TW Piscis Austrini

"Fomalhaut B" redirects here. For the former exoplanet candidate, see Fomalhaut b.

TW Piscis Austrini (also known as Fomalhaut B) is a main sequence star in the constellation Piscis Austrinus. It lies relatively close to the Sun, at an estimated distance of 24.8 light-years (7.6 parsecs). 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.^[8]

A visual band light curve for TW Piscis Austrini, adapted from Busko and Torres (1978).^[9] The error bar shown on the left-most point applies to all points.

TW Piscis Austrini

Location of TW Piscis Austrini (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Piscis Austrinus
Right ascension	22h 56m 24.05256s[1]
Declination	−31° 33′ 56.0306″[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)	+6.79±0.12[1] km/s
Proper motion (μ)	RA: 330.203 mas/yr[1] Dec.: −158.602 mas/yr[1]
Parallax (π)	131.5525 ± 0.0275 mas[1]
Distance	24.793 ± 0.005 ly (7.602 ± 0.002 pc)
Absolute magnitude (MV)	7.08[5]
Details[5]
Mass	0.73+0.02 −0.01 M☉
Radius	0.688±0.034 R☉
Luminosity	0.189±0.013 L☉
Temperature	4,594±80 K
Metallicity [Fe/H]	−0.07±0.03[6] dex
Rotation	10.3 days
Rotational velocity (v sin i)	2.93[3] km/s
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[7]
Database references
SIMBAD	data
ARICNS	data

The name TW Piscis Austrini 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.^[10] 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.^[5] 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.^[8]

Planetary system

In 2019, an exoplanet candidate around Fomalhaut B was detected by astrometry, but this remains unconfirmed.^[11][12]

The Fomalhaut B planetary system^[12]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (years)	Eccentricity	Inclination	Radius
b (unconfirmed)	1.2+0.7 −0.6 MJ	—	25+52 −21	—	—	—

References

1. Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 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, Brice-Olivier; Ségransan, Damien; Forveille, Thierry; Queloz, Didier; Beuzit, Jean-Luc; Delfosse, Xavier; Di Folco, Emmanuel; Kervella, Pierre; Le Bouquin, Jean-Baptiste; Perrier, Christian; Benisty, Myriam; Duvert, Gilles; Hofmann, Karl-Heinz; Lopez, Bruno; Petrov, Romain (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. Mamajek, E.E. (August 2012). "On the Age and Binarity of Fomalhaut". Astrophysical Journal Letters. 754 (2): L20. arXiv:1206.6353. Bibcode:2012ApJ...754L..20M. doi:10.1088/2041-8205/754/2/L20. S2CID 119191190.
6. Gomes da Silva, J.; Santos, N. C.; Adibekyan, V.; Sousa, S. G.; Campante, T. L.; Figueira, P.; Bossini, D.; Delgado-Mena, E.; Monteiro, M. J. P. F. G.; de Laverny, P.; Recio-Blanco, A.; Lovis, C. (2021-02-01). "Stellar chromospheric activity of 1674 FGK stars from the AMBRE-HARPS sample. I. A catalogue of homogeneous chromospheric activity". Astronomy and Astrophysics. 646: A77. arXiv:2012.10199. Bibcode:2021A&A...646A..77G. doi:10.1051/0004-6361/202039765. ISSN 0004-6361. Fomalhaut B's database entry at VizieR.
7. "V* TW PsA -- Variable of BY Dra type". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2010-01-20.
8. Bob King (2014-10-01). "Fomalhaut: A crazy-wide triple stat". Sky & Telescope. 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-year.
9. Busko, I. C.; Torres, C. A. O. (March 1978). "Flare activity by BY Draconis stars". Astronomy and Astrophysics. 64: 153–160. Bibcode:1978A&A....64..153B. Retrieved 23 February 2022.
10. 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.
11. Joe Rao (2019-09-23). "Fomalhaut, 'Royal' Star of Autumn, Swims with the Southern Fish". Space.com. 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.
12. 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.

External links

• TW Piscis Austrini at SolStation.