Hot, dense material surrounds O-type star with largest magnetic field known

Observations using NASA's Chandra X-ray Observatory revealed that the unusually large magnetosphere around an O-type star called NGC 1624-2 contains a raging storm of extreme stellar winds and dense plasma that gobbles up X-rays before they can escape into space.

Findings from a team of researchers led by Florida Institute of Technology Assistant Professor Véronique Petit may help scientists better understand the lifecycle of certain massive stars, which are essential for creating metals needed for the formation of other stars and planets.

The findings will be published Sept. 23 in the journal Monthly Notices of the Royal Astronomical Society from Oxford University Press.

The massive O-type star – the hottest and brightest type of star in the universe – has the largest magnetosphere known in its class. Petit found NGC 1624-2's magnetic field traps gas trying to escape from the star and those gases absorb their own X-rays. The star's powerful stellar winds are three to five times faster and at least 100,000 times denser than our Sun's solar wind. Those winds grapple violently with the magnetic field and the trapped particles create the star's huge aura of hot, very dense plasma.

“The magnetic field isn't letting its stellar wind get away from the star, so you get these big flows that are forced to collide head on at the magnetic equator, creating gas shock-heated to 10 million Kelvin and plenty of X-rays,” said Petit, who was part of a team of scientists that discovered the star in 2012. “But the magnetosphere is so large that nearly 80 percent of these X-rays get absorbed before being able to escape into free space and reach the Chandra telescope.”

The magnetic field at the surface of NGC 1624-2 is 20,000 times stronger than at the surface of our Sun. If NGC 1624-2 was in the center of our solar system, loops of dense, hot plasma would extend nearly to the orbit of Venus.

Only one in 10 massive stars have a magnetic field. Unlike smaller stars like our sun that generate magnetism with an internal dynamo, magnetic fields in massive stars are “fossils” left over from some event in its early life, perhaps from a collision with another star.

Petit and her team, including Florida Tech graduate student Rebecca MacInnis, will know even more about the NGC 1624-2 in October after getting data back from the Hubble Space Telescope that will explore the dynamics of its trapped wind.

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Other scientists who contributed to the research were: David Cohen, Swarthmore College; Gregg Wade, Royal Military College of Canada; Yael Nazé, L'Université de Liège; Stanley Owocki, University of Delaware; Jon Sundqvist, University of Delaware; Asif ud-Doula, Penn State Worthington Scranton; Alex Fullerton, Space Telescope Science Institute; Maurice Leutenegger, NASA/Goddard Space Flight Center and University of Maryland; Marc Gagné, West Chester University.

The paper will be available at midnight Sept. 23 here: http://mnras.oxfordjournals.org/lookup/doi/10.1093/mnras/stv1741