Concept
HL Tauri (abbreviated HL Tau) is a young T Tauri star in the constellation Taurus, approximately from Earth in the Taurus Molecular Cloud. The luminosity and effective temperature of HL Tauri imply that its age is less than 100,000 years. At apparent magnitude 15.1, it is too faint to be seen with the unaided eye. It is surrounded by a protoplanetary disk marked by dark bands visible in submillimeter radiation that may indicate a number of planets in the process of formation. It is accompanied by the Herbig–Haro object HH 150, a jet of gas emitted along the rotational axis of the disk that is colliding with nearby interstellar dust and gas. Indications of a protoplanetary disk were first presented in 1975 with infrared spectral observations in wavelengths between 2 and 4 microns, which were made possible by the recent invention of the indium antimonide photovoltaic detector. Of 29 very young stars examined, only HL Tauri showed a strong absorption feature centered on the expected 3.07 micron absorption of ice particles, which authors attributed to the ν1, ν3, and 2ν2 vibrational frequencies of the O–H bond. A 1982 survey identified HL Tauri as one of the most highly polarized T Tauri stars known, along with DG Tauri and V536 Aquilae. A gas disk was discovered by interferometric observation of carbon monoxide (CO) emissions in 1986. Based on observation data in 1985 and 1986 from the Millimeter Wave Interferometer of the Owens Valley Radio Observatory, the circumstellar disk was estimated to have a mass between and , with a best fit of , and a radius of about 200 AU. The temperature of the gas and grains of the disk are probably of the order of a few tens of kelvins. The gas was found to be bound to and in Keplerian rotation around a star with a mass of about . Bipolar outflow of molecules such as carbon monoxide (CO) and diatomic hydrogen (H2) have been observed. The element iron has also been noted in the outflow in its Fe(II) oxidation state, also called Fe2+ or ferrous iron.
