TW Piscis Austrini

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"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 24.05327s[1]
Declination −31° 33′ 56.0351″[1]
Apparent magnitude (V) 6.48[2]
Spectral type K5Vp[3]
U−B color index 1.02[4]
B−V color index 1.10[4]
Variable type BY Draconis
Radial velocity (Rv) +6[2] km/s
Proper motion (μ) RA: −331.11[1] mas/yr
Dec.: −158.98[1] mas/yr
Parallax (π) 131.42 ± 0.62[1] mas
Distance 24.8 ± 0.1 ly
(7.61 ± 0.04 pc)
Absolute magnitude (MV) 7.08[5]
Mass 0.725 ± 0.036[3] M
Radius 0.629 ± 0.051[3] R
Luminosity 0.19[5] L
Temperature 4,711 ± 134[3] K
Rotational velocity (v sin i) 2.93[3] km/s
Age 4.4 × 108[5] years
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

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 24.9 light years.

The TW in the name is astronomical nomenclature for a variable star. This is a flare star of the type known as a BY Draconis variable. It varies slightly in apparent magnitude, ranging from 6.44 to 6.49 over a 10.3 day period. While smaller than the Sun, it is relatively large for a flare star. Most flare stars are red M-type dwarfs.

TW Piscis Austrini lies within a light year of the much brighter star Fomalhaut,[6] and the two may be travelling companions that share a common motion through space.

A 2012 study[5] showed that TW PsA and Fomalhaut are approximately only 0.28 parsecs (57,000 AU) apart, and share a space velocity within 0.1±0.5 km/s, consistent with constituting a physical binary system. Independent age estimates for Fomalhaut and TW PsA yield similar results, and the age for the system is 440±40 million years.[5]


  1. ^ a b c d e 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. ^ a b "V* TW PsA -- Variable of BY Dra type". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2010-01-20. 
  3. ^ a b c d e 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 
  4. ^ a b Johnson, H. L.; Iriarte, B.; Mitchell, R. I.; Wisniewskj, W. Z. (1966). "UBVRIJKL photometry of the bright stars". Communications of the Lunar and Planetary Laboratory 4 (99). Bibcode:1966CoLPL...4...99J. 
  5. ^ a b c d e 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. 
  6. ^ p. 237, The Brightest Stars: Discovering The Universe Through The Sky's Most Brilliant Stars, Fred Schaaf, Hoboken, NJ: John Wiley & Sons, Inc., 2008, ISBN 978-0-471-70410-2.

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