Dusty Crescents Around a Baby Star

Did vortices sculpt the crescent-shaped clumps of dust around the young star MWC 758? Using data from the Atacama Large Millimeter/submillimeter Array (ALMA), researchers have mapped the motions of the dust clumps and weighed in on the vortex hypothesis.

Complicated Disks

How planets form is one of the most pressing questions in astronomy. Observations increasingly show that protoplanetary disks, the sites of planet formation, are complex objects. These disks feature rings, gaps, spirals, and vortices, any of which might signal the presence of baby planets.

Among the intriguing features seen in protoplanetary disks are crescents: asymmetric regions where the density of dust is enhanced. These regions are readily visible in observations by ALMA and have been found in several protoplanetary disks.

Researchers suspect that crescents are caused by swirling regions called vortices. Like debris caught in an eddy in a stream, dust could theoretically become trapped in a vortex, forming the clumps seen in images — and potentially creating a perfect dusty ecosystem for planets to form.

The Causes of Crescents

Recently, a team led by I-Hsuan Genevieve Kuo (Academia Sinica Institute of Astronomy and Astrophysics, Taiwan; University of Arizona) investigated the causes of crescents in the protoplanetary disk around the star MWC 758, also called HD 36112. MWC 758 is a 1.5–2-solar-mass star that is about 3.5 million years old and less than 500 light-years away. Its disk sports several intriguing features, including two crescents and a spiral. Previous research has attributed these features to the presence of one or more planets.

Vortex theory predicts that vortices in a protoplanetary disk will revolve around the central star at the Keplerian velocity (i.e., following the predictions of Kepler’s laws of motion). To test this theory, Kuo’s team used ALMA data from 2017 and 2021 to measure the motion of the two crescents. They found that the crescents moved in the direction of the disk’s rotation, with the inner crescent moving 50% slower than expected for the vortex hypothesis and the outer crescent moving 33% faster than expected.

Spiral vs. Vortex

Does this finding necessarily rule out the vortex hypothesis? Kuo and coauthors first investigated and eliminated the possibility that imperfections in the disk, like warps or eccentricity, were the cause of the mismatch with theory. They then noted that the motion of the crescents matches the Keplerian velocity at a radius of about 0.46 arcsecond. If a planet were present at that radius, there’s a chance it could throw the system off-kilter and produce the observed behavior — but MWC 758’s putative planets are located well inside and outside this radius.

Instead, the non-Keplerian rotation of the dust clumps might be caused by the interaction of vortices and spirals. In this case, the vortices themselves, which are invisible to us, are moving in the way predicted by theory, but the spiral knocks the (visible) dust off course.

Luckily, this prediction is testable. If vortices and spirals are vying for control of the dust crescents in MWC 758’s disk, their power struggle would be less effective on large dust grains than on small dust grains. In other words, large dust grains should adhere more closely to the expected Keplerian velocity than small dust grains do. Future high-resolution observations at different wavelengths, which probe grains of different sizes, may provide an answer.

Citation

“ALMA Observations of Proper Motions of the Dust Clumps in the Protoplanetary Disk MWC 758,” I-Hsuan Genevieve Kuo et al 2024 ApJL 975 L33. doi:10.3847/2041-8213/ad86c1