Largest-ever Magnetic Field Found Around Massive Star
September 12, 2012

Astronomers Measure Largest Magnetic Field Of Star Yet

Lee Rannals for - Your Universe Online

Astronomers have measured the largest-ever magnetic field around a massive star and reported their findings in the journal Monthly Notices of the Royal Astronomical Society.

The team used the Hobby-Eberly Telescope (HET) at The University of Texas McDonald Observatory and the Canada-France Hawaii Telescope (CFHT) on Hawaii's Mauna Kea to determine that the star's magnetic field is 20,000 times stronger than the Sun's.

The magnetic field of the O-type star NGC 1624-2 is nearly 10 times stronger than other high-mass stars.

The star lies in the open star cluster NGC 1624, which is about 20,000 light-years away in the constellation Perseus.

The star will help astronomers better understand all massive stars, which play an important role in the evolution of galaxies.

"Understanding the evolution of massive stars, those that explode as core-collapse supernovae, is really important," team member Anne Pellerin of Canada's Mount Allison University said in a statement.

She said once these stars explode, the heavy chemical elements born in the cores are scattered into space.

"In the big picture, the Sun is born from the debris of a supernova that exploded – that's how we get iron," she said.

The short lives of O-type stars help to shape the galaxies in which they live. NGC 1624-2 is estimated by scientists to only live about five million years, which is one-tenth of one percent the Sun's current age at midlife.

"Their strong winds, intense radiation fields, and dramatic supernova explosions make them the primary sculptors of the structure, chemistry, and evolution of galaxies," said Gregg Wade of the Royal Military College of Canada.

Scientists do not have a good grasp yet of the extreme magnetic fields of massive stars.

"The most important consequence of the strong magnetic field is that it binds and controls the stellar wind of NGC 1624-2 to a very large distance from the star – 11.4 times the star's radius," Wade said. "The huge volume of this magnetosphere is remarkable. It's more than four times wider than that of any other comparable massive star, and in terms of volume it is around 80 times larger."

The magnetic field can strongly influence a massive star's life, but because these fields are poorly understood, models of stellar evolution are incomplete.

"We need observations of stars like NGC 1624-2 to teach us what's really going on," Wade said.

The team wanted to have a better understanding of the nature of this star, but because it is surrounded by dust and lies at such a distance, they needed a telescope with huge light-tethering power. The astronomers turned to the 30-foot HET coupled with its High Resolution Spectrograph instrument for the job.

"This star is hard to observe because it's highly extinguished by dust," Pellerin said. "That makes it fainter, so it takes a bigger telescope mirror."

The astronomers were able to observe the star's rotation by studying repeating patterns in its spectrum from HET.

"The winds of massive stars are very dense, especially compared to the Sun's," which is called the solar wind, Pellerin said. "These stars are losing a lot of mass through their winds – up to 30 percent over their entire lives. The wind is a plasma, made up of charged particles that follow the lines of the magnetic field."

She said these wind features in the star's light allowed them to figure out that the star is rotating slowly, taking about 160 Earth days to rotate once on its axis. The Sun takes about 25 days to rotate on its axis.

"We think that the star is slowed down because it has to drag its wind around – because the wind is bound to the magnetic field," Wade added. "This is something that has to be tested, but it looks very likely."

In order to measure the strength of the magnetic field, the team used the CFHT coupled with an instrument known as ESPaDOnS.

They measured small biases in the direction of rotation of the electromagnetic waves absorbed or emitted by atoms located in the field.

"An excess of clockwise-rotating waves indicates a magnetic field pointing towards us, while an excess of counterclockwise-rotating waves indicates a magnetic field pointing away from us," Wade said. "The larger the excess, the larger the magnetic field. These excesses are usually very tiny, requiring many observations or careful processing of the data to tease out the signal. But in the case of NGC 1624-2, it was obvious from our very first observations that a remarkably strong magnetic field was present."