Tau Boötis 2

The Tau Boötis 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 Boötes, the Bear Driver -- southwest of Muphrid (Eta Boötis) and Arcturus (Alpha Boötis) and northwest of Nu Boötis. In 1996, astronomers announced the discovery of a Jupiter-like planet around this Sun-like star (Butler et al, 1997) -- details below).

© James B. Kaler, UIUC -- more information
(Photo from Stars, Planet Project, and Tau Boötis; used with permission)

Tau Boötis AB System

Tau Boötis 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).

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	Orbital Distance (a=AUs)	Orbital Period (P=years)	Orbital Eccentricity (e)	Orbital Inclination (i=degrees)	Mass (Earths)	Diameter (Earths)	Density (Earths)	Surface Gravity (Earths)	Metallicity (Solar)
AB Mass Center	0.0	...	...	...	...	...	...	...	...
Tau Boötis A	30	~2,000	0.91	50.68	420,000	159	...	...	2.09
Planet "b"	0.05	0.009	0.23	44.5	1,900	13	...	...	...
Disrupted H.Z. A?	1.8	2.2	0	44.5	...	...	...	...	...
Tau Boötis B	194	~2,000	0.91	50.68	70,000	63	...	...	...
H.Z. B	0.09	0.06	0	...	...	...	...	...	...

Tau Boötis A

Digitized Sky Survey 2,
ESO





Larger and jumbo images.





Tau Boötis 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 Boötis 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 Boötis may be between 1.3 and two billion years old. Tau Boötis 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, STScI/NASA


See animations of planetary
transits by a "hot Jupiter."


Tau Boötis 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).

David Aguilar,
CfA


Larger
illustration.


Tau Boötis A
has a magnetic
field in complex
interaction with
planet b (more).

---------------------------------------------- [Guide] -- [Larger] ----------------------------------------------

	Orbital Distance (a=AUs)	Orbital Period (P=years)	Orbital Eccentricity (e)	Orbital Inclination (i=degrees)	Mass (Earths)	Diameter (Earths)	Density (Earths)	Surface Gravity (Earths)	Metallicity (Solar)
Tau Boötis A	0.0	...	...	...	430,000	150	...	...	2.09
Planet "b"	0.05	0.009	0.23	44.5	~1,900	13	...	...	...
Disrupted H.Z.?	1.8	2.2	0	44.5	...	...	...	...	...

Planet "b"

© John Whatmough -- larger image
(Artwork from Extrasolar Visions, used with permission).

Planet b's atmosphere evaporating in a comet-like trail with several moons,
as imagined by Whatmough.

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).

© Christoph Kulmann -- larger image

Artwork from Exoplaneten.de
(used with permission).

Another view of planet b losing its atmosphere,
as imagined by Kulmann (more).

Although planet b orbits Tau Boötis 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.

© Walter Myers -- larger image
(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 Boötis A planetary system only, click here.)

Tau Boötis B

NASA -- larger image

Tau Boötis B is a dim red dwarf star, like
Gliese 623 A (M2.5V) and B (M5.8Ve) at lower right.