TW Piscis Austrini

TW Piscis Austrini (also Fomalhaut B) is a dwarf star in the constellation Piscis Austrinus. It lies relatively close to the Sun, at an estimated distance of 24.8 light years. To an observer on Earth the star is visually separated from its larger companion Fomalhaut A by 2 degrees - the width of four full moons.[7]

"Fomalhaut B" redirects here. For the extrasolar planet candidate, see Fomalhaut b.
TW Piscis Austrini
Observation data
Epoch J2000      Equinox J2000
Constellation Piscis Austrinus
Right ascension 22h 56m 24.0533s[1]
Declination −31° 33′ 56.0351″[1]
Apparent magnitude (V) 5.44 - 6.51[2]
Characteristics
Spectral type K5Vp[3]
U−B color index 1.02[4]
B−V color index 1.10[4]
Variable type BY Draconis[2]
Astrometry
Radial velocity (Rv)+6[5] km/s
Proper motion (μ) RA: 329.577±0.116[1] mas/yr
Dec.: −158.293±0.100[1] mas/yr
Parallax (π)131.4380 ± 0.0856[1] mas
Distance24.81 ± 0.02 ly
(7.608 ± 0.005 pc)
Absolute magnitude (MV)7.08[6]
Details
Mass0.725 ± 0.036[3] M☉
Radius0.629 ± 0.051[3] R☉
Luminosity0.19[6] L☉
Temperature4,711 ± 134[3] K
Rotational velocity (v sin i)2.93[3] km/s
Age440[6] Myr
Other designations
TW PsA, Gl 879, HR 8721, CD−32°17321, HD 216803, LTT 9283, GCTP 5562.00, SAO 214197, CP(D)−32°6550, HIP 113283
Database references
SIMBADdata
ARICNSdata

The TW in the name is a variable star designation. This is a variable star of the type known as a BY Draconis variable, with surface brightness variations causing the changes as the star rotates. It varies slightly in apparent magnitude, ranging from 6.44 to 6.51 over a 10.3 day period.[2]

TW Piscis Austrini lies within a light year of Fomalhaut A.[8] Due to sharing the same proper motion, and the same estimated age of approximately 440±40 million years, astronomers now consider them to be elements of a multiple star system.[6] A third star, dimmer, and more widely separated, Fomalhaut C, gives the system the widest visual separation, to observers from Earth, at approximately 6 degrees.[7]

In 2019, an exoplanet candidate around Fomalhaut B was detected by astrometry.[9][10]

The Fomalhaut B planetary system[11]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
Bb (unconfirmed) — — <30000 days — — —

References
  1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  2. Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1: B/gcvs. Bibcode:2009yCat....102025S.
  3. Demory, B.-O.; et al. (October 2009), "Mass-radius relation of low and very low-mass stars revisited with the VLTI", Astronomy and Astrophysics, 505 (1): 205–215, arXiv:0906.0602, Bibcode:2009A&A...505..205D, doi:10.1051/0004-6361/200911976, S2CID 14786643
  4. Johnson, H. L.; Mitchell, R. I.; Iriarte, B.; Wisniewski, W. Z. (1966). "UBVRIJKL photometry of the bright stars". Communications of the Lunar and Planetary Laboratory. 4 (99): 99. Bibcode:1966CoLPL...4...99J.
  5. "V* TW PsA -- Variable of BY Dra type". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2010-01-20.
  6. Mamajek, E.E. (August 2012). "On the Age and Binarity of Fomalhaut". Astrophysical Journal Letters. 754 (2): L20. arXiv:1206.6353. Bibcode:2012ApJL..754...20M. doi:10.1088/2041-8205/754/2/L20. S2CID 119191190.
  7. Bob King (2014-10-01). "Fomalhaut: A crazy-wide triple stat". Sky and Telescope magazine. Retrieved 2020-06-13. Though it may appear isolated in the barren October sky, Fomalhaut has company. It feels the gravitational tug of the magnitude +6.5 star TW Piscis Austrini, 2° to the south. Both are 25 light-years distant and move in tandem across space, partaking of the same proper motion. They form a true double star with an actual separation of 5.5 trillion miles, or 0.91 light-years.
  8. Fred Schaaf (31 March 2008). The Brightest Stars: Discovering the Universe through the Sky's Most Brilliant Stars. Wiley. ISBN 978-0-470-24917-8.
  9. Joe Rao (2019-09-23). "Fomalhaut, 'Royal' Star of Autumn, Swims with the Southern Fish". Space magazine. Retrieved 2020-06-13. Believe it or not, an extrasolar planet might also be circling TW Piscis Austrini. NASA's Transiting Exoplanet Survey Satellite (TESS), a space telescope that's searching for planets around the brightest stars in Earth's night sky, recently found a possible candidate circling this star. It's almost the same size as our Earth, and orbits the star about every 10 days at a distance of 7.5 million miles from it.
  10. De Rosa, Robert J.; Esposito, Thomas M.; Hirsch, Lea A.; Nielsen, Eric L.; Marley, Mark S.; Kalas, Paul; Wang, Jason J.; Macintosh, Bruce (7 October 2019). "The Possible Astrometric Signature of a Planetary-mass Companion to the Nearby Young Star TW Piscis Austrini (Fomalhaut B): Constraints from Astrometry, Radial Velocities, and Direct Imaging". Astronomical Journal. 158 (6): 225. arXiv:1910.02965. Bibcode:2019AJ....158..225D. doi:10.3847/1538-3881/ab4c9b. S2CID 203902656.
  11. "Exo.MAST". exo.mast.stsci.edu. Retrieved 11 December 2018.
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