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Spot evolution on the red giant star XX Triangulum

Solar spots can be observed in great detail and their evolution can be studied in real-time. Their decay rate is of great interest as it is directly related to the magnetic diffusivity, which itself is a key quantity for the length of a magnetic-activity cycle. Is a linear spot decay also detectable on other stars, and is this in agreement with the large range of solar and stellar activity cycle lengths?

Written by A. Künstler

Tuesday, 24 March 2015

We investigated the evolution of starspots on the K0 giant XX Triangulum by using consecutive time-series Doppler images. XX Tri is a member of the RS CVn class of magnetically active components of close binaries in a synchronized SB1-type system with a period of Prot = Porb = 24.0 days.

With our 1.2-m robotic STELLA telescope on Tenerife, continuous high-resolution and phase-resolved spectra were obtained over six years, and continuing. Between July 2006 and April 2012 we reconstructed 36 independent Doppler images with our line-profile inversion code iMