TW Piscis Austrini
Star in the constellation Piscis Austrinus
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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]
 | |
| Observation data Epoch J2000 Equinox ICRS | |
|---|---|
| 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 | |
| Evolutionary stage | main sequence[1] |
| 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] |
| Orbit | |
| Primary | Fomalhaut |
| Name | TW Piscis Austrini |
| Period (P) | 5.8+3.8 −2.2 × 106 yr |
| Semi-major axis (a) | 4.5+1.8 −1.2 × 104 AU |
| Inclination (i) | 65+30 −29° |
| 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
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]
| 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 |
— | — | — |