Tau Bo÷tis 2
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On June 27, 2010, a team of astronomers using the European Southern Observatory's Very Large Telescope were able to directly capture infrared light from the hot Jupiter-class planet "b" orbiting Tau Bo÷tis A, although the planet does not transit the star. By studying carbon dioxide in planetĺs atmosphere via spectroscopy and measuring its orbit and mass more precisely for the first time since its discovery some 15 years ago, the team found that the planetĺs atmosphere appears to be cooler higher up, contrary to what was expected. The planet was found to have around 5.95 +/- 0.28 Jupiter-masses and an orbit inclined about 44.5 +/- 1.5 degrees from the perspective of an observer on Earth (ESO news release; and Brogi et al, 2012).
The Tau Bo÷tis system is located about 50.9 light-years from Sol. It lies in the southwestern part (13:47:15.7+17:27:24.9, ICRS 2000.0) of Constellation Bo÷tes, the Bear Driver -- southwest of Muphrid (Eta Bo÷tis) and Arcturus (Alpha Bo÷tis) and northwest of Nu Bo÷tis. In 1996, astronomers announced the discovery of a Jupiter-like planet around this Sun-like star (Butler et al, 1997) -- details below).
Tau Bo÷tis AB System
Tau Bo÷tis A has a widely separated, dim companion Star ("B"). This star is located about 224 AUs away on average (a semi-major axis of 14.39" at a HIPPARCOS distance of 50.87 light-years) with an orbital period around 2,000 years and an extremely high orbital eccentricity of 0.91. This orbit is tilted by 50.68░ from the perspective of an observer on Earth (Alan Hale, 1994, see HD 120136; and the Sixth Catalog of Visual Orbits of Binary Stars).
|AB Mass Center||0.0||...||...||...||...||...||...||...||...|
|Tau Bo÷tis A||30||~2,000||0.91||50.68||420,000||159||...||...||2.09|
|Disrupted H.Z. A?||1.8||2.2||0||44.5||...||...||...||...||...|
|Tau Bo÷tis B||194||~2,000||0.91||50.68||70,000||63||...||...||...|
Digitized Sky Survey 2,
Larger and jumbo images.
Tau Bo÷tis A is a spectral
type F star that is somewhat
bluer, brighter, and larger
than Sol, and is visible to
the unaided Human eyes
from Earth's night sky (more).
Tau Bo÷tis A is a yellowish main sequence dwarf star of spectral and luminosity type F7 V, although it has also been classified as a F6 IV subgiant. The star has about 1.27 times the mass of Sol (NASA Exoplanet Archive; and exoplanets.org), 1.46 +/- 0.5 times its diameter (Brogi et al, 2012; and Baliunas et al, 1997), and around 3.4 times its luminosity. It appears to be more than twice (2.09 times) as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (exoplanets.org). Tau Bo÷tis may be between 1.3 and two billion years old. Tau Bo÷tis A is a New Suspected Variable (NSV) 6444. Useful catalogue numbers and designations for the star include: Tau Boo A, 4 Boo A, HR 5185*, Gl 527 A, Hip 67275, HD 120136, BD+18 2782, SAO 100706, FK5 507, LTT 14021, and ADS 9025 A.
Bryan Preston, Max-Q Digital,
See animations of planetary
transits by a "hot Jupiter."
Tau Bo÷tis A has a planet "b"
in a "torch" inner orbit, that
is massive enough to be a
gas giant bigger than Jupiter.
The star exhibits strong differential rotation, which varies from 3.0 days at the equator to 3.9 days at the poles (Brogi et al, 2012). The finding of a 3.3-day stellar rotation period for Star A that is estimated to be very similar to that of its recently discovered planetary companion moving in a very close "torch" orbit suggests that the planetary object may have tidally locked the star (Brogi et al, 2012; and Butler et al, 1997). In May 2005, a team of astronomers (led by Jaymie Matthews) using Canada's first space telescope, the Microvariability and Oscillations of Stars (MOST) satellite, announced that Star A has constantly varying starspots and that its planetary companion was close and massive enough to be tugging on the star's outer layer and affecting its rotation and brightness every 3.3 days (Maggie McKee, New Scientist, May 18, 2005). On November 27, 2006, a team of astronomers using the ESPaDOnS spectro-polarimeter on the Canada-France-Hawaii Telescope announced the discovery of a magnetic field around Star A in complex interaction with close-in planet b (CFHT press release; and Catala et al, 2006). On June 24, 2009, astronomers submitted a paper summarizing observations which reveals that Star A reverses its magnetic poles every two years (some 11 times faster than our Sun, Sol), possibly because of the close orbit of its Jupiter-class planetary companion (Fares et al, 2009; and Unknown, New Scientist, July 13, 2009).
