Most Magnetic Massive Star Found

An international team of astronomers has reported the discovery of the strongest magnetic field ever found around a massive star.

This image shows the open star cluster NGC 1624. The blue arrow pinpoints the star NGC 1624-2 (Centre de Données astronomiques de Strasbourg / SIMBAD)

NGC 1624-2, also known as 2MASS J04403728+5027410, is an O-type star with a mass of 30 times that of the Sun. The star lies in the open star cluster NGC 1624 some 20,000 light-years away in the constellation Perseus.

The star’s magnetic field is 20,000 times stronger than the Sun’s, and almost 10 times stronger than that detected around any other high-mass star.

“Understanding the evolution of massive stars, those that explode as core-collapse supernovae, is really important,” said Dr Anne Pellerin of Canada’s Mount Allison University, a co-author of a paper published in the journal Monthly Notices of the Royal Astronomical Society (arXiv.org version).

“When the stars explode, the heavy chemical elements born in the cores are scattered into space,” she explained. “In the big picture, the Sun is born from the debris of a supernova that exploded – that’s how we get iron.”

Additionally, despite their short lives – NGC 1624-2 will live only about five million years – massive stars shape the galaxies in which they live. “Their strong winds, intense radiation fields, and dramatic supernova explosions make them the primary sculptors of the structure, chemistry, and evolution of galaxies,” said lead author Dr Gregg Wade of the Royal Military College of Canada.

“But massive stars are rare,” Dr Pellerin added. “Anything we can do to get to know them is good. The extreme magnetic fields of massive stars aren’t well understood.”

“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,” Dr 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 star’s magnetic field also influences the internal structure of NGC 1624-2.”

Thus the magnetic field can strongly influence a massive star’s life, from birth to supernova death. But because these magnetic 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,” Dr Wade said.

The team wanted to better understand the nature of this monster star, but it is so distant, and surrounded by dust, that they needed a large telescope with immense light-gathering power to study its light in detail.

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

The team used the Hobby-Eberly Telescope (HET) coupled with its High Resolution Spectrograph instrument. They teased out the star’s rotation by studying repeating patterns in the star’s spectrum from HET. The patterns in the spectra are caused by winds coming off of the star.

“The winds of massive stars are very dense, especially compared to the Sun’s, which is called the solar wind,” Dr 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. It creates some weird features in the spectra.”

The repetition of such ‘weird features’ in the star’s light allowed the team to figure out that the star is rotating quite slowly. It takes this star about 160 Earth days to rotate once on its axis. For comparison, it takes the Sun 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,” Dr Wade said. “This is something that has to be tested, but it looks very likely.”

To measure the strength of star’s magnetic field, the team used the Canada-France-Hawaii Telescope coupled with an instrument called ESPaDOnS. Specifically, they measured small biases in the direction of rotation of the electromagnetic waves absorbed or emitted by atoms located in the field.

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Bibliographic information: Wade G.A. et al. 2012. NGC 1624-2: a slowly rotating, X-ray luminous O-type star with an extraordinarily strong magnetic field. Monthly Notices of the Royal Astronomical Society, volume 425, issue 2, pages 1278–1293; doi: 10.1111/j.1365-2966.2012.21523.x