The Sun's spots exhibit a systematic cyclical pattern driven by its internal magnetic dynamo. However, capturing similar behavior on other stars has been a rare opportunity due to observational limitations. Leveraging the STELLA observatory, astronomers observed XX Trianguli, one of the most "spotted" stars in the sky, nearly every clear night since 2006. This effort produced a long-term dataset comparable to those available for solar studies.

Researchers applied Doppler imaging, an indirect surface reconstruction technique, to analyze the star's surface over time. The project resulted in 99 independent surface images of XX Trianguli, enabling the reconstruction of a 16-year-long movie. Unlike the Sun's predictable dynamo cycles, XX Trianguli's star spots exhibited chaotic, nonperiodic activity, suggesting a distinctly different dynamo mechanism.

"STELLA is our home-made observatory: designed, constructed, built, and operated remotely from Potsdam," explained Professor Klaus G. Strassmeier, lead author of the study and Principal Investigator of STELLA. The observatory's high-resolution Echelle Spectrograph (SES) captured spectral line profiles across multiple phases of the star's 24-day rotational cycle. These profiles were then mathematically inverted to generate detailed two-dimensional surface maps.

The analysis revealed dark spots on XX Trianguli's surface that caused its photocenter - the perceived "center of light" - to shift by up to 24 microarcseconds, about 10% of the star's visible radius. This erratic movement contrasts with the Sun's predictable activity cycles and highlights a challenge for detecting exoplanets. Such spot-induced photocenter variations can mimic or obscure the subtle astrometric signals caused by orbiting planets, complicating exoplanet detection.

XX Trianguli (HD 12545) is a giant star approximately 10% more massive than the Sun, with a radius 10 times larger and an effective temperature of 4,630 Kelvin. Its rotation period of 24 days is synchronized with the orbital period of its binary companion. The star's gigantic star spot, covering an area 10,000 times that of the Sun's largest spot group, underscores the complexity of its magnetic activity.

Prof. Strassmeier emphasized the implications of these findings: "XX Tri's maximum photocenter shift with the stellar rotation period is certainly a large value for spotted stars but comparable to the expected astrometric displacement for a star with a Saturn-mass planet in a one-year orbit around it at about 300 lightyears distance." The study reveals that these spot-induced shifts are significantly larger than those expected from short-period exoplanets, complicating the interpretation of astrometric data, such as that from ESA's Gaia mission.

The research marks the first direct measurement of a stellar photocenter influenced by star spots. The quantified offsets for XX Tri highlight the challenges in distinguishing between the effects of rotationally modulated spots and exoplanetary signals, particularly when their periodicities overlap.

Research Report:Long-term Doppler imaging of the star XX Trianguli indicates chaotic non-periodic dynamo

Related Links
Leibniz Institute for Astrophysics Potsdam
Stellar Chemistry, The Universe And All Within It