Planetary Disks Slow Stellar Rotation

[Fraser]

New data from NASA's Spitzer Space Telescope are giving astronomers a sense of how protoplanetary disks might act as a brake to slow stellar rotation. Young stars spin very quickly, often completing a rotation in less than a day. They could spin even faster, but something is slowing them down. Spitzer gathered data on 500 young stars in the Orion Nebula. The fastest spinning stars don't have planetary disks around then. It might be that the magnetic field of the star interacts with the planetary disk, slowing the star down.

Read the full blog entry

[Jerry]

Developing stars spin so fast that, left unchecked, they would never fully contract and become stars. Prior to the new study, astronomers had theorized that disks might be slowing the super speedy stars by yanking on their magnetic fields. When a star’s fields pass through a disk, they are thought to get bogged down like a spoon in molasses. This locks a star’s rotation to the slower-turning disk, so the shrinking star can’t spin faster.

Shouldn't this also increase the velocity of the disk material nearest the star, and by extension, shouldn't we also expect a coupling of the Sun's magnetic field with the planets in our own solar system? Or are the planet's orbital periods harmonic multiples of the solar magnetic day, naturally dampening the elliptical eccentricities of the planetary orbits?

[iantresman]

Is this related to Alfvén's hypothesis of "magnetic braking", H. Alfvén, "Remarks on the rotation of a magnetized sphere, with application to solar rotation," Arkiv Mat. Astron. Fys., vol. 28A, no. 6, p. 1, 1942, and related to that of V. C. A. Ferraro, "The non-uniform rotation of the sun and its magnetic field,” Mon. Not. R. Astron. Soc., vol. 97, p. 458, 1937.

Regards,
Ian Tresman

[George]

Interesting story.

Question...Cresswell admitted that their model (see #1 below) still did not explain gas giants due to the time frame problem; protogiants need more time. Yet, using Jerry's idea of an angular momentum transfer to the disk, wouldn't that buy valuable time for these protogiants (in disk systems), or would it be too miniscule a difference?


Also, the article mentions the possibility of planets falling into the stars as a means to reduce rotation. Is it me, or are there a lot of ideas about the possiblity of planets crashing into their nascent host? Here are three:

1) Cresswell and Nelson's solar system model which has more initial planets, some of which migrate and merge with the sun.

2) V838 Monocerotis and at least one other as a new class of stars which flash because of planetary migrations into their host. [visual hiccups! ]

3) This article suggesting the possibility that swallowed planets slow stellar rotation. [paraphrased]

This is quite interesting to me.