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]

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)	6.44–6.51[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
Age	440±40 Myr
Other designations
Fomalhaut B, TW PsA, CD−32°17321, CPD−32°6550, GJ 879, HD 216803, HIP 113283, HR 8721, SAO 214197, LTT 9283, PLX 5562.00[7]
Database references
SIMBAD	data
ARICNS	data

Stellar properties

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.

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	—	—	—

See also

• List of nearest K-type stars