|Tau Bo÷tis A||0.0||...||...||...||430,000||150||...||...||2.09|
In 1996, a team of astronomers (including Eric Williams, Heather M. Hauser, and Phil Shirts) led by Geoffrey W. Marcy and R. Paul Butler announced the discovery of a Jupiter-class planet around Star A using highly sensitive radial-velocity methods (Butler et al, 1997). Following 15 years of observation and high-resolution infrared spectroscopy, planet b is now throught to have 5.95 +/- 0.28 Jupiter-masses. It moves around Star A at an average distance of less than 0.05 AUs (a semi-major axis well within Mercury's orbital distance) in a near circular orbit (e= 0.23 +/- 0.015) that takes 3.312 days to complete. This orbit appears to be inclined by 44.5 +/- 1.5░ from the perspective of an observer on Earth (ESO news release; Brogi et al, 2012; and Baliunas et al, 1997). Assuming a Jupiter-like composition, its radius is projected to be about 1.2 times that of Jupiter, enlarged relative to Jupiter because of greater absorbed stellar radiation in its inner orbit. However, the object has an albedo significantly darker than the gas giant planets of the Solar System (Charbonneau et al, 1999).
Although planet b orbits Tau Bo÷tis A quite closely, this star is even hotter than Sol (as an F spectral type). Hence, b may have the the highest surface temperature of any known planet. In combination with strong stellar winds, the planet's temperature may be so high that its atmosphere may be evaporating and eroding in a comet-like trail.
ę Walter Myers --
(Artwork from Computergraphic Vistas, used with permission)
View of planet b from a baked and cracked moon with very dim
companion Star B at upper right, as imagined by Myers (more).
The orbit of an Earth-like planet (with liquid water) around Star A may be centered as close as 1.8 AU -- between the orbital distances of Mars and the Main Asteroid Belt in the Solar System -- with an orbital period of 2.2 years. However, the inward migration of planet b's orbit to less than 0.05 AU could have disrupted the development of Earth-type planet in the water zone. Astronomers would find it very difficult to detect an Earth-type planet in the water zone of this star using present methods. (See animations of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics. To "jump" to a close-up animation of the Tau Bo÷tis A planetary system only, click here.)
NASA -- larger image
Tau Bo÷tis B is a dim red dwarf star, like
Gliese 623 A (M2.5V) and B (M5.8Ve) at lower right.
Star "B" is a red main sequence dwarf star of spectral and luminosity type M2 V, with about one fifth of Sol's mass, 58 percent of its diameter, and 84/10,000th of its luminosity. As mentioned above, this stellar companion is widely separated in an extremely elliptical orbit. Useful catalogue numbers and designations for the star include: Tau Boo B, 4 Boo B, Gl 527 B, HD 120136 B, BD+18 2782 B, and ADS 9025 B.
The orbit of an Earth-like planet (with liquid water) around Star B may be centered as close as 0.09 AU -- well within the orbit distance of Mercury -- with an orbital period of just around 22 days. Unfortunately, tidal locking of a planet in such a close orbit would result in perpetual day on one side (and perpetual night on the other). Astronomers would find it very difficult to detect an Earth-type planet in the water zone of this star using present methods.
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 Tau Bo÷tis.
|Star System||Spectra &|
|Wolf 515||M2 V||3.1|
|GJ 1171||K0 V||5.6|
|AC+18 1204-96 AB||M3.5 Ve |
|HR 5273 AB||G8 V |
|Wolf 497||M1 V||8.4|
|Diadem ABC||F5 V |
|Ross 837||M3 V||9.1|
|LP 440-38||M V||9.2|
|Hip 70536||M2 V||9.8|
|BD+16 2658||M2.5 V||9.8|
|Ross 1020||M3.5 V||9.9|
|70 Virginis||G2.5-5 V||9.9|
|* plus bright stars *||. . .|
|59 Virginis||G0 Vs||12|
|Muphrid 2?||G0 IV |
|BD+3 2765 AB||G5 V |
|Sigma Bo÷tis||F3 Vw||14|
|BD+03 2766||G5 V||15|
|HR 4864||G7 V||15|
|HR 5534 AB||G0-2 V |
|HR 5384||G1 V||19|
|Chien 112||F6 V||19|
Try Professor Jim Kaler's Stars site for other information about Tau Bo÷tis at the University of Illinois' Department of Astronomy. The late John Whatmough created illustrated web pages on this system in Extrasolar Visions. For another illustrated discussion, see Christoph Kulmann's web page on Tau Bo÷tis.
Up-to-date technical summaries on these stars can be found at: Jean Schneiders's Extrasolar Planets Encyclopaedia; the Astronomiches Rechen-Institut at Heidelberg's ARICNS (Star A and Star B), the NASA Exoplanet Archive and the Research Consortium on Nearby Stars (RECONS). Additional information may be available at Roger Wilcox's Internet Stellar Database.
Most of the stars in Bo÷tes form a kite-shaped figure near the Big Dipper's handle (or Big Bear). Hence, a nightwatcher can imagine that Bo÷tes is chasing the bears (Constellations Ursa Major and Minor) around the North Pole with a pair of hunting dogs (Constellation Canes Venacti). For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Bo÷tes. For another illustration, see David Haworth's Bo÷tes.
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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