FU Tauri

FU Tauri is a brown dwarf binary system in the constellation of Taurus about 429 light years away. The secondary is very close to the lower limit for brown dwarfs and several databases list it as a distant massive exoplanet.

FU Tauri

FU Tauri and its companion (lower left) Credit: Image Credit: NASA/ESA/CSA JWST; Ya-Lin Wu et al.; processing: Meli_thev
Observation data Epoch J2000      Equinox J2000
Constellation	Taurus
A
Right ascension	04h 23m 35.3912s[1]
Declination	+25° 03′ 02.74″[1]
B
Right ascension	04h 23m 35.3912s[2]
Declination	+25° 03′ 02.74″[2]
Characteristics
Spectral type	M7.25 + M9.25[3]
Apparent magnitude (G)	15.2[1] + 20.5[2]
Variable type	T Tau[4]
Astrometry
A
Proper motion (μ)	RA: 6.895[1] mas/yr Dec.: −21.026[1] mas/yr
Parallax (π)	7.5981±0.1497 mas[1]
Distance	429 ± 8 ly (132 ± 3 pc)
B
Proper motion (μ)	RA: 12.450[2] mas/yr Dec.: −21.761[2] mas/yr
Parallax (π)	7.4909±1.2887 mas[2]
Distance	approx. 440 ly (approx. 130 pc)
Details
A
Mass	0.05[5] M☉
Radius	1.8[5] R☉
Luminosity	0.2[3] L☉
Temperature	2,838[3] K
Age	≤1[5] Myr
B
Mass	0.015[5] M☉
Luminosity	0.0039[3] L☉
Temperature	2,375[3] K
Age	1[5] Myr
Other designations
Gaia DR2 149629483705467008, WDS J04236+2503A, 2MASS J04233539+2503026
Database references
SIMBAD	A
	B

System

The two stars of the FU Tauri system are separated by 5.7″, equivalent to 800 AU at the distance of FU Tauri.^[6] The primary is a brown dwarf with a mass of 0.05 M_☉, while the secondary has a mass of 0.015 M_☉. The secondary mass of 15 M_J is close to the dividing line between brown dwarfs and exoplanets, and it is often treated as an exoplanet.^[7]

Properties

Both members of the binary are low-mass objects still contracting towards the main sequence. Comparison with theoretical evolutionary tracks gives them ages of one Myr or less. However, the primary is more luminous than expected even for this age and it may be younger than the secondary.^[5] The primary has a temperature of 2,838 K, a radius of 1.8 R_☉, and a bolometric luminosity of 0.2 L_☉. The secondary has a temperature of 2,375 K and a bolometric luminosity of 0.0039 L_☉.^[5]

Variability

A red band light curve for FU Tauri, adapted from Scholz et al. (2012)^[8]

FU Tauri varies in brightness. The primary star is a T Tauri variable, a type of irregular pre-main-sequence star. Its brightness has been observed to vary from a photovisual magnitude of 16.0 to fainter than 17.0.^[9] Its photographic magnitude has been measured to vary between magnitude 15.1 and below magnitude 17.6.^[10]