The process of mergers in binary systems is responsible for the magnetic field of massive stars
Massive stars acquire their magnetic fields through collisions and mergers with other stars, according to new data obtained from studying a binary system shrouded in a dusty, element-rich nebula.
In the midst of this nebula, known under the dual designation NGC 6164/6165, lies the star system HD 148937. This system, located approximately 3800 light years away in the constellation Norma southern hemisphere, contains two massive stars orbiting a common center of mass. One of these stars, the brightest and hottest of them all, has a magnetic field. This is of interest to scientists, since, according to current ideas about the internal structure of stars, massive stars should not have magnetic fields.
We usually consider the Sun as a typical magnetic star, which is characterized by such phenomena as sunspots, flares and solar wind. The Sun's magnetic field is generated at the boundary between the inner radiation layer and the outer convection layer. In the radiative layer, energy is transferred from the core by gamma radiation sources, and in the convective layer, this energy is converted into a stream of hot plasma, which rises to the visible surface of the Sun, where the energy is released in the form of light and heat.
However, as the star's mass increases, the convective layers become smaller, making the generation of magnetic fields more difficult. Red dwarfs are almost entirely convective and magnetically active. However, the more massive the star, the smaller the convective layer. This means that the most massive stars without a convective layer cannot generate a magnetic field.
However, observations show that about 7% of the most massive stars do have magnetic fields, which poses a mystery to astronomers. The answer to this phenomenon probably lies in the star HD 148937.
A team of researchers led by Frost and Hugues Sana of the University of Leuven in Belgium conducted an extensive study of HD 148937 using nine years of data collected by the ESO telescope interferometer in Chile.
Their analysis revealed that the more massive of the two stars appears significantly younger than its companion, which is contrary to expectations, since both stars should have formed at the same time. The larger of the two stars has a mass of 50 to 60 times the mass of the Sun, and it has a magnetic field. However, it appears to be 1.5 million years younger than its companion, a significant difference for massive stars that typically live only a few million years before going supernova.
Additionally, scientists paid attention to the nebula surrounding the HD 148937 system. Its shape indicates that it arose as a result of the explosion of one of the stars in this system just 7,500 years ago. The nebula contains high concentrations of carbon, nitrogen and oxygen — elements commonly found inside stars but not outside them.
Frost and Sana's observations were able to piece together this stellar puzzle of the system HD 148937. The fact that one of the stars in this binary system appears younger than its companion, indicating a possible merger that occurred in this system in the last few thousand years .
«We assume that initially there were at least three stars in this system. Two of them were close to each other, while the third star was much further away», — noted scientists.
According to this theory, HD 148937 was in the past a compact binary system around which there was a third star. Two nearby stars merged into one star, ejecting excess material along their new axis of rotation, resulting in the formation of a nebula. Inside only the new star, changes are still taking place: the stellar material of the two stars is mixed, giving the new star a younger appearance, and the turbulent and convective environment in its interior is capable of generating a magnetic field with a strength of 1 kilogauss (the magnetic field of the Sun is on average 1 Gauss) .
The magnetic field will not last long — The inner regions of the star will eventually become completely mixed, after which the magnetic field will «turn off». This not only indicates that the stellar merger must have occurred relatively recently, but also projects the 7% of massive stars with magnetic fields into the context of merger rates in close binary systems.