPF will include two complementary observatory groups: a visible-light coronagraph; and a "formation-flying" infrared interferometer, of which both are indefinitely postponed, while Darwin will launch a flotilla of three mid-infrared telescopes and a fourth communications hub but has been similarly postponed.
Life Around a Flare Star
Many dim, red (M) dwarf stars exhibit unusually violent flare activity for their size and brightness. These flare stars are actually common because red dwarfs make up more than half of all stars in our galaxy. Although flares do occur on our Sun every so often, the amount of energy released in a solar flare is small compared to the total amount of energy that Sol produces. However, a flare the size of a solar flare occurring on a red dwarf star (40 Eridani C) that is more than ten thousand times dimmer than our Sun would emit about as much or more light as the red dwarf does normally.
High resolution and jumbo images (Benz et al, 1998).
Star C is a flare star, like UV Ceti (Luyten 726-8 B)
shown flaring at left. UV Ceti is an extreme example
of a flare star that can boost its brightness by five times
in less than a minute, then fall somewhat slower back
down to normal luminosity within two or three minutes
before flaring suddenly again after several hours.
Flare stars erupt sporadically, with successive flares spaced anywhere from an hour to a few days apart. A flare only takes a few minutes to reach peak brightness, and more than one flare can occur at a time. Moreover, in addition to bursts of light and radio waves, flares on dim red dwarfs may emit up to 10,000 times as many X-rays as a comparably-sized solar flare on our own Sun, and so flares would be lethal to Earth-type life on planets near the flare star. Hence, Earth-type life around flare stars may be unlikely because their planets must be located very close to dim red dwarfs to be warmed sufficiently by star light to have liquid water (about 0.03 AU for Star C), which makes flares even more dangerous around such stars. In any case, the light emitted by red dwarfs may be too red in color for Earth-type plant life to perform photosynthesis efficiently.
The following star systems are located within 10 ly of 40 Eridani.
|Star System||Spectra &|
|LP 656-38||M3.5 V||3.8|
|BD-03 1123||M1.5 V||6.2|
|Epsilon Eridani||K2 V||6.4|
|LTT 17897||M4 V||8.1|
|(LP 944-20)||brown dwarf||8.2|
|Teegarden's Star||M6.5 V||~8.7|
|L 730-18 ABC||M3 V |
|G 99-44||DZ9 /VII||9.1|
|Ross 614 AB||M4.5 Ve |
|Ross 47||M4 V||9.3|
|Gliese 229||M1 Ve||9.8|
Up-to-date technical summaries on this star can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARICNS for Star A, Star B, and Star C; the NASA Star and Exoplanet Database for Stars A, B, and C; 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.
Eridanus, the river, wends its way from the Hunter's foot of Orion then southwest to the southern circumpolar zone to enclose a larger area of sky than any other constellation. For more information on stars and other objects in Constellation Eridanus and an illustration, go to Christine Kronberg's Eridanus. For another illustration, see David Haworth's Eridanus.
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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