Gliese 570 / HR 5568 ABC
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Hurt, NASA, JPL, CalTech,
Larger system image with moon,
as imagined by Hurt.
The Gliese 570 triple star system has
a very cool, methane brown dwarf "d"
in a wide orbit around an orange-red
dwarf and a tight binary of red dwarf
stars (press release and more graphics).
Also known as HR 5568 for its relative bright primary, this triple star system is located 19.3 (ly) away from Sol. It lies in the southwestern part (14:57:28.0-21:24:55.7 for Star A, and 14:57:26.5-21:24:41.5 for Star B, 14:57:26.5-21:24:41 for Star C, and 14:57:15.2-21:21:50 for Brown Dwarf d, ICRS 2000.0) of Constellation Libra, the Scales -- southwest of Zebenelgenubi (Alpha2 Librae) and northwest of Zubenhakrabi (Sigma Librae). Many astronomers now refer to this star system by its designation in the famous Gliese Catalogue of Nearby Stars (CNS, now ARICNS database) of Wilhelm Gliese (1915-93), who was a longtime astronomer at the Astronomiches Rechen-Institut at Heidelberg (even when it was at Berlin).
Due to Gliese 570's relative proximity and rough similarity of spectral type to Sol, the star has been an object of intense interest among astronomers. Eta Cassiopeiae became one of the top 100 target stars for NASA's proposed Terrestrial Planet Finder (TPF), but the project has been postponed indefinitely. It was also selected as a "Tier 1" target star for NASA's optical Space Interferometry Mission (SIM) to detect a planet as small as three Earth-masses within two AUs of its host star (and so some summary system information and images of Gliese 570 A may still be available from the SIM Teams), but the SIM project manager announced on November 8, 2010 that the mission was indefinitely postponed due to withdrawal of NASA funding.
Astronomers have identified Gliese 570 A
as a prime target for the Terrestrial Planet
Finder (TPF), and the Space Interferometry
Mission (SIM), now both indefinitely postponed.
As a relatively bright star in Earth's night sky, Star A is catalogued as Harvard Revised (HR) 5568, a numbering system derived from the 1908 Revised Harvard Photometry catalogue of stars visible to many Humans with the naked eye. The HR system has been preserved through its successor, the Yale Bright Star Catalogue -- revised and expanded through the hard work of E. Dorrit Hoffleit (1907-2007) and others. (More discussion on star names and catalogue numbers is available from Alan MacRobert at Sky and Telescope and from Professor James B. Kaler's Star Names.)
In the 1990s, the European HIPPARCOS satellite mission measured a parallax for the close binary Stars B and C that suggested a distance of 24.4 ly from Sol with a relatively large error (Plx=133.63 mas, +/- 33.56), about five ly farther than the estimated distance to Star A. Earth-based parallax and orbit observations made over a longer time frame, however, suggest that the two stars are actually bound to Star A and so must actually lie at the same distance from Sol (Staffan Soderhjelm, 1999; and Yale Parallax Catalogue). On January 15, 2000, astronomers announced the discovery of a cool methane brown dwarf in a very wide orbit (separation of over 1,500 AUs) around the three stars (Burgasser et al, 2000; press release; additional graphics; and more details below).
A-BC Multiple Star System
According to the new Sixth Catalog of Orbits of Visual Binaries, Star A and the close binary pair BC have a wide average separation of about 190 AUs (semi-major axis of 32.34"), moving in an eccentric orbit (e= 0.20) that lasts some 2,130 years and is inclined from the perspective of an observer on Earth at 72.53° (Alan Hale, 1994, pp. 312, 314, and 317; and Duquennoy and Mayor, 1988). In addition, according to the new Sixth Catalog of Orbits of Visual Binaries, the B and C pair of stars have an average separation around 0.79 AUs (semi-major axis of 0.133" at a distance of 19.26 ly) in a highly eccentric orbit (e= 0.765) lasting about 309 days with an inclination from the perspective of an observer on Earth of about 110° (Dmitri Pourbaix, 2000; and Duquennoy and Mayor, 1988). The triple star system is said to exhibit moderate chromospheric activity (Duquennoy and Mayor, 1988). It has a galactic motion (eccentricity of 0.195 and inclination of sin i = 0.553) that is consistent with an assignation to the old disk population (Augensen and Buscombe, 1978; and Glenn J. Veeder, 1974). If the stars are as young as their brown dwarf companion "d," on the other hand, then they may only be between two and five billion years old (Geballe et al, 2001).
