Xi Aquilae

Xi Aquilae (ξ Aquilae, abbreviated Xi Aql, ξ Aql), officially named Libertas /ˈlɪbərtæs/,[10] is a red clump giant star located at a distance of 184 light-years (56 parsecs) from the Sun in the equatorial constellation of Aquila. As of 2008, an extrasolar planet (designated Xi Aquilae b, later named Fortitudo) has been confirmed in orbit around the star.[11]

Xi Aquilae
Location of ξ Aquilae (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Aquila
Right ascension  19h 54m 14.88184s[1]
Declination +08° 27′ 41.2299″[1]
Apparent magnitude (V) 4.722[2]
Characteristics
Spectral type G9 III[3]
U−B color index +0.89[4]
B−V color index +1.049[5]
R−I color index 0.57
Astrometry
Radial velocity (Rv)-42.10 ± 0.14[5] km/s
Proper motion (μ) RA: +101.91[1] mas/yr
Dec.: –81.20[1] mas/yr
Parallax (π)17.77 ± 0.29[1] mas
Distance184 ± 3 ly
(56.3 ± 0.9 pc)
Absolute magnitude (MV)+0.96[6]
Details
Mass1.16 ± 0.28[7] M☉
Radius10.45 ± 0.18[8] R☉
Luminosity49[7] L☉
Surface gravity (log g)2.66[7] cgs
Temperature4,783[7] K
Metallicity [Fe/H]–0.13[7] dex
Rotational velocity (v sin i)5.2[5] km/s
Age4.63 ± 2.88[7] Gyr
Other designations
Libertas, BD+08 4261, 59 Aquilae, HD 188310, HIP 97938, HR 7595, SAO 125210.[9]
Database references
SIMBADdata

Nomenclature

ξ Aquilae (Latinised to Xi Aquilae) is the star's Bayer designation. Following its discovery the planet was designated Xi Aquilae b.

In July 2014 the International Astronomical Union launched a process for giving proper names to certain exoplanets and their host stars.[12] The process involved public nomination and voting for the new names.[13] In December 2015, the IAU announced the winning names were Libertas for this star and Fortitudo for its planet.[14]

The winning names were those submitted by Libertyer, a student club at Hosei University of Tokyo, Japan. The names which were originally proposed were in English and were 'Liberty' and 'Fortitude', but to comply with the IAU's rules they were modified to be Latin versions of the same words, and so the final names became 'Libertas' and 'Fortitudo' respectively. 'Aquila' is Latin for 'eagle', a popular symbol of liberty and embodiment of fortitude - emotional and mental strength in the face of adversity.[15]

In 2016, the IAU organized a Working Group on Star Names (WGSN)[16] to catalog and standardize proper names for stars. In its first bulletin of July 2016,[17] the WGSN explicitly recognized the names of exoplanets and their host stars approved by the Executive Committee Working Group Public Naming of Planets and Planetary Satellites, including the names of stars adopted during the 2015 NameExoWorlds campaign. This star is now so entered in the IAU Catalog of Star Names.[10]

Properties

This star has an apparent visual magnitude of 4.722,[2] which, according to the Bortle Dark-Sky Scale, is bright enough to be viewed with the naked eye from dark suburban skies. The orbital motion of the Earth causes this star to undergo an annual parallax shift of 17.77 milliarcseconds. From this measurement, the distance to this star can be determined, yielding an estimate of approximately 184 light years with a 2% margin of error.[1] The magnitude of the star is diminished by 0.09 from the extinction caused by interstellar gas and dust.[7]

The spectrum of this star matches a stellar classification of G9 III,[3] where the G9 means that it belongs to the category of G-type stars while the luminosity class of III indicates that, at an estimated age of nearly five billion years,[7] is an evolved star that has reached the giant stage. It is in the red clump,[18] meaning it is generating energy through the fusion of helium into carbon at its core.

