T Tauri

T Tauri is a trinary variable star in the constellation Taurus, the prototype of the T Tauri stars. It was discovered in October 1852 by John Russell Hind. T Tauri appears from Earth amongst the Hyades cluster, not far from ε Tauri, but it is actually 318 light-years behind it and not a member of the cluster. The cloud to the west of the system is NGC 1555, known more commonly as Hind's Variable Nebula. Although this system is considered to be the prototype of T Tauri stars, a later phase in a protostar's formation, it is a very atypical T Tauri star.

Orbital characteristics and mass The system has three stars: T Tauri North (T Tau N), T Tauri South A (T Tau Sa), and T Tauri South B (T Tau Sb). T Tau N is estimated to be approximately 300 AU away from the southern binary, with the separation of the binary believed to be approximately 7 AU with an orbital period of 27.2±0.7 years. The orbit of T Tau N about the southern binary is poorly constrained, with estimates of the period ranging from 400 years to 14,000 years as of 2020. T Tau N has a mass of ~2.1 M☉, T Tau Sa is estimated to be 2.0–2.3 M☉, and T Tau Sb is estimated to be approximately 0.4–0.5 M☉.

Variability and optical extinction

The southern binary is visible mainly in infrared, which is likely due to a circumbinary ring that is blocking the optical light (if there is any optical light leaking through, it must be at a magnitude of less than 19.6), while the accretion disk of T Tau N is believed to be nearly perpendicular to our line of sight, thus allowing us to see T Tau N in the optical. The southern binary's brightness varies dramatically over seemingly short timescales in the infrared. It is believed this variability is due to both the matter in the circumbinary ring not being uniform, thus varying the light let through as it orbits the binary, and due to the individual components of the binary flaring up as they accrete matter. It is unknown which mechanism contributes the most to the variability. The T Tau S system is seen moving towards the north-west and two works predicted that the circumbinary ring around T Tau S will move in front of T Tau N within 100 years. Based on AAVSO observations it was found that between 2015 and 2024 the star did fade by around 2 magnitudes. This is interpreted as the beginning of the great dimming of T Tau N. The dimming will last 60 to 70 years or more and T Tau N might disappear in the optical when the dense mid-plane of the ring moves in front of T Tau N.

Outflow system

All three stars are in their T Tauri phase. During this phase, a star does not undergo nuclear fusion within its core; it shines due to the residual heat given off by its collapse. This causes a T Tauri star to vary in brightness over the course of weeks or months as they accrete matter. An important mechanic in star formation are the jets that are formed by the accretion, which function similarly to the jets of a quasar or an active galactic nucleus (AGN). These jets form due to the magnetic fields formed in the accretion disk, and as a side effect, they carry away excess angular momentum from the star. Without this mechanism, a star would not be able to accrete to more than 0.05 M☉. As of 2020, T Tau Sb is passing through the plane of the T Tau S circumbinary ring, and is currently dimming as the ring blocks its light. The T Tauri system has been of particular interest to astronomers because it is by no means a typical T Tauri star. Specifically, it appears that T Tau N is actually supposed to still be an embedded protostar, but it was likely ejected from the dense cloud it was born in sometime in the past few thousand years. It is almost certainly still gravitationally bound to the other two stars. Its spectra is exactly that of a Classic T Tauri Star (CTTS), but evolutionary speaking it is not a T Tauri star. The complex outflow system created by the stars is poorly understood, particularly in how it evolves over time. It is believed there are two bipolar outflows, with one coming from T Tau N, and the other coming from T Tau S. Since the two stars of T Tau S are so close, their individual outflows appear to either merge or T Tau Sb does not produce much of an outflow. The two outflows seem to be interacting somewhat, and it is believed this interaction will only become more intense in the future.

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