An international team of scientists led by Chin-Fei Li at the Academy of Sciences' Institute of Astronomy and Astrophysics (ASIAA) have discovered a very small accretion disk formed around one of the youngest protostars. The discovery was made with the Atacama massive millimeter / submillimeter array (ALMA). This discovery creates a limitation on the existing theory of disc formation, as it confirms the theory that the disc can form much earlier than previously thought. In addition, a compact rotating outflow was found. This allowed astronomers to track the disk wind, which carries angular momentum from the disk and thus facilitates disk formation.
“ALMA is so powerful that it can observe an accretion disk with a radius of up to 15 astronomical units (AU),” Chin-Fei Lee told ASIAA. 'Since this disk is several times younger than the previously observed similar, the result of our observations imposes a strong limitation on the current theory of disk formation, since now we know that the time of disk formation may be several times earlier. In addition, along with the previous observation of old disks, our observational result favors a model where the disk radius grows linearly with protostellar mass and thus supports an 'early onset, slow growth' scenario that conflicts with the 'slow onset, fast growth 'previously taken as a model for the formation of an accretion disk around a protostar'.
HH 211 is one of the youngest protostellar systems in the constellation Perseus, about 770 light-years from Earth. The central protostars are about 10,000 years old and less than 0.05 solar masses. It controls a powerful bipolar jet and therefore must absorb material efficiently.
A previous search with a resolution of about 50 AA found only a small disk of dust near the protostar. Now with ALMA with a resolution of 7 AU, which is about 7 times smaller, it was possible to identify a dusty disk at a submillimeter wavelength. It is currently the smallest accretion disk feeding the central protostar and having a radius of about 15 AU. At the same time, the disk is wide enough, which indicates that the submillimeter light-emitting particles have not yet settled on the middle plane. Unlike the previously observed disc HH 212, which looks like a large 'hamburger', this young disc looks more like a small 'bun'. Thus, it seems that the outer disc will grow from a small 'bun' to a large 'hamburger' at a later stage. In addition, a compact rotating outflow has been discovered, and it can trace the direction of the disk wind, carrying angular momentum from the disk and thus facilitating the formation of the disk.
Observations provide scientists with an exciting opportunity to directly detect and characterize the small disks around the youngest protostars, allowing for a complete redefinition of disk formation and, therefore, star formation itself.