Xi Aquilae has an estimated 116% of the Sun's mass,[7] while its outer atmosphere has expanded to more than ten times the radius of the Sun.[8] It is radiating 49 times the Sun's luminosity at an effective temperature of 4,783 K,[7] giving it the golden-hued glow of a G-type star.[19] The possibility of a binary stellar companion can be ruled out based upon observations with the CHARA array.[3]

Planetary system

In 2008, the presence of a planetary companion was announced, based upon Doppler spectroscopy results from the Okayama Astrophysical Observatory. This object, designated as Xi Aquilae b, has at least 2.8 Jupiter masses and is orbiting at an estimated 0.68 astronomical units from the star with a period of 136.75 days.[11] Any planets that once orbited to the interior of this object may have been consumed as the star entered the red giant stage and expanded in radius.[20]

The Xi Aquilae planetary system[11]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b (Fortitudo) >2.8 MJ 0.68 136.75 ± 0.25 0 (fixed) — —
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References
  1. van Leeuwen, F. (November 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.
  2. Soubiran, C.; Le Campion, J.-F.; Cayrel de Strobel, G.; Caillo, A. (June 2010), "The PASTEL catalogue of stellar parameters", Astronomy and Astrophysics, 515: A111, arXiv:1004.1069, Bibcode:2010A&A...515A.111S, doi:10.1051/0004-6361/201014247.
  3. Baines, Ellyn K.; et al. (July 2010), "Ruling Out Possible Secondary Stars to Exoplanet Host Stars Using the CHARA Array", The Astronomical Journal, 140 (1): 167–176, arXiv:1005.2930, Bibcode:2010AJ....140..167B, doi:10.1088/0004-6256/140/1/167.
  4. Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99), Bibcode:1966CoLPL...4...99J.
  5. Massarotti, Alessandro; et al. (January 2008), "Rotational and Radial Velocities for a Sample of 761 HIPPARCOS Giants and the Role of Binarity", The Astronomical Journal, 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209.
  6. Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015.
  7. Ghezzi, L.; et al. (December 2010), "Metallicities of Planet-hosting Stars: A Sample of Giants and Subgiants", The Astrophysical Journal, 725 (1): 721–733, arXiv:1008.3539, Bibcode:2010ApJ...725..721G, doi:10.1088/0004-637X/725/1/721.
  8. Baines, Ellyn K.; et al. (August 2009), "Eleven Exoplanet Host Star Angular Diameters from the Chara Array", The Astrophysical Journal, 701 (1): 154–162, arXiv:0906.2702, Bibcode:2009ApJ...701..154B, doi:10.1088/0004-637X/701/1/154.
  9. "ksi Aql -- Star", SIMBAD Astronomical Database, Centre de Données astronomiques de Strasbourg, retrieved 2012-07-20.
  10. "IAU Catalog of Star Names". Retrieved 28 July 2016.
  11. Sato, Bun'ei; et al. (2008). "Planetary Companions around Three Intermediate-Mass G and K Giants: 18 Delphini, ξ Aquilae and HD 81688". Publications of the Astronomical Society of Japan. 60 (3): 539–550. arXiv:0802.2590. Bibcode:2008PASJ...60..539S. doi:10.1093/pasj/60.3.539.
  12. NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars. IAU.org. 9 July 2014
  13. NameExoWorlds The Process
  14. Final Results of NameExoWorlds Public Vote Released, International Astronomical Union, 15 December 2015.
  15. NameExoWorlds The Approved Names
  16. "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  17. "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
  18. Puzeras, E.; et al. (October 2010), "High-resolution spectroscopic study of red clump stars in the Galaxy: iron-group elements", Monthly Notices of the Royal Astronomical Society, 408 (2): 1225–1232, arXiv:1006.3857, Bibcode:2010MNRAS.408.1225P, doi:10.1111/j.1365-2966.2010.17195.x.
  19. "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, retrieved 2012-01-16.
  20. Kunitomo, M.; et al. (August 2011), "Planet Engulfment by ~1.5-3 M sun Red Giants", The Astrophysical Journal, 737 (2): 66, arXiv:1106.2251, Bibcode:2011ApJ...737...66K, doi:10.1088/0004-637X/737/2/66.

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