What puts the brakes on madly spinning stars?

Keying off new observations, astronomers are turning to an old idea of what puts the brakes on young, rapidly rotating stars, some of which spin so fast that astronomers are amazed they simply don’t fly apart.

The results, reported by a team of university astronomers and their colleagues in the February 2001 edition of Astronomical Journal, are important not only because they may help explain a long-standing puzzle of stellar rotation, but because they help flesh out our picture of the origins of the sun and other stars like it.

“The puzzle is not where stellar spin comes from,” says Keivan Stassun, a UW–Madison astronomer and the lead author of the study. “The puzzle is how stars get rid of that spin.”

That stars rotate at all was a surprise when the phenomenon was first discovered by the great 16th century Italian physicist and astronomer Galileo Galilei who observed the sun spinning on its axis. Astronomers now believe that stars are set in motion when spinning clouds of gas and dust collapse to form new stars.

But astronomers have always been perplexed by the reality of spinning stars because they seem to defy one of nature’s inviolable laws: the conservation of angular momentum.

“Like the figure skater who spins faster and faster by pulling his arms inward, newly-formed stars should be forced to spin extremely rapidly as a result of their collapse from an initially immense cloud of gas and dust,” says Stassun.

Nascent stars, in fact, should end up spinning so fast that they fly apart. But they don’t, and astronomers have struggled for decades to understand what forces might be acting on stars to slow them down. The prevailing theory, says Stassun, is that a star’s magnetic field acts like a leash, exerting influence on the disks of dust and gas that surround many young stars.

“The idea is that the star’s magnetic field latches on to the slowly rotating accretion disk to slow itself down,” Stassun says. The idea is known to astronomers as magnetic disk-locking.

To test the idea, Stassun and collaborators Robert Mathieu, also a UW–Madison astronomer, Frederick Vrba of the U.S. Naval Observatory and Tsevi Mazeh of Tel Aviv University, Israel, used an infrared camera and the National Science Foundation’s 4-meter Cerro-Tololo Telescope in Chile to search for disks around very young stars. Such disks are only observable from Earth in infrared light.

Probing two stellar nurseries in the Milky Way — one in the constellation Orion and another in the constellation Taurus — Stassun and his colleagues searched for disks around 32 very young stars most of whose spin rates had already been calculated by William Herbst of Wesleyan University.

To its surprise, Stassun’s team discovered that many of the stars had no disks at all, a finding that seriously undermines the idea that the interplay of a star’s magnetic field and its accretion disk is the brake that slows its spin by soaking up the rotational energy that would otherwise be conserved through angular momentum.

“A lot of the slowly rotating stars we looked at simply don’t have disks,” says Vrba, a staff astronomer at the U.S. Naval Observatory in Flagstaff, Ariz.

Moreover, Stassun’s team also failed to find disks among several stars already spinning at the maximum speed possible.

Vrba says something other than disks must eventually slow down these stars because there are no disks there to put on the brakes. That finding, he says, raises questions about whether magnetic disk-locking occurs to the extent envisioned by current theories.

An alternative explanation is one that was set aside by astronomers years ago but now seems more plausible, Stassun says. That idea holds that stellar winds — streams of gas that blow off star surfaces — carry angular momentum away from stars and cause them to spin more slowly. Currently, that idea takes a back seat to star-formation theories that depend on magnetic disk-locking, but the new results may revitalize that old notion.