A Keplerian disk with a four-arm spiral birthing an episodically accreting high-mass protostar
Methanol masers at 6.7 GHz have been frequently targeted by VLBI users for their bright emission and multiple uses in radio astronomy. In addition to their uses in trigonometric annual parallax measurements, it is their reliable and exclusive association with intermediate- and high-mass protostars which make them a favourite maser transition among star formation astronomers investigating the higher mass regimes. VLBI images of the 6.7 GHz methanol maser often show rings of up to 100s of AU around protostars, or arcs of maser clouds, or seemingly random distributions.
