Braking news: Disks slow down stars

Left to their own devices, young stars would twirl so fast that they’d fly apart. Astronomers have long suspected that the planet-forming disks of gas and dust that surround many newborn stars put the brakes on these whirling dervishes. Now, researchers have the first clear-cut evidence that the young stars’ rotations are indeed slowed by their disks.

SLOWDOWN. A dusty disk surrounds a young star with a large magnetic field (green, spidery lines) in this artist’s depiction. Interactions between the field and particles in the disk seem to slow the rotations of such stars. R. Hurt (SSC), Caltech, JPL/NASA

To investigate, Luisa Rebull of the California Institute of Technology in Pasadena and her colleagues used the infrared Spitzer Space Telescope to study about 500 young stars in a densely packed stellar nursery, the Orion nebula. The telescope can easily find disks because the dust within them absorbs visible light from their parent stars and reemits the radiation at infrared wavelengths.

Rebull and her collaborators divided the Orion stars into those that take more than 1.8 days to complete one rotation and those that take longer. The slow spinners are five times as likely as their faster siblings to have disks, strongly suggesting that disks control the spin, the researchers report in the July 20 Astrophysical Journal.

Astronomers have proposed that the slowdown occurs because the strong magnetic fields that emanate from a young star extend into its surrounding disk. Charged particles within the disk drag on the magnetic field, slowing the star.

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