Planets could put the brakes on young stars

Lindsay Lohan, take note

The very early stages of planet formation could be responsible for putting the brakes on fast-spinning young stars, according to astronomers using the Spitzer infrared space telescope.

The researchers have found that slow spinning stars are five times more likely than their speedier cousins to be encircled by a disk of proto-planetary dust.

Dr Luisa Rebull of NASA's Spitzer Science Centre, lead author on the study, commented: "We knew that something must be keeping the stars' speed in check. Disks were the most logical answer, but we had to wait for Spitzer to see the disks."

Young stars spin around incredibly fast, some making a complete revolution in less than half a day. This is because of how they form: clouds of spinning gas collapse in on themselves, spinning faster and faster, just as an ice skater will when he pulls his arms in as he spins.

As it spins, excess dust and gas will flatten and form disks around the newly ignited star. The disk rotates much more slowly, and astronomers had suggested that the disk might interact with the star's magnetic field somehow and act as a brake.

We know something has to. Left to itself, a star spinning at a rate of a revolution every half day will not form planets. In addition, every single star that has been observed with planets so far, has been a relatively slow spinner, like our sun.

To test the idea that the disks do slow stars down, Rebull trained the Spitzer telescope on a region of the Orion Nebula, and surveyed 500 young stars.

She split the stars into fast spinners and slow spinners, and then used Spitzer to determine which were surrounded by disks. Slower stars turned out to be five times more likely to have disks than their faster colleagues.

Rebull concedes that this is not conclusive evidence for the exact mechanism of braking, but stresses that at least: "We can now say that disks play some kind of role in slowing down stars in at least one region."

She adds that other factors could be operating in tandem with the braking effect of a disk, and that different stars in different environments could behave differently.

The research is published in the 20 July issue of the Astrophysical Journal. ®

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