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Massive Stars Magnetism
Recent research has unveiled that massive stars acquire their magnetism from collisions and mergers with other stars. This discovery stems from a peculiar binary system surrounded by a dusty nebula filled with elements. The bipolar nebula, known as NGC 6164/6165, encompasses the star system HD 148937. Situated approximately 3,800 light-years away in the Norma constellation’s southern hemisphere, HD 148937 comprises two massive stars in mutual orbit. Notably, one of these stars harbors a magnetic field, making it the most luminous and hottest magnetic massive star on record. This peculiarity contrasts with existing knowledge suggesting that massive stars lack magnetic fields.
Conventionally, the sun is characterized as a magnetic star, governing solar events like sunspots, flares, and prominences. The sun generates a magnetic dynamo at the interface between its inner radiative layer and outer convection layer. Whereas small stars predominantly possess convection layers, massive stars have larger radiative layers, precluding magnetic field generation.
However, about 7% of the most massive stars exhibit magnetic fields, posing a conundrum for astronomers. Addressing this mystery, HD 148937 emerges as a critical subject for investigation. Scientists led by Abigail Frost from the European Southern Observatory and Hugues Sana from KU Leuven conducted an in-depth study of HD 148937 utilizing data collected over nine years from the ESO’s Very Large Telescope Interferometer, enhancing our understanding of this intriguing system.
HD 148937 System
Frost and Sana’s analysis unveiled surprising insights about HD 148937. Despite these stars’ expected simultaneous formation, the larger one, boasting 50 to 60 solar masses, appears significantly younger than its companion by 1.5 million years. This age dissonance, in a system where stars typically endure a few million years before supernova, hints at a turbulent history.
Furthermore, the nebula’s bipolar structure indicates a violent outburst less than 7,500 years ago stemming from the HD 148937 system. Noteworthy is the nebula’s high concentrations of carbon, nitrogen, and oxygen, elements primarily confined within stellar interiors. Frost and Sana’s findings indicate that the binary system incurred a stellar merger within the past few thousand years, triggering this unique astronomical event.
Stellar Merger Hypothesis
According to Sana, the system’s original configuration likely involved three stars, with two in close proximity and a third more distant. The primary theory posits a scenario where the two close stars merged violently, ejecting material that formed the nebula. This event also altered the orbits, leading to the binary system observed today.
Eventually, the merged star’s magnetic strength will deteriorate as its interior homogenizes into a fully radiative state. This evolution provides further evidence of a recent merger and sheds light on the prevalence of magnetic fields in massive stars. The study detailing these findings was published in the journal Science, contributing significantly to our comprehension of the magnetic properties of massive stars.
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