Astronomers have discovered that certain stars have significantly stronger surface magnetic fields than previously believed, challenging current models of their evolution.
In stars like our sun, surface magnetism is closely tied to stellar spin, similar to the mechanism found in hand-cranked flashlights. Strong magnetic fields are typically observed in the core of magnetic sunspot regions and can cause various space weather phenomena. Until now, low-mass stars (celestial bodies with lower mass than our sun that can rotate rapidly or slowly) were thought to have very low levels of magnetic activity. This assumption made them ideal candidates for hosting potentially habitable planets.
In a recent study published in The Astrophysical Journal Letters, researchers from The Ohio State University propose a new internal mechanism called core-envelope decoupling. This mechanism suggests that the surface and core of cool stars start spinning at the same rate but then gradually drift apart. This process may enhance magnetic fields in these stars, leading to increased radiation that can endure for billions of years and potentially affect the habitability of nearby exoplanets.
This research was made possible by a technique developed earlier this year by Lyra Cao, the lead author and a graduate student in astronomy at Ohio State, and co-author Marc Pinsonneault, a professor of astronomy at Ohio State. This technique allowed the team to measure and characterize starspots and magnetic fields.
Although low-mass stars are the most common type in the Milky Way and often host exoplanets, our knowledge about them is limited. For years, it was assumed that physical processes in lower mass stars align with those in solar-type stars. Astronomers use stellar spin as a tool to understand the nature of a star’s physical processes and its interactions with companions and surroundings. However, there are instances where stellar rotation appears to halt, as Cao points out.
The team analyzed data from the Sloan Digital Sky Survey, focusing on a sample of 136 stars in M44, also known as Praesepe or the Beehive cluster. They found that the magnetic fields of the low-mass stars in the region were much stronger than current models could explain.
While previous research had already demonstrated that the Beehive cluster contains stars that defy current theories of rotational evolution, one of Cao’s team’s most exciting findings was the uncovering of unusually strong magnetic fields in these stars, which surpassed predictions based on current models.
Cao expressed her excitement about the link between magnetic enhancement and rotational anomalies, stating that it indicates the potential presence of intriguing physics. The team also put forth the hypothesis that the process of synchronizing a star’s core with its envelope might generate the distinct magnetism observed in these stars, which differs from that observed in the sun.
“We have found evidence suggesting the existence of a different type of dynamo mechanism driving the magnetism in these stars,” said Cao. “This research demonstrates that stellar physics can have unexpected implications for other fields.”
According to the study, these findings have significant implications for astrophysics, particularly in the search for extraterrestrial life. Cao stated that stars with enhanced magnetism are likely to subject their planets to high-energy radiation. This effect may persist for billions of years in some stars, underscoring the importance of understanding its impact on our notions of habitability.
However, these findings should not discourage the quest for extraplanetary life. Further research on the team’s discovery could lead to a better understanding of where to search for planetary systems capable of hosting life. On Earth, Cao believes that these discoveries could contribute to improved simulations and theoretical models of stellar evolution.
“The next step is to verify that enhanced magnetism occurs on a broader scale,” Cao added. “If we can comprehend the internal dynamics of these stars as they experience shear-enhanced magnetism, it will propel scientific exploration in a new direction.”
The study received support from The Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the National Science Foundation. Jennifer van Saders from the University of Hawaii also served as a co-author.