Delving into the Enigma of Magnetic Massive Star Systems

Through the use of spectropolarimetric data, researchers have discovered that magnetic fields are more prevalent in star systems that have massive blue stars than was previously thought. This finding has improved our understanding of the role of these stars in the universe and their evolution and explosive deaths.

A recent study has shown that magnetic fields are common in large blue star systems, challenging previous beliefs and providing new insights into the evolution and explosive nature of these massive stars.

Astronomers from the European Southern Observatory (ESO), the Leibniz Institute for Astrophysics Potsdam (AIP), and the MIT Kavli Institute and Department of Physics have found that magnetic fields are much more common in multi-star systems that have at least one giant hot blue star than scientists previously thought. This discovery has significantly advanced our understanding of these stars' role as progenitors of supernova explosions.

Blue, or O-type stars, are the most massive stars in the universe, with masses more than 18 times that of the sun. Four of the 90 brightest stars visible from Earth are O-type, and despite their rarity, they have a significant impact on galaxies' structure due to the physical processes they drive and their ability to enrich interstellar space chemically. These stars are typically found in areas with active star formation, like the arms of a spiral galaxy or in colliding or merging galaxies.

Because massive stars end their evolution explosively as a supernova, leaving behind a compact remnant like a neutron star or black hole, they are of special interest for magnetic studies.

Binary systems, or systems of two stars bound together by gravity that orbit each other, can become a compact binary object if both stars are O-type. These stars may end their lives as neutron stars or black holes after exploding as a supernova, and such binary systems can result in a pair of neutron stars, a neutron star and a black hole, or two black holes. The orbits of these objects degrade due to the emission of gravitational waves, which can be detected.

The magnetosphere, an area around an astronomical object where charged particles are influenced by the object's magnetic field, is formed by white lines representing magnetic field lines. The color brightness denotes higher gas density distribution. These magnetic fields can cause the concentrated explosion of massive stars, which is important for understanding long-duration gamma-ray bursts, X-ray flashes, and other supernovae features.

Although a theoretical explanation for the influence of magnetic fields on supernovae or gamma-ray bursts has existed for some time, only eleven O-type stars had been reported to have magnetic fields. Most of these stars were either singular or part of wide binaries. As more than 90% of O-type stars are part of multi-star systems, the paucity of detected magnetic fields in massive stars was puzzling.

To solve this discrepancy, the authors carried out a magnetic survey using spectropolarimetric observations of stellar systems with at least one O-type component. Spectropolarimetry measures light polarization, giving information about a star's magnetic field. The data used were collected by the HARPS spectropolarimeter installed at the ESO 3.6 m telescope on La Silla/Chile, and ESPaDOnS at the Canada-France-Hawaii telescope on Mauna Kea. A special procedure was developed to measure the magnetic field from the data.

“To our surprise, the results showed a very high occurrence rate of magnetism in these multiple systems. 22 out of the 36 systems studied have definitely detected magnetic fields, while only three systems did not show any sign of a magnetic field,” explains Dr Silva Järvinen from AIP’s Stellar Physics and Exoplanets section.

“The large number of systems with magnetic components presents a mystery, but probably indicates that the fact that these stars grew up in binaries plays a defining role in the generation of magnetic fields in massive stars through interaction between the system components, such as mass transfer between two of the stars, or even a merging event of two stars. This work is also the first ever observational confirmation of the previously suggested theoretical scenario for how a star’s magnetic field affects its death, letting it explode faster and more energetically,” continues Dr. Swetlana Hubrig.