|A-BC Mass Center||0.0||...||...||...||...||...||...||...||...|
|Gliese 570 A||103||2,130||0.20||72.53||0.76-0.80||0.77||...||...||1.02|
|Inner H.Z. Edge A?||0.473||0.373||0||72.53||...||...||...||...||...|
|Outer H.Z. Edge A?||0.930||1.029||0||72.53||...||...||...||...||...|
|BC Mass Center||87||2,130||0.20||72.53||...||...||...||...||...|
|Gliese 570 B||0.31||0.846||0.765||110||0.55||0.65||...||...||1.02?|
|Inner H.Z. Edge B?||~0.20||~0.12||0||110||...||...||...||...||...|
|Outer H.Z. Edge B?||~0.35||~0.27||0||110||...||...||...||...||...|
|Gliese 570 C||0.48||0.846||0.765||110||0.35||?||...||...||1.02?|
|Inner H.Z. Edge C?||~0.07||~0.03||0||110||...||...||...||...||...|
|Outer H.Z. Edge C?||~0.13||~0.08||0||110||...||...||...||...||...|
This main sequence, orange-red dwarf (K4-5 Ve) may have just over three fourths (76 percent) to 80 percent of Sol's mass (RECONS; and Demory et al, 2009), about 77 percent of its diameter, (Alan Hale, 1994, page 320; and Soderblom et al, 1991), only 15.6 percent of its luminosity and around 26 percent of its bolometric luminosity (NASA Star and Exoplanet Database, based on Kenneth R. Lang, 1980), and a rotational period of 48.3 days. It appears to be as enriched (102 percent) as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Cayrel de Strobel et al, 1991, page 297). The star has been identified as a BY-Draconis variable and given the variable designation KX Librae. Some alternative useful star catalogue numbers for the star are: KX Lib, HR 5568 A, Gl 570 A, Hip 73184, HD 131977, BD-20 4125, SAO 183040, FK5 1391, LHS 387, LTT 5949, LFT 1161, LPM 551, Vys or McC 726 A, and ADS 9446 A.
© Torben Krogh & Mogens Winther,
(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)
Gliese 570 A is an orange-red dwarf
star, like Epsilon Eridani at left
center of meteor. (See a 2MASS
Survey image of Gliese 570 A from
the NASA Star and Exoplanet
Star A's late spectral type and dim luminosity puts it possibly close to the lower limit of habitability for (multicellular) Earth-type plant and animal life, given the redness of its light and the increased risk of tidal locking from the closeness of the orbit necessary for liquid water on a planetary surface. The NASA Star and Exoplanet Database has calculated a habitable zone between 0.473 and 0.930 AUs around Proxima. Accounting for infrared radiation, the distance from Gliese 570 A where an Earth-type planet could have liquid water on its surface is around 0.702 around Venus' orbital distance of about 0.7 AU from Sol -- with a corresponding orbital period of 246 days (just over two-thirds of an Earth year). In any case, the rotation of such a planet would probably be tidally locked so that one side would be in perpetual daylight and the other in darkness.
This is a red dwarf of spectral and luminosity type M1 V. It has about 55 percent of Sol's mass (RECONS), perhaps less than 65 percent of its diameter, and 1.9 percent of its visual and and less than 3.8 percent of its bolometric luminosity (NASA Star and Exoplanet Database, based on Kenneth R. Lang, 1980). The star is a double-lined spectroscopic binary. Speckle interferometry resolved that Star B has a stellar companion "C" (with a possible mass ratio of 0.71 +/- 0.02) (Duquennoy and Mayor, 1988). Some alternative useful star catalogue numbers for the star are: HR 5568 B, Gl 570 B, Hip 73182, HD 131976, BD-20 4123, SAO 183039, Vys or McC 726 B, LHS 386, LTT 5948, LFT 1160, LPM 550, and ADS 9446 B.
