Tau Boötis

Tau Boötis

A star chart of the constellation of Boötes showing the position of Tau Boötis (circled)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Boötes
Right ascension	13h 47m 15.74s
Declination	+17° 27′ 24.9″
Apparent magnitude (V)	4.50
Characteristics
Spectral type	F6 IV
U−B color index	0.04
B−V color index	0.48
R−I color index	0.24
Variable type	Variable star
Astrometry
Radial velocity (Rv)	-15.6 km/s
Proper motion (μ)	RA: -479.53 ± 0.16 mas/yr Dec.: 53.49 ± 0.13 mas/yr
Parallax (π)	64.03 ± 0.19 mas
Distance	50.9 ± 0.2 ly (15.62 ± 0.05 pc)
Absolute magnitude (MV)	3.53
Details
Mass	1.3 / 0.4 M☉
Radius	1.331 ± 0,027 R☉
Luminosity	3.0 L☉
Surface gravity (log g)	4.30 cgs
Temperature	6,360 K
Metallicity	210%
Rotation	3.31 days
Rotational velocity (v sin i)	14 km/s
Age	1.6–2.3  Gyr
Other designations
4 Boötis, ADS 9025, BD+18°2782, CCDM 13473+1727, FK5 507, GC 18637, GCTP 3144.00, GJ 527, HD 120136, HIP 67275, HR 5185, LTT 14021, SAO 100706
Database references
SIMBAD	data
Data sources:
Hipparcos Catalogue, CCDM (2002), Bright Star Catalogue (5th rev. ed.)
Database references
SIMBAD	data

Tau Boötis (τ Boo, τ Boötis) is an F-type main-sequence star approximately 51 light-years away in the constellation of Boötes. The system is also a binary star system, with the secondary star being a red dwarf. As of 1999, an extrasolar planet has been confirmed to be orbiting the primary star.

Stellar components

The system is a binary. The primary component is a yellow-white dwarf (spectral type F7 V) and secondary is a dim red dwarf (spectral type M2 V). The system is relatively nearby, distance being about 51 light years. The primary star should be easily visible to the unaided eye under dark skies.

The primary star, Tau Boötis A is a yellow-white dwarf. It is 20% more massive than our Sun and thus is somewhat brighter and hotter. It has a radius 1.9 times solar, and is probably about 1.3 billion years old. Since it is more massive than the Sun, its lifespan is shorter - less than 6 billion years. Tau Bootis is the first star apart from the sun to be observed changing the polarity of its magnetic field. It is also listed as a suspected variable star.

Tau Boötis B (with a capital B, as opposed to the planet) is a dim red dwarf orbiting the primary star at a distance of 240 AU. One orbit around the primary would take thousands of years to complete.

Planetary system

In 1996 a planet, designated as Tau Boötis b, was discovered orbiting the primary star. There are also some indications of another, more distant, planet orbiting the star. In an unusual case of role-reversal, it appears that Tau Boötis' rotation has been tidally locked to Tau Boötis b. The planet was discovered by a team of astronomers led by Geoff Marcy and R. Paul Butler.

The Tau Boötis A planetary system

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b	6±0.28 MJ	0.0481 ± 0.028	3.312463 ± 0.000014	0.023 ± 0.015	44.5 ± 1.5°	—

The planet and its host star is one of the planetary systems selected by the International Astronomical Union as part of their public process for giving proper names to exoplanets and their host star (where no proper name already exists). The process involved public nomination and voting for the new names, and the IAU planned to announce the new names in mid-December 2015. However, the IAU annulled the vote as the winning name ("Shri Ram Matt" for the star, "Bhagavatideva" for the planet) was judged not to conform with the IAU rules for naming exoplanets.