Starspot activity of the red giant XX Trianguli indicates non-periodic, chaotic dynamo

In a study published in Nature Communications, researchers from the Leibniz-Institut for Astrophysics Potsdam (AIP) and the HUN-REN Research Centre for Astronomy and Earth Sciences (HUN-REN CSFK) have reconstructed the temporal changes in the distribution of surface spots on the red giant star XX Trianguli. The research is based on a unique series of spectroscopic observations carried out by the STELLA robotic observatory over 16 years.

“Sunspots are the most well-known manifestations of solar magnetic activity, which, together with many other phenomena, such as solar flares or the solar cycle, can be linked to the dynamo mechanism operating inside the sun,” explains Dr. Zsolt Kővári, scientific advisor at the Konkoly Observatory of the HUN-REN CSFK, and a member of the research team.

“Starspots are phenomena similar to sunspots, but on the surface of distant stars. However, usually we cannot observe the surface of stars directly. Therefore, we applied an indirect tomographic technique called Doppler imaging to the data of XX Tri,”

“The large-amplitude variations in the brightness of the red giant XX Tri have been observed before, so it was also known that the variations were caused by dark spots, coming in and out of view as the star rotates around its axis in 24 days. These spots are even larger than the entire surface of our sun—which is why XX Tri has been dubbed ‘the most spotted star in the sky,'” adds Dr. Kővári.

The researchers used more than 2,000 high-resolution spectra collected over 16 years with AIP’s STELLA robotic telescopes in Tenerife, an unprecedented amount of homogeneous spectroscopic data from a star. STELLA robotic observatory—whose principal investigator is Prof. Klaus G. Strassmeier, lead author of the study—is AIP’s “homemade observatory.” The twin telescopes were designed, built, and operated remotely in Potsdam. From the data, 99 time-series Doppler images were reconstructed, showing the spot evolution on the stellar surface between 2006 and 2022.

One of the main findings of the study is that the surface changes of XX Tri do not show sun-like magnetic cycles, based on which the authors conclude that the star’s dynamo is non-periodic, most likely chaotic in nature.

Data visualization experts at Moholy-Nagy University of Art and Design Budapest (MOME) created a spectacular movie from the Doppler images. Moreover, seven additional videos were produced as supplementary material to the publication thanks to the cooperation with MOME.

“The solar dynamo shows a cyclic behavior in the number and distribution of sunspots that repeats every 11 years on average, known as the butterfly diagram. However, XX Tri is a red giant star about ten times larger than the sun and its internal structure differs significantly from the sun’s, which is a main sequence dwarf. We found no evidence of similar cycles for XX Tri,” says Dr. Kővári.

“By studying stellar dynamos, we can also get closer to understanding the solar dynamo. Moreover, dynamo operation is not only responsible for the magnetic features on the sun or other spotted stars, but also the Earth’s magnetic field is created by a similar mechanism, which—on the other hand—happens to protect our technical civilization from harmful solar storms.

“Our goal is to be able to observe the manifestations of the dynamo mechanism in as many stars with different characteristics as possible, since by knowing the similarities and differences, we will ultimately better understand the processes that shape our own environment.”

This study is the first to demonstrate how the huge starspots cause a tiny displacement of XX Tri in the sky, which appears virtually as a point source when observed from Earth. The reason for this is that while the photocenter of a homogeneous (unspotted) stellar disk is the same as the geometric center of the star, huge starspots on the stellar disk shift the photocenter in the opposite direction to the spots.

In the case of XX Tri, which is 630 light-years away, the photocenter of the stellar disk can shift by up to 10% of the star’s radius relative to the geometric center, causing an apparent displacement of 24 micro-arcseconds in the celestial position of the star (the diameter of a hair at a distance of 1000 km).

However, with the help of the Gaia astrometric space observatory, even such small displacements can now be measured. This is similar to the expected astrometric displacement caused by a Saturn-mass planet in a one-year orbit around a sun-like star at about 300 lightyears in distance. Therefore, separating the effects of spots and exoplanets can be very challenging, if not impossible, especially in cases of similar periodicities, the researchers conclude.

Observations of XX Tri with STELLA are continuing, so according to Dr. Kővári, the new data may reveal further details about the star’s dynamo operation.