Hubble: Magnetic fields weaving rings around stars
Posted: July 16, 2001

Animation Still: Greg Bacon (STScI/AVL)
There are stars with planets. Stars with companion stars. Stars with pancake-shaped disks of rocky debris. But how about young, hot, hefty stars embedded in large inner tube-shaped clouds of shimmering gas?

Astronomers had suspected that the thick rings are the signatures of stars with strong magnetic fields. Sometimes, the surfaces of those "magnetic stars" possess peculiar chemical compositions, namely low amounts of "heavy elements" like iron. Now a team of astronomers analyzing archival information on four stars provides convincing evidence of the link between rings and magnetic fields. The team also suggests that rings around massive stars are more common than scientists thought. The study shows that magnetic stars with normal chemical abundances can have rings, too.

"It is a milestone in astronomy when one can point to a unifying characteristic of different types of stars that brings them all under one roof," says Myron Smith of the Space Telescope Science Institute in Baltimore, MD. "We're finding that there may be at least twice as many massive stars with rings as astronomers once thought. In all of these stars, the rings are created and maintained by the same mechanism: stable magnetic fields. The rings form from the collision of particles flowing along the field lines." Smith's results were recently published in the June II issue of the journal Astronomy and Astrophysics.

Smith and his colleague Detlef Groote of the University of Hamburg, Germany, analyzed 16 years of archival data from four stars observed in ultraviolet light with the International Ultraviolet Explorer (IUE), a satellite that observed the heavens from 1978 to 1996. The IUE digital library is stored in the Multi-Mission Archive at the Space Telescope Science Institute. Two of the stars' surfaces showed normal chemical abundances, consisting of a mixture of hydrogen, helium, and heavy elements such as iron. The other pair exhibited low amounts of heavy elements. All the stars are about 10 million years old and several times more massive than the Sun.

The astronomers could not observe the magnetic fields in all the stars directly, but they inferred their existence by studying the colors (spectral signatures) of the rings. In their analysis, Smith and Groote found that the rings absorbed light at many wavelengths, especially ultraviolet. Scientists who previously studied these ringed stars noticed that the starlight dimmed at periodic intervals. Smith and Groote show that the rings passing in front of the stars produced the dimming. They also were surprised that the rings revealed strong spectral signatures of hot nitrogen and carbon gases. Normally, those gases are not hot enough to be seen. The temperature of the gases was 90,000 degrees Fahrenheit (50,000 degrees Kelvin), twice as hot as the surfaces of the stars.

"We knew that some mechanism within the star was heating some of the gas in the ring," Smith says. "The presence of the ring led us to believe that a magnetic field was responsible for the glowing nitrogen."

The astronomers believe that each star contains a simple "bar-like" magnetic field like the Earth's. The field extends from a magnetic north to a south pole, looping around the star in a pattern resembling a freeway cloverleaf. Gas trapped in the field lines would look like an inner tube-shaped cage around the star. The magnetic field's axis is tilted relative to the star's rotation. So, anyone observing the star will notice that the inner tube appears to wobble as the star spins.

This simple bar magnetic field is a key player in forming the ring system. According to a popular theory, the magnetic field directs the flow of particles streaming off the star in the form of "stellar winds" -- streams of charged particles that travel about 2 million miles per hour (4 million kilometers per hour). All hot stars have stellar winds, but those without strong magnetic fields release streams of particles in all directions. A star with a magnetic field positioned along the poles -- like the ones in Smith's and Groote's study -- channels the wind only from the two poles. The wind particles, especially material like iron, are driven by the star's intense radiation and travel on the cloverleaf track over the star. A few hours later, they slam into streams of matter traveling from the opposite direction, somewhere along the star's tilted magnetic equator.

The collisions resemble a series of head-on car accidents along a congested highway. As the particles slam into each other, they stop and congregate along the equator, eventually forming a thick ring. The particle collisions also cause the material to glow and radiate at high energies, detected in ultraviolet and perhaps X-ray wavelengths.

Smith and Groote found that the thick rings are around several, perhaps all, types of massive stars with strong magnetic fields.

"The rings of various types of massive stars may have different spectral properties [chemical compositions]," Smith explains. "For example, in some stars, iron-like particles are lost into space, creating rings with various chemical compositions. But scientists now understand that the rings -- although chemically different, depending on the type of star -- were created by the same kind of interplay between a star's magnetic field and its wind. The new concept unifies apparently different groups of stars."

The Space Telescope Science Institute (STScI) is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).

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