Astronomical splendor with a bright story

The beautiful HD 148937 system has a dramatic development history. © ESO/VPHAS+ team. Acknowledgment: CASU

An exciting pair of stars in their sights: Researchers have gained insight into the formation of a binary star system shrouded in nebula, thereby shedding light on a stellar magnetism puzzle. Their investigations revealed surprising differences between the two partners: one star appears younger and has a magnetic field, the other does not. According to model simulations, this can be explained by the fact that the system originally consisted of three stars until two merged to form the special partner. The study shows that the strong magnetic fields of some stars could be due to mergers, say astronomers.

Beautiful and mysterious: HD 148937 has been in the sights of astronomy for some time. It is a binary system about 3,800 light-years away from us that is enveloped by a cloud of gas and dust. It was already known that it consists of two stars more massive than our Sun. At the beginning of the study by Abigail Frost’s team from the European Southern Observatory (ESO) in Santiago, the question was how this system could have come about. “A nebula surrounding two massive stars is a rarity, and it was suspected that something extraordinary must have happened in this system,” says Frost.

Analytical look at a mysterious couple

The research team has now evaluated observation data from the HD 148937 system collected over the last nine years. Instruments were primarily used at ESO’s Very Large Telescope Interferometer in the Chilean Atacama Desert. They provided spectral data of the light from the two celestial bodies, which made it possible to draw conclusions about their characteristics. As Frost and her colleagues report, their data analyzes yielded surprising results: Stars in binary systems are usually very similar, because they are like twins that emerged at the same time from the same cloud of matter. But in the case of HD 148937, the slightly larger of its two partners emits a pattern of rays that make it appear younger than the other star.

Another peculiarity of the larger star also emerged: certain clues in its radiation pattern clearly show that, unlike its partner, it has a magnetic field. Our sun also has this feature. But given the size of the star in the HD 148937 system, a magnetic field appears unusual. The researchers explain: The magnetic fields of low-mass stars such as the Sun are generated by a dynamo effect. It is created when mass movement driven by convection takes place inside the star. However, for stars that have eight or more solar masses, this process should actually come to a standstill due to their physical properties. However, it was previously found that, contrary to expectations, around seven percent of massive stars have magnetic fields. What caused this was previously unclear. The study now sheds light on exactly this question, say the researchers. According to them, the answer is reflected in the history of the creation of the HD 148937 system, which they reconstructed using model simulations.

A trio became the unlikely duo

“We now assume that this system originally consisted of at least three stars, two of which were very close to each other at some point in their orbit, while another star was much further away. The two inner stars then merged in a violent manner,” says co-author Hugues Sana from KU Leuven. During the merger, the material was also thrown into the surrounding space, now forming the nebula. “The more distant star then formed a new orbit with the merged star, forming the binary star we see today at the center of the nebula,” says Sana. As the astronomers explain, the union gave the newly formed star a radiation pattern that appeared tapered compared to its partner. The magnetic field can also be traced back to the violent union: it set off a dynamo effect, the team explains.

Video: This animation illustrates the creation process of the HD 148937 system. © ESO/L. Calçada, M. Kornmesser/VPHAS+ team. Acknowledgment: CASU

For the first time, the study provides direct evidence for the previous assumption that magnetic fields in massive stars can be traced back to a previous merger, say the scientists. However, it can be assumed that the magnetism is not maintained for very long compared to the lifespan of the star. In the case of HD 148937, this means: “It appears that we observed this rare event very shortly after its formation,” says Frost.

The researchers now want to continue to keep an eye on this exciting system. ESO’s Extremely Large Telescope, which is currently being built in the Chilean Atacama Desert, could soon provide more detailed insights.

Source: European Southern Observatory, specialist article: Science, doi: 10.1126/science.adg7700

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