Young Stars Grow Via Violent Interactions With Protoplanetary Disks

This infrared image 950 light-years away has an unusual variable object that has the typical signature of a protostar. NASA/ESA/J. Muzerolle/E. Furlan/K. Flaherty/Z. Balog/R. Gutermuth

Stars are born in chaotic collapsing clouds, and astronomers have now found evidence that the tumultuous times don’t stop at birth, but continue throughout their infancy.

A new study indicates that young stars may undergo an intense and messy growth process, during which they steal mass from the surrounding protoplanetary disk. The research, published in Science Advances, suggests that the growth process is not steady but intermittent, with powerful and quick mass accretion events.

The international team of astronomers used observational data from the Subaru Telescope’s near-infrared imaging instrument. They looked at four very young stars from a class called FU Orionis that exhibit peculiar jumps in brightness. For example, in 1937, the namesake of the class – which is one of the four stars in the study – became almost 10 million times brighter, and it remains as bright to this day.

The study indicates that the sudden increase in luminosity exhibited by these stars is due to chaotic events in the protoplanetary disk. The disk can undergo fragmentation due to gravitational instabilities, and those dense gaseous clumps can migrate towards the center and eventually be accreted by the star.

Intensity images of the selected FU Orionis objects. Liu et al./Science Advances

The team has singled out several asymmetric features in the infrared images of these four objects. These features are arms and arcs, and they stretch for billion of kilometers through the gas clouds surrounding the stars.

Although the system is turbulent, the features can persist for several thousand years. Astronomers have compared these findings with simulations of young systems and they believe this can explain why some protostars, like FU Orionis, go through sudden bursts of luminosity.

The findings have consequences beyond the small class of variable stars. Disk instabilities could play a pivotal role in the formation of all stars, and also explain some peculiar configurations in exoplanetary systems. We have observed giant gas planets orbiting very close to their parent stars, and disk instabilities and migration towards the star could explain their mysterious formation. 

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