NASA -- larger image
Gliese 570 B and C are dim red dwarf stars, like
Gliese 623 A (M2.5V) and B (M5.8Ve) at lower right.
Estimates provided by the NASA Star and Exoplanet Database -- where the inner edge of Star B's habitable zone could be located at around 0.563 AUs from the star, while the outer edge lies farther out at around 1.108 AUs -- appear to be incorrect for this spectral class M star. Instead, estimates from Groombridge 34 A (spectral type M1.5) can applied as a rough proxy, which indicated that the inner edge of Star B's habitable zone could be located around 0.2 AU from the star, while the outer edge lies around 0.35 AUs. Accounting for infrared heating, the distance from Groombridge 34 A where an Earth-type planet should have liquid water on its surface is centered around only 0.25 AU -- well within the orbital distance of Mercury in the Solar System, where a planet's period would be less than 62 days (or just under 9 weeks). Tidal locking of such a closely orbiting planet could result in perpetual day on one side and perpetual night on the other.
A rather dim red dwarf, Star C is of spectral and luminosity type M3 V, with only about 35 percent of Sol's mass (RECONS) and slightly over 0.3 percent of its visual luminosity. It appears to have a high rotational velocity (Duquennoy and Mayor, 1988). Some alternative useful star catalogue numbers for the star are: HR 5568 C, Gl 570 C, HD 131976 B, BD-20 4123 B, SAO 183039 B, Vys or McC 726 C, and LHS 386 B, and ADS 9446 C.
Ross 154 (spectral type M3.5) can be used as a rough proxy for Gliese 570 C (spectral type 3). According to one type of model calculations performed for the NASA Star and Exoplanet Database, the inner edge of Ross 154's habitable zone should be located very close to the star, at around 0.07 AU from the star, while the outer edge lies around 0.13 AUs. Accounting for the great infrared output of M-stars like Ross 154, the equivalent orbital distance for an Earth-type planet be only around 0.2 AUs (Lisa Kaltenegger, 2005, page 48). At around half of Mercury's orbital distance in the Solar System, however, the rotation of the planet could become tidally locked with the star so that one side would have perpetual daylight with the other in darkness. Assuming that Gliese 570 C has about 35 percent of Sol's mass, a small Earth-type rocky planet would complete its orbit the star in about 55 days (just under 8 weeks).
On January 15, 2000, astronomers at the Infrared Processing and Analysis Center (IPAC) on the Caltech campus in Pasadena, California (including Adam Burgasser and Davy Kirkpatrick) announced that they had found one of the coolest brown dwarfs then known around Gliese ABC. The object was observed at a wide separation of more than 1,500 AUs (258.3"+/-0.4" at 19.26 ly) from the triple star system. It has an estimated mass of 50 +/- 20 Jupiters (see Burgasser et al, 2000; press release; and more graphics).
IPAC (2MASS Atlas Image
Gliese 570 d is a cool methane brown
dwarf -- larger and boxed field images.
Atlas Image mosaic obtained as part of the
Two Micron All Sky Survey (2MASS), a joint
project of the University of Massachusetts
and the Infrared Processing and Analysis
Center/California Institute of Technology,
funded by the National Aeronautics and
Space Administration and the National
The status of object "d" as a methane brown dwarf was confirmed by taking its spectral fingerprint with the 4-meter telescope at the Cerro Tololo Interamerican Observatory in Chile. Found as one of 12 objects discovered since 1995 in the 2-Micron All Sky Survey (a collaborative project between IPAC and the University of Massachusetts known as 2MASS), the surface temperature of this substellar object was found to be a relatively cool 900° F (500° C), compared with Sol's 10,000° F (5,500°C). Gliese 570 d is significantly cooler (by 160° K) and less luminous than any other known brown dwarf, including the prototype "T" dwarf, Gliese 229 b. Classed as T7.5 V (Burgasser et al, 2006), it is estimated to be between two and five billion years old (Geballe et al, 2001). Ammonia (NH3), evidence of non-equilibrium chemistry, has also been detected in its atmosphere through analysis of its infrared spectrum and may be evidence of vertical mixing (Saumon et al, 2006).
