GG Tauri

GG Tauri, often abbreviated as GG Tau, is a quadruple or quintuple star system in the constellation Taurus. At a distance of about 450 light years (140 parsecs) away, it is located within the Taurus-Auriga Star Forming Region. The system comprises two (possibly three) stars orbiting each other in a hierarchical triple system, and another binary star system more distant from the central system.
The system is unusual because it contains two distinct circumstellar disks: one surrounding the entire system, and another surrounding the primary component of the system.
Properties.
GG Tauri consists of four (possibly five) stars, which are T Tauri stars – a class of variable stars that show irregular changes in brightness. These stars are extremely young and more luminous than their main sequence counterparts, because they have not condensed into the normal size yet. The four components of GG Tauri stars are relatively cool K-type or M-type stars, with these spectral types: K7 for GG Tauri Aa, M0.5 for GG Tauri Ab, M5 for GG Tauri Ba, and M7 for GG Tauri Bb; the age of the system is estimated to be 1.5 million years.
A dynamical study of the system found the masses of the four stars to be: for GG Tauri Aa, for GG Tauri Ab, for GG Tauri Ba, and for GG Tauri Bb. At , GG Tauri Bb has a substellar mass and is a brown dwarf. Orbital motion has been detected in the central system, but not in the outer pair (as its separation is too high); a preliminary orbit for GG Tauri A has been calculated.
Interferometric techniques have been used to observe GG Tauri Ab, the lower-mass component of the central system. GG Tauri Ab may actually be a double star system comprising two red dwarfs (Ab1 = M2V, Ab2 = M3V), with a separation of about 4.5 AU. Its orbital period is currently estimated to be around 16 years. This would explain why the GG Tauri Ab's spectrum suggests an unusually low-mass star instead of the higher mass that was measured.
Circumstellar disks.
T Tauri stars are usually surrounded by circumstellar disks. These disks coalesce into protoplanets and then into planets. The inner disk around GG Tauri has a mass of about , or about the mass of Jupiter, at a temperature of about 20 to 30 K. However, because mass is currently accreting into the stars themselves, the inner disk must have been consumed, or another larger, circumstellar disk must have been supplying mass into the smaller disk.
A search for diatomic hydrogen gas near GG Tauri was conducted; H2 gas could be found up to 100 AU away from the center of the system, with significant emission also being detected 30 AU away. This emission was detected where a previous survey found gas streaming from the outer disk to the inner disk, so it was assumed that the emission resulted from mass falling from the inner disk to the outer disk. Observations taken in 2014 showed similar results. Indeed, observations of the circumstellar disk around GG Tauri found "sharply-defined features" around the disk: simulations of GG Tauri's disk indicate that these are likely due to the gravitational forces from the two stellar components.
A "gap" in the larger circumstellar disk has been detected at the three-o'clock position, at a position angle of about 268°. First seen in 2002, it was confirmed to be the result of interstellar material shadowing part of the disk.