This study, led by Yue Xiang, focuses on SZ Piscium (SZ Psc), a binary star system known for its magnetic activity. SZ Psc consists of two stars: a cooler, magnetically active K-type subgiant and a hotter F-type star. Starspots, similar to sunspots, are areas on a star's surface where magnetic activity is strong. By studying these spots, astronomers can learn more about the magnetic processes happening inside stars. Doppler imaging, a technique used to map a star's surface based on how its light shifts, was applied here to study the starspot activity of SZ Psc and measure its differential rotation, which refers to how different parts of the star rotate at different speeds.

Observations and Data Collection

The team collected high-resolution spectra (light data) of SZ Psc using telescopes in China over the span of 2014 to 2018. They used a technique called Least-Squares Deconvolution (LSD) to improve the quality of the data by averaging multiple spectra together. This process revealed detailed profiles of SZ Psc, highlighting distortions in the light caused by starspots. One of the key features detected was a stable, large starspot near the star's pole, which appeared consistently over the years.

Starspot Mapping with Doppler Imaging

Using Doppler imaging, the team reconstructed the surface of the K-type subgiant star, revealing widespread starspots across various latitudes. In all maps, a prominent polar starspot was found, which may be a stable feature on this star. However, starspots at lower latitudes were more dynamic, evolving over short timescales. These spots changed noticeably even within a single month, indicating that starspot patterns on SZ Psc are quite complex and rapidly shifting. This detailed surface mapping provided insights into how the star's magnetic field affects its surface.

Measuring Differential Rotation

Differential rotation is a key factor in understanding a star’s magnetic activity. It refers to the difference in rotation speed between a star’s equator and poles. The researchers measured SZ Psc’s differential rotation by comparing Doppler images taken ten days apart in late 2017. Their results showed that the equator of the K star rotates faster than the poles, similar to how the Sun rotates. This differential rotation is essential for driving the star’s magnetic dynamo, which generates magnetic fields and starspots.

The Third Star in SZ Piscium’s System

SZ Psc is part of a triple star system, meaning it has a third companion star. By analyzing the radial velocity (the speed at which stars move toward or away from us), the team confirmed the presence of this third star. They calculated that this third star orbits the binary system with a period of about 1530 days (a little over 4 years) and has a mass of around 0.75 times that of the Sun.

Conclusion

The study provided new insights into the magnetic activity and rotation of SZ Psc. The star’s surface is highly active, with starspots evolving on a short timescale. The detection of differential rotation on the K star, with the equator rotating faster than the poles, adds valuable information to the study of stellar magnetic fields. The discovery of the third star’s orbit also improves our understanding of this triple system. These findings are crucial for building a broader picture of how magnetic activity functions in stars other than the Sun.

Source: Xiang