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An international team, led by Instituto de Astrofísica e Ciências do Espaço (IA) researcher Ângela Santos, has made the first measurement of the magnetic field of β Hydri, a nearby aged solar analog subgiant star. This result, in Astronomy & Astrophysics, uncovered an unexpectedly strong magnetic braking, which supports the idea of a "born-again dynamo" in subgiant stars.

Santos (IA & Dep. of Â鶹ÒùÔºics and Astronomy of the Science Faculty of the University of Porto—DFA-FCUP), the lead author of the paper, says, "In recent years, we have discovered that the evolution of stellar magnetic activity is significantly more complex than previously thought. Studying solar-like stars, like β Hydri, particularly at different evolutionary states, is key to build a comprehensive picture of how stars and their systems age."

β Hydri is one of the brightest stars visible to the naked eye in the southern sky, and one of the most studied stars besides the sun. Slightly more massive than our star and further along in its life cycle, it has long been used as a model for understanding the sun's eventual evolution into a subgiant.

Despite its age, β Hydri shows signs of a magnetic activity cycle similar to that of the sun—an unexpected feature for a star at this point in its evolution.

The team obtained high-precision spectropolarimetric data, using the HARPSpol instrument at the European Southern Observatory's (ESO) 3.6-meter telescope in La Silla (Chile). These observations allowed, for the first time, a direct estimate of the star's surface magnetic field.

By combining this magnetic field measurement with stellar parameters derived from previous studies, including recent photometric data from NASA's TESS space mission, the researchers calculated β Hydri's current magnetic braking, the process by which a star loses angular momentum due to its magnetic wind.

"Our analysis shows that β Hydri is undergoing magnetic braking, several times stronger than that of the sun," says Santos. "This is a clear sign that the star hosts a large-scale dynamo, which is surprising for a subgiant."

This finding lends support to the emerging "born-again dynamo" scenario, in which subgiant stars—after a quieter magnetic phase typical of middle age—can experience a reactivation of magnetic activity as their outer layers expand.

"This kind of behavior challenges our conventional understanding of stellar magnetic evolution, and makes β Hydri a benchmark for testing and refining models of magnetic braking and dynamo evolution in old solar-like stars," says Tiago Campante, leader of the Stellar Astrophysics Research Group at IA and professor at DFA-FCUP.

Co-author Ricardo Gafeira (IA & Faculdade de CIências e Tecnologia da Universidade de Coimbra—FCTUC) adds, "It also has implications for planetary environments because β Hydri seems to have reignited its magnetic engine, potentially increasing space weather hazards for any nearby planets."

As the sun's elder twin, β Hydri offers a glimpse into the magnetic future of our own star and, perhaps, a cautionary tale for any planets orbiting such "retired" suns.

The study highlights the scientific leadership of the IA in solar-stellar connection research, which involves different fields, combining stellar evolution, asteroseismology, and magnetic field modeling.