Stellar magnetic fields are constantly changing!
An international study involving the Universe and Particles Laboratory in Montpellier (UM – CNRS) has just shown that the magnetic field of the red dwarf star AD Leonis is heading toward a polarity reversal, a first for this type of star. These results, published in the journal Astronomy and Astrophysics, were obtained through 15 years of observations using the Canada-France-Hawaii Telescope and the Bernard Lyot Telescope.
The 11-year solar activity cycle is a well-known phenomenon during which the intensity of the Sun’s magnetic field varies and its polarities reverse. Over the past 30 years, astronomers have identified similar behavior in several stars similar to the Sun. But until now, no reversal of magnetic polarities has been observed in their cooler counterparts, the red dwarf stars.
An international team including scientists from IRAP, LUPM, IPAG, IAP, and LAM has recently shown that the magnetic field of the notoriously active red dwarf AD Leonis may be approaching a polarity reversal, based on observations spanning 15 years. These data were obtained using the ESPaDOnS and SPIRou instruments at the Canada-France-Hawaii Telescope (CFHT) as well as NARVAL at the Bernard Lyot Telescope (BLT).
AD Leonis is a notoriously active red dwarf star; its magnetic field is about 1,000 times stronger than that of the Sun. More generally, the magnetic fields of red dwarfs differ greatly from the Sun’s magnetic field, and it is not yet known whether they can exhibit magnetic cycles, although there are indications of activity cycles.
Red dwarf stars and exoplanets
AD Leonis was regularly observed between 2019 and 2020 using the SPIRou instrument as part of the Legacy Survey. The study led by Stefano Bellotti, a PhD student at IRAP, shows that not only has the magnetic field strength been steadily decreasing since observations conducted starting in 2006 with the ESPaDOnS and NARVAL instruments, but also that the star’s magnetic poles have begun to flip. Although a polarity reversal did not occur during the SPIRou observations, these results indicate that red dwarfs like AD Leonis may undergo magnetic cycles, similar to the Sun. This encourages astronomers to continue observing AD Leonis for several more years.
This finding provides a better understanding of how the magnetic fields of stars cooler than the Sun are generated. Furthermore, studying the magnetic fields of red dwarfs—prime targets for the detection of Earth-like rocky exoplanets—such as AD Leonis is essential for understanding the space environment in which rocky exoplanets orbit. Indeed, a star’s magnetic field regulates the space weather in its vicinity, namely the energetic phenomena associated with stellar activity—such as flares and coronal mass ejections—which play a critical role in an exoplanet’s ability to retain an atmosphere over the course of its evolution, and thus to remain habitable.
Read the study " Monitoring the Large-Scale Magnetic Field of AD Leo with SPIRou, ESPaDOnS, and Narval"