About 15 of the methane brown, or "T" dwarfs, had been discovered by early 2000. David Golimowski, a co-discoverer of the first methane brown dwarf, Gliese 229 b, recently speculated that such cool, dim objects could be as plentiful as the stars found in Sol's neighborhood -- as many as "one for every star." 2MASS astronomers believe that there may be twice as many brown dwarfs (L and T types) as stars in the Milky Way galaxy but that they may sum to only 15 percent of its stellar mass.
Hunt for Planetary Companions
No gas giant or smaller planets have been detected in this triple star system thus far. Astronomers would find it very difficult to detect Earth-type planets around these stars 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.
© American Scientist
(Artwork by Linda Huff for Martin et al, 1997; used with permission)
Although brown dwarfs lack sufficient mass (at least 75-80 Jupiters) to
ignite core hydrogen fusion, the smallest true stars (red dwarfs) can
have such cool atmospheric temperatures (below 4,000° K) that it is
difficult to distinguish them from brown dwarfs. While Jupiter-class planets
may be much less massive than brown dwarfs, they are about the same
diameter and may contain many of the same atmospheric molecules.
University of California at Berkeley astronomer Ben R. Oppenheimer, who helped to discover 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.
While brown dwarfs have too little mass to fuse "regular" hydrogen (which has a single proton nucleus), virtually all of the ones discovered until 1999 were too hot -- that is "young" -- to show evidence of methane which is destroyed by stellar temperatures. In fact, while methane is a atmospheric characteristic of giant gas planets like Jupiter, the only brown dwarf found to even have a trace of methane was Gliese 229 b.
© John Whatmough (Artwork from Extrasolar Visions, used with permission)
The brown dwarf companion to Gliese 229 -- with its own dark satellite,
as imagined by Whatmough
In Spring 1999, however, two very dim and reddish brown dwarfs were found as solitary objects (one 30 light-years away in Ophiuchus and another also relatively nearby in Virgo). Analysis of their spectra indicated that both have atmospheres that are rich in methane. In addition, four similar objects that are too cool to be observed in visible light were found using near-infrared telescopes also to have the methane fingerprint of extremely cool (that is "old") brown dwarfs. These discoveries represent strong evidence that, although hard for astronomers to detect, faint brown dwarfs which have had billions of years to cool may represent a significant population of the universe. Some astronomers speculate that these objects may well be as numerous as the stars, reviving theories of stellar formation that suggest the existence of uncountably numerous brown dwarfs, rather than the relatively few easy-to-detect, bright ones found thus far.
The following star systems are located within 10 ly of Gliese 570.
|Star System||Spectra &|
|LP 914-54||M8 V||1.9|
|BD-11 3759||M3.5 V||3.6|
|Hip 72509||M V||4.8|
|Gl 581/HO Librae||M2.5 V||5.2|
|CD-25 10553 AB||M3 V |
|CD-40 9712||M0-3 V||7.1|
|L 768-119||M3.5-5 V||8.6|
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 and B; the SIMBAD Astronomical Database for Stars A, B, and C, and brown dwarf d; 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.
The ancient Greeks grouped the stars of Libra with Constellation Scorpius, the Scorpion. To the later Romans, however, Constellation Libra represent "the Scales of Justice" held by Julius Caesar. For later peoples, these scales became associated with Virgo as the Goddess of Justice, proving that political power is indeed fleeting. For more information about the stars and objects in this constellation, go to Christine Kronberg's Libra. For an illustration, see David Haworth's Libra.
For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.
Note: Nominated as a "notable nearby star" by Mike Stevens.
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