XX Tri: Watching the Sky’s Most Spotted Star Evolve Over 40 Years
In a detailed new study, Zs. Kővári and collaborators explore the curious case of XX Trianguli (XX Tri), a red giant star that has earned the nickname "the most spotted star" due to its extreme surface activity. Over the last four decades, this team has monitored XX Tri’s brightness with incredible detail, collecting photometric data that reveals how the star’s surface spots and brightness levels change over time. Their goal: to understand the star’s magnetic behavior and long-term activity cycles in a system where a close binary partner influences how the star evolves.
Magnetism and Rotation in RS CVn Stars
The study begins by introducing RS CVn-type binary stars, which are systems where one star, often a cool giant, shows dramatic changes in brightness due to magnetic activity on its surface. In the case of XX Tri, it’s the presence of dark starspots, similar to sunspots, that dominate the surface, causing the star’s light to dim as it rotates. Unlike the Sun, however, XX Tri rotates relatively quickly for a giant star, thanks to the influence of its companion star. This fast rotation plays a key role in keeping its magnetic activity strong. Earlier models of XX Tri already hinted at large, cool spots, but this new study compiles a much longer history of data, making it possible to study these surface features on both seasonal and decadal timescales.
Tracking Rotation Through Fourier Analysis
To dig deeper into XX Tri’s changing brightness, the researchers split the full dataset into four time intervals and analyzed the rotation-related signals. Using Fourier analysis, they identified several close-but-distinct rotation periods over time. These suggest that the surface of the star is rotating at different speeds depending on latitude, a phenomenon called differential rotation. This is similar to what happens on the Sun, but in XX Tri, the difference between the equator and poles is smaller. The dominant rotation periods were close to the star’s orbital period (about 24 days), and the changes in these periods hinted at evolving spot locations across different latitudes.
Cycles Within Cycles: Long-Term Variability
Beyond rotation, the team also uncovered long-term cycles in the star’s brightness. Using time-frequency analysis tools, they found a strong and persistent ~4-year cycle, along with weaker cycles lasting 5–6 and even 11 years. Interestingly, the 4-year cycle didn’t just appear in the brightness data, it also showed up in the star’s temperature changes and spot rearrangements. This behavior is consistent with a phenomenon called "flip-flop" cycles, where groups of spots seem to shift positions on the star's surface over time. Instead of a simple back-and-forth swap between two regions, the researchers found more complex rearrangements among two to three active longitudes.
Heating Up: Migration Across the H-R Diagram
Another important finding was that XX Tri has been gradually getting brighter and hotter over the last 40 years. This was shown by measuring changes in color indices like B–V and V–IC, which relate directly to surface temperature. These color shifts suggest that the entire star, not just its spots, is evolving. The authors interpret this long-term brightening as more than just a change in how many spots the star has. Instead, they argue that the star’s overall surface, even the parts without spots, has become hotter, possibly due to long-term effects of magnetic activity that block and then slowly release energy.
Differential Rotation and Spot Dynamics
By comparing rotation periods across seasons and modeling spot movement, the team concluded that XX Tri rotates differentially, with spot-induced signals changing as activity centers migrate across latitudes. Their analysis, confirmed by Doppler imaging and a technique called ACCORD, found a weak but measurable sun-like surface shear. The results fit with expectations for a giant star in a tidally locked binary system, more sheared than rapidly rotating dwarfs, but less so than single giants or the Sun.
A Star That Defies Simplified Models
Finally, the paper ties all this together by confirming that XX Tri’s magnetic activity is dominated by starspots, which are responsible for both the short-term rotational variability and the longer-term changes. The fact that its brightness has increased while spot coverage has decreased suggests a deeper connection between the surface magnetism and the star’s internal processes. This challenges the common assumption in light curve models that a star’s “unspotted” brightness stays constant over time.
A Laboratory for Stellar Magnetism
Through meticulous observations and careful analysis, this study paints a rich and evolving picture of XX Tri. The work not only documents how the star has changed over four decades but also provides clues about the dynamo processes, those that drive magnetic activity, not just in this star, but potentially in many other active stars across the galaxy.
Source: Kővári
