FU Orionis is avariable andbinary star[7] system in theconstellation ofOrion, that in 1937 rose in apparent visual magnitude from 16.5 to 9.6, and has since been around magnitude 9.[8][1] The nameFU Orionis is avariable star designation in theArgelander system, which are assigned sequentially as new variables are discovered.[9] FU Orionis is about 1,360 light years distant and is associated with the molecular cloudBarnard 35.[4][10]
For a long time this variable was considered unique, but in 1970 a similar star,V1057 Cygni, was discovered, and a number of additional examples have been discovered since then. These stars constitute theFU Orionis class of variable stars, GCVS typeFU, often nicknamedFUors. These stars arepre–main sequence stars which display an extreme change in magnitude and spectral type.
Theblue bandlight curve for FU Orionis, adapted from Clarkeet al. (2005).[11] The inset plot, adapted from Siwak,et al. (2013),[12] illustrates the short timescale variability.
FU Orionis consists of two components, both surrounded by acircumstellar disk. Both disks were resolved withALMA. The primary is surrounded by a dust disk with a radius of 11 astronomical units and the secondary disk has a similar inclination and size. The disks are separated by about 250 au. The12CO emission show a complex kinematic environment and signatures of disk rotation, which are asymmetric. The asymmetry of the disk rotation is explained with interactions of the disks during astellar flyby.[5]
The primary, called FU Orionis north has a mass of 0.6 M☉ and accretesM☉ per year.[5] The primary has an uncertain spectral type and luminosity class.[2] FU Orionis stars do not show strong emission lines during the outburst and have spectral features that resemble F- or G-typesupergiants during the maximum. The outer parts of FU Orionis stars produce a K-M supergiant spectrum, which can be observed in thenear-infrared.[13] The secondary, called FU Orionis south could be the more massive component in the system with 1.2M☉ and a spectral type of about K5.[2]
A 2023 study proposed that FU Orionis north might be accreting matter from an evaporating planet about 6 times the mass of Jupiter.[15] Simulations predict an extremely large radius of14RJ or around 1.4 R☉ at the beginning of the extreme evaporation event.[16]
