The red dwarf television AU Microscopii (AU Mic for short), 32 light years from Earth in the constellation Microscopium (the Microscope), is a small, dim, cool star only about one-tenth the mass of our Sun. Like our Sun, red dwarfs are subject to periods of intense magnetic activity, including starspots and flares.
A team of astronomers led by Yuta Notsu, a JSPS Overseas Research Fellow at the Subaru Telescope and the National Astronomical Observatory of Japan, wanted to investigate the mechanisms that drive the activity cycles in red dwarfs. One particular area of interest was whether red dwarfs experience magnetic field reversals, akin to what is observed in the Sun.
Magnetic reversals occur when a star's north and south magnetic poles switch places, essentially flipping the entire magnetic field. The Sun's magnetic field has an 11-year cycle, and reverses polarity every 5.5 to 6 years.
However, it's unknown how magnetic reversals occur in red dwarfs. Most previous observations have focused on younger, more rapidly rotating red dwarfs that could be hiding complex and short period field oscillations, similar to how a rapidly spinning top wobbles. This makes understanding the true rotation rate and hence magnetic cycle of these stars difficult.
AU Mic offered a unique opportunity for the astronomers to study this phenomenon because it is a relatively old (5 to 10 billion years), and hence slow-rotating, red dwarf. Notsu explained that "AU Mic is known to have a rotation period of 4.8 days, which is long enough that the complex rotation and magnetic activity can be separated."
The astronomers obtained four years of high-cadence TESS photometric data. Combined with nearly eight years of spectroscopic observations using the Subaru Telescope's High Dispersion Spectrograph (HDS), and additional measurements made using the EXPRES spectrograph on the Lowell Discovery Telescope in Arizona, they carefully disentangled the contributions from the star's rotation and magnetic spots to better measure AU Mic's magnetic cycle and track the surface activity.
The researchers detected three full magnetic cycles spanning the four-year TESS timebase, and found significant changes during the last cycle. Notsu commented, "The amplitude of spots gradually changed, and the hemispheric asymmetry of the spot distribution also changed sign, suggesting that the magnetic polarity of the spots may have reversed."
By analyzing the data further, the team estimated the full magnetic cycle of AU Mic to be about 13.5 years. AU Mic's activity also showed characteristics similar to sunspots, including strong magnetic fields exceeding several thousand Gauss, the emergence and decay over two to three months, and a tendency for spot activity to concentrate at specific latitudes on the star.
The research suggests that polar field reversals could also be occurring on AU Mic, although a longer observational baseline would be needed to conclusively confirm this. Future studies using larger telescopes, such as the Thirty Meter Telescope, currently under construction on Maunakea in Hawaii, may enable the astronomers to capture evidence of a polarity reversal and measure the precise nature of the cycle that drives the activity.
