Asteroseismology, specifically helioseismology when studying the Sun, examines the oscillations of stars. "To understand it, you must imagine a star as a big ball of gas in constant motion. Inside this star, there are waves or pulsations that make it vibrate, rather like the sound that resonates in a musical instrument," explained Jerome Betrisey, a postdoctoral researcher in the Astronomy Department of the Faculty of Science at UNIGE and the study's lead author. "These vibrations cause the star's surface to move slightly and change luminosity regularly. Thanks to very precise instruments, we can detect these variations in luminosity from Earth or from space," he continued.
By analyzing these variations, scientists can gain insight into a star's internal structure, determining its size, age, chemical composition, and evolutionary stage. Understanding these characteristics is crucial not only for studying the stars themselves but also for learning about the planets that orbit them and the history of the Milky Way.
Despite the successes of asteroseismology in recent decades, significant differences remain between observational data and the predictions made by theoretical models. Various methods have been developed over the years to reduce these discrepancies, though none have fully accounted for the role of magnetic activity, which was previously assumed to have a negligible impact.
The international team led by Jerome Betrisey has now shown that the Sun's seismic age, as determined through helioseismology, fluctuates significantly in relation to the solar activity cycle. The Sun is estimated to be around 4.6 billion years old, with observed variations of up to 300 million years depending on the phase of the solar cycle. While these variations might seem small relative to the Sun's total age, they are critical given the level of precision expected from upcoming space missions.
To investigate the impact of magnetic activity, the researchers analyzed 26.5 years of solar data spanning two full activity cycles. They broke this data into roughly 90 smaller series, each covering one year and spaced three months apart. For each series, a seismic analysis was conducted to measure how the Sun's fundamental properties, such as mass, radius, and age, evolved over time.
The robustness of the results was confirmed using two independent datasets: one from the University of Birmingham's BiSON (Birmingham Solar Oscillations Network) and the other from the GOLF (Global Oscillations at Low Frequencies) instrument aboard the SOHO (Solar and Heliospheric Observatory) satellite, which has been monitoring the Sun since the mid-1990s.
The findings indicate that the Sun's seismic age correlates strongly with the solar activity cycle, with variations of about 6% observed between solar minimum and maximum periods. This is highly significant for future space missions like PLATO (PLAnetary Transits and Oscillations of stars), which aims for a 10% precision in age determination for stars similar to the Sun.
The study also revealed that the impact of the solar activity cycle on seismic age is more pronounced during the more active of the two cycles examined. "However, the Sun is not a particularly active star, which suggests that the impact of magnetic activity could be very significant for more active stars such as those that PLATO will detect," added Betrisey.
The study highlights the challenges that stellar magnetic activity poses for future space missions like PLATO, particularly in characterizing the most active stars. However, it also opens up exciting new avenues for research into the interaction between magnetic fields and stellar oscillations. Understanding these interactions could advance our knowledge of stellar physics and improve our comprehension of stellar activity cycles, which are analogous to the Sun's solar cycles.
Research Report:Imprint of the magnetic activity cycle on solar asteroseismic characterisation based on 26 years of GOLF and BiSON data
