Tau Boötis

Tau Boötis, Latinised from τ Boötis, is an F-type main-sequence star approximately 51 light-years away[1] in the constellation of Boötes. It is a binary star system, with the secondary star being a red dwarf. In 1999, an extrasolar planet was detected orbiting the primary star. In December 2020, astronomers may have observed, for the first time, radio emissions from a planet beyond the Solar System. According to the researchers: "The signal is from the Tau Boötis system, which contains a binary star and an exoplanet. We make the case for an emission by the planet itself."[8][9]

Not to be confused with T Boötis.
τ Boötis

Location of τ Boötis (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Boötes
Right ascension 13h 47m 15.74s[1]
Declination +17° 27′ 24.9″[1]
Apparent magnitude (V) 4.50[2] (4.46 to 4.52)[3]
Characteristics
Spectral type F6V + M2[4]
B−V color index 0.48[2]
Variable type Suspected[3]
Astrometry
Proper motion (μ) RA: −479.53±0.16[1] mas/yr
Dec.: +53.49±0.13[1] mas/yr
Parallax (π)64.03 ± 0.19 mas[1]
Distance50.9 ± 0.2 ly
(15.62 ± 0.05 pc)
Absolute magnitude (MV)3.38[5]
Orbit[4]
Primaryτ Boo A
Companionτ Boo B
Period (P)2,420+2,587
−947 yr
Semi-major axis (a)14.1+8.8
−3.9″
Eccentricity (e)0.87±0.04
Inclination (i)47.2+2.7
−3.7°
Longitude of the node (Ω)191.8+3.3
−4.7°
Argument of periastron (ω)
(secondary)		290.7+13
−10°
Details
τ Boo A
Mass1.35±0.03[4] M☉
Radius1.42±0.02[4] R☉
Luminosity3.06±0.16[6] L☉
Surface gravity (log g)4.26±0.06[4] cgs
Temperature6,387±44[4] K
Metallicity [Fe/H]0.25±0.03[4] dex
Rotation3.2±0.2 days[4]
Rotational velocity (v sin i)14.27±0.06[6] km/s
Age1.3+0.4
−0.6[4] Gyr
τ Boo B
Mass0.49±0.02[4] M☉
Radius0.48±0.05[4] R☉
Surface gravity (log g)4.90[7] cgs
Temperature3,580±90[4] K
Metallicity [Fe/H]+0.21[7] dex
Rotational velocity (v sin i)5.0[7] km/s
Other designations
τ Boo, 4 Boötis, NSV 6444, BD+18°2782, FK5 507, GC 18637, GJ 527, HD 120136, HIP 67275, HR 5185, SAO 100706, ADS 9025, CCDM 13473+1727, LTT 14021
Database references
SIMBADdata
A
B

Stellar components

The system is a binary. The primary component is a yellow-white dwarf (spectral type F6V) and secondary is a dim red dwarf (spectral type M2).[4] 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 35 percent more massive and 42 percent larger than the Sun and thus is somewhat brighter and hotter. It is about 1.3 billion years old, making it younger than the Sun as well.[4] Since it is more massive than the Sun, its lifespan is shorter—less than 6 billion years. Tau Boötis is the first star apart from the Sun to be observed changing the polarity of its magnetic field.[10] It is also listed as a suspected variable star. The magnetic activity cycle for this star shows a period of 122 days—much shorter than the solar cycle.[11]

Tau Boötis B (with a capital B, as opposed to the planet) is a dim, 11 mag red dwarf with only about half the mass and radius of the Sun. It orbits the primary star at an average distance of about 220 AU (14 arcseconds) but comes as close as about 28 AU to the primary, giving its orbit a very high eccentricity of about 0.87. One orbit around the primary would take approximately 2400 years to complete.[4]

Planetary system

In 1996 the planet Tau Boötis b was discovered orbiting the primary star by a team of astronomers led by R. Paul Butler.[12] There are also some indications of another planet orbiting the star with a period of roughly 5,000 days; however, this could be due to an instrumental effect or a stellar magnetic activity cycle.[13] Tau Boötis and its planet appear to be tidally locked to each other.[14]

The Tau Boötis A planetary system[15]
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° —

Naming controversy

The planet and its host star was one of the planetary systems selected by the International Astronomical Union as part of NameExoWorlds, their public process for giving proper names to exoplanets and their host star (where no proper name already exists).[16][17] The process involved public nomination and voting for the new names, and the IAU announced the new names in mid-December 2015.[18] However, the IAU annulled the vote for the system, as the winning names ("Shri Ram Matt" for the star and "Bhagavatidevi" for the planet)[17] were judged not to conform with the IAU rules for naming exoplanets due to the political activities of the namesake people.[19] The names garnered the majority of the votes cast for the system, and also making up a significant proportion of all votes cast as part of the contest.[17]

