Massive stars gain their magnetism by colliding and merging with other stars, according to evidence from a strange binary system surrounded by a dusty, element-rich nebula.
This bipolar nebula, which has the double designation NGC 6164/6165, contains the star system HD 148937. light years Far away in the southern constellation Norma, HD 148937 contains two large bodies stars orbit around each other. One of these stars is magnetic and is actually the brightest and hottest known massive star. Magnetic field. This is quite confusing because, based on what we know about the interiors of stars, massive stars Negative It has magnetic fields.
We are used to thinking Sun as a magnetic star that directs events such as sunspotsflares, protrusions and solar wind. Magnetic dynamo was produced in the sun, at the boundary between the inner radiation layer and the outer convection layer. In the radiative layer, energy is transported from the core of the star in the following ways: gamma ray photons; In the convective layer, this energy is converted into a hot plasma stream that rises up to the sun’s rays. visible surfaceWhere energy is released as light and heat.
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The less mass a star has, the more of its volume the convective layer occupies. Smallest red dwarfs They are almost entirely convective and as a result are highly magnetically active. But the more massive a star is, the smaller the convective layer. This means that the most massive stars are purely radiative and cannot generate a magnetic field without a convective layer.
However, it has been observed that approximately 7% of the largest stars somehow have a magnetic field. The question that puzzles astronomers is: How?
The answer to this mystery may lie in HD 148937.
“As I was doing background research, I realized how special this system seemed,” said Abigail Frost of the European Southern Observatory. press release.
Frost and Hugues Sana from KU Leuven in Belgium led efforts to examine HD 148937 more closely using ESO’s nine years of observations. Very Large Telescope Interferometer combining the powers of four eight-meter telescopes in Chile.
“After detailed analysis, we were able to determine that the more massive star appears to be much younger than its companion, which makes little sense since they would have to form at the same time,” Frost said.
The larger of the two stars is between 50 and 60 times its size mass of the sun, is the magnetic one. Judging by its temperature, it appears to be 1.5 million years younger than its companion. This is a significant age difference for massive stars, which typically live only a few million years before fading. supernova.
There’s also the nebula to consider; Its bipolar shape suggests its origin in some kind of explosion from one of HD 148937’s stars only within the last 7,500 years. nebula It contains high concentrations of carbon, nitrogen, and oxygen, elements that are usually found inside stars but not outside stars.
Frost and Sana’s observations were able to piece together this particular stellar puzzle. The fact that a star in a binary system is younger than its companion is a sure sign that a merger of two stars has occurred in the last few thousand years.
“We think this system initially had at least three stars,” Sana said. “While the two should have been close together at one point in their orbit, another star was much further away.”
We can imagine HD 148937 as it once was: a close binary star in a short orbit with a third star around them. Two nearby stars came together and became a single star. They created the nebula by ejecting the excess material along new spin axes. Meanwhile, inside the newly merged star, events were, and still are, in flux. Stellar material from the two stars churns, giving the merging star a younger, rejuvenated skin, while inside a turbulent and convective environment is created that can produce the dynamo that gives the star a magnetic field strength of one kilogauss (for example, by comparison, the average strength of the sun’s magnetic field is 1 gauss).
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“The two inner stars violently merged, forming a magnetic star and ejecting some of the material that formed the nebula,” Sana said. “The more distant star formed a new orbit with the newly merged, now magnetic star, creating the binary we see today at the center of the nebula.”
The magnetic field will not last long; The interior of the star will eventually settle to become completely mixed and fully radiative. From there the magnetic dynamo will turn off. Not only is this further evidence that mergers must have occurred relatively recently, but it also puts the figure of 7% of massive stars having magnetic fields into context and points to the rate of mergers between nearby binary stars.
The findings were reported in a paper published April 12 in the journal. Science.