Fragments falling onto the Sun
Young low-mass stars are thought to interact with their circumstellar disks via magnetic funnels. Hot gas plasma accretes along these funnels, falling onto the stellar surface at velocities of hundreds of kilometers per second . Most of the evidence for this accretion comes from excess emission seen at infrared, optical, ultraviolet, and even X-ray wavelengths. Current models suggest that an impact region is rather complex because of the interplay between the radiation and the hot gas. According to models, the infalling material, after colliding back onto the surface, is heated to millions of degrees and partially sinks into the star's chromosphere. The impact can also drive strong motions and feed material back into surrounding coronal structures. The streams could be highly structured in both density and velocity, and result in inhomogeneous impact spots.
All these ideas are now being tested, thanks in part to the remarkable Solar Dynamics Observatory satellite that was launched in 2010 with an instrument team that included SAO scientists. In the latest issue of Science Express, SAO astronomer Paola Testa and five colleagues report discovering that fragments of ejected material from a solar flare fell back onto the Sun's surface and produced intense bursts of emission resembling those thought to occur in young stars. The scientists simulated the infall, and found good agreement between the models and observations. It seems likely that studies of our own mature Sun will help unravel a mystery about how young stars develop.