References
  1. van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600. Vizier catalog entry
  2. Mallik, Sushma V. (December 1999). "Lithium abundance and mass". Astronomy and Astrophysics. 352: 495–507. Bibcode:1999A&A...352..495M.
  3. Samus, N. N.; et al. (2017). "General Catalogue of Variable Stars". Astronomy Reports. 5.1. 61 (1): 80–88. Bibcode:2017ARep...61...80S. doi:10.1134/S1063772917010085. S2CID 125853869. Search for: NSV 6444.
  4. Justesen, A. B.; Albrecht, S. (2019). "Constraining the orbit of the planet-hosting binary τ Boötis". Astronomy & Astrophysics. EDP Sciences. 625: A59. arXiv:1812.05885. Bibcode:2019A&A...625A..59J. doi:10.1051/0004-6361/201834368. ISSN 0004-6361.
  5. Reiners, Ansgar (January 2006). "Rotation- and temperature-dependence of stellar latitudinal differential rotation". Astronomy and Astrophysics. 446 (1): 267–277. arXiv:astro-ph/0509399. Bibcode:2006A&A...446..267R. doi:10.1051/0004-6361:20053911. S2CID 8642707.
  6. Borsa, F.; Scandariato, G.; Rainer, M.; Bignamini, A.; Maggio, A.; Poretti, E.; Lanza, A. F.; Di Mauro, M. P.; Benatti, S.; Biazzo, K.; Bonomo, A. S.; Damasso, M.; Esposito, M.; Gratton, R.; Affer, L.; Barbieri, M.; Boccato, C.; Claudi, R. U.; Cosentino, R.; Covino, E.; Desidera, S.; Fiorenzano, A. F. M.; Gandolfi, D.; Harutyunyan, A.; Maldonado, J.; Micela, G.; Molaro, P.; Molinari, E.; Pagano, I.; et al. (2015). "The GAPS programme with HARPS-N at TNG. VII. Putting exoplanets in the stellar context: Magnetic activity and asteroseismology of τ Bootis A". Astronomy and Astrophysics. 578: A64. arXiv:1504.00491. Bibcode:2015A&A...578A..64B. doi:10.1051/0004-6361/201525741. S2CID 53490623.
  7. Lindgren, Sara; Heiter, Ulrike; Seifahrt, Andreas (February 2016). "Metallicity determination of M dwarfs". Astronomy & Astrophysics. 586: A100. arXiv:1510.06642. Bibcode:2016A&A...586A.100L. doi:10.1051/0004-6361/201526602. eISSN 1432-0746. ISSN 0004-6361.
  8. Friedlander, Blaine (16 December 2020). "Astronomers detect possible radio emission from exoplanet". Phys.org. Retrieved 16 December 2020.
  9. Turner, J.D.; et al. (2020). "The search for radio emission from the exoplanetary systems 55 Cancri, upsilon Andromedae, and tau Boötis using LOFAR beam-formed observations". Astronomy & Astrophysics. 645: A59. arXiv:2012.07926. Bibcode:2021A&A...645A..59T. doi:10.1051/0004-6361/201937201.
  10. Donati, J.-F.; et al. (2008). "Magnetic cycles of the planet-hosting star Tau Boötis". Monthly Notices of the Royal Astronomical Society. 385 (3): 1179–1185. arXiv:0802.1584. Bibcode:2008MNRAS.385.1179D. doi:10.1111/j.1365-2966.2008.12946.x. S2CID 119089082.
  11. Mittag, M.; et al. (April 2017). "Four-month chromospheric and coronal activity cycle in τ Boötis". Astronomy & Astrophysics. 600: 9. Bibcode:2017A&A...600A.119M. doi:10.1051/0004-6361/201629156. A119.
  12. Butler, R. Paul; et al. (1997). "Three New 51 Pegasi Type Planets". The Astrophysical Journal Letters. 474 (2): L115–L118. Bibcode:1997ApJ...474L.115B. doi:10.1086/310444.
  13. Howard, Andrew W.; Fulton, Benjamin J. (2016). "Limits on Planetary Companions from Doppler Surveys of Nearby Stars". Publications of the Astronomical Society of the Pacific. 128 (969). 114401. arXiv:1606.03134. Bibcode:2016PASP..128k4401H. doi:10.1088/1538-3873/128/969/114401. S2CID 118503912.
  14. Walker, G. A. H.; et al. (2008). "MOST detects variability on tau Bootis possibly induced by its planetary companion". Astronomy and Astrophysics. 482 (2): 691–697. arXiv:0802.2732. Bibcode:2008A&A...482..691W. doi:10.1051/0004-6361:20078952. S2CID 56317105.
  15. Butler, R. P.; et al. (2006). "Catalog of Nearby Exoplanets". The Astrophysical Journal. 646 (1): 505–522. arXiv:astro-ph/0607493. Bibcode:2006ApJ...646..505B. doi:10.1086/504701. S2CID 119067572.
  16. "NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars". International Astronomical Union. 9 July 2014. Retrieved 8 January 2016.
  17. "The ExoWorlds". NameExoWorlds. International Astronomical Union. n.d. Retrieved 8 January 2016.
  18. "The Process". NameExoWorlds. International Astronomical Union. 30 November 2015. Archived from the original on 15 August 2015. Retrieved 8 January 2016.
  19. "Final Results of NameExoWorlds Public Vote Released". International Astronomical Union. 15 December 2015. Retrieved 8 January 2016.
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