XX Trianguli, a bright K0-type giant star in a binary system located in the constellation of Triangulum, exhibits chaotic, non-periodic star spot behavior, according to a team of astronomers from the Leibniz-Institute for Astrophysics Potsdam, the University of Potsdam and Konkoly Observatory.
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Starspots on the surface of XX Trianguli. Image credit: HUN-REN RCAES / Zs. Kővári, MOME / Á. Radványi, AIP / K. Strassmeier.
“Among the observables from the spatially resolved solar disk are the number, size, and morphology of sunspots, their growth and decay, and their migration in latitude and longitude,” said lead author Professor Klaus Strassmeier, an astronomer at the Leibniz-Institute for Astrophysics Potsdam and the University of Potsdam, and his colleagues.
“Such spots are also seen on other stars, then referred to as star spots.”
“We employ an indirect surface imaging technique to invert the spectral line profiles into images of the stellar surface.”
“Typically, only occasional snapshots of spots on stellar surfaces are obtainable while it is well-known that spots systematically change with time and, like on the Sun, only then tell us about the interior dynamo and structure of the target in question.”
“We picked one of the most spotted stars in the sky, XX Trianguli, for such a more continuous application of Doppler imaging.”
XX Trianguli lies approximately 640 light-years away in the constellation of Triangulum.
Also known as XX Tri or HD 12545, the star has a mass just 10 % more massive than the Sun, a radius of 10 solar radii, and an effective temperature of 4630 K.
It has a rotation period of 24 days synchronized to the orbital period of the binary system.
XX Trianguli was previously found to have a gigantic star spot with physical dimensions 10,000 times the area of the largest spot group ever seen on the Sun, equivalent to 10 times the projected solar disk.
Using an indirect surface imaging technique called Doppler imaging, Professor Strassmeier and co-authors captured 99 independent images of the star.
“Dark spots on the star’s surface caused shifts in its photocenter — essentially the point that represents the ‘center of light’ of the star — by up to 24 micro arc seconds, equivalent to about 10% of the star’s visible disk radius,” they said.
“These shifts occur because dark spots reduce the brightness in certain areas of the star, causing the perceived center of light to move slightly.”
“However, unlike the Sun’s predictable activity cycle, these photocenter displacements did not follow a periodic pattern, suggesting a mostly chaotic and likely non-periodic dynamo very different to the Sun’s.”
“This phenomenon also highlights a challenge for detecting exoplanets, as these spot-induced variations in the photocenter can mimic or obscure the tiny motions caused by orbiting planets, adding an intrinsic limitation for such astrometric exoplanet catches.”
The findings appear in the journal Nature Communications.
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K.G. Strassmeier et al. 2024. Long-term Doppler imaging of the star XX Trianguli indicates chaotic non-periodic dynamo. Nat Commun 15, 9986; doi: 10.1038/s41467-024-54329-4
