“Astronomy & Astrophysics publishes the results of an international team of scientists with leading Bulgarian participation

The Institute of Astronomy with the NAO-BAS has identified the study of magnetic activity in the red giant star RZ from the constellation Aries (RZ Ari) due to the engulfment of a planet as its most significant scientific achievement for 2024. The results obtained by the team with Bulgarian lead researcher Prof. Dr. Renada Konstantinova-Antova and four other Bulgarian scientists were published in the prestigious European journal “Astronomy & Astrophysics” 681, A36, 2024. In addition to the five Bulgarians, the team includes researchers from France, USA, Brazil and Chile.

By investigating the causes of RZ Ari’s magnetic activity, scientists found that this star evolved to the tip of the red giant branch, or even further – beyond the helium burning phase. The evolutionary stage at which the object is located does not suggest that the activity was driven by a solar-type dynamo (alpha-omega). Analysis of the stellar giant’s parameters – rotation, chemical abundances, magnetic field strength, etc., suggest that another type of dynamo may be at work. The rotation of the star is accelerated, and the content of the chemical element lithium is atypically high.

These parameters, together with the fact that the star on the Hertzsprung-Russell diagram is far from the so-called “second magnetic strip”, where the previously known pro-evolved red giants with a magnetic field concentrate, indicate that the likely cause of the magnetic dynamo action in RZ Ari is planetary engulfment. This is the first time that the magnetic field at the surface of a star at such an advanced evolutionary stage has been mapped, showing the presence of poloidal and toroidal components, in agreement with dynamo theory.

There are also indications of the presence of large convective structures on the surface of the giant which may suggest the action of a local dynamo, similar to the supergiant Betelgeuse. However, analysis of the spectropolarimetric and photometric data shows that even if there is such an effect, its contribution to the magnetic field and activity at RZ Ari is not major.

The study was partially funded by the Bulgarian National Science Fund under DN 18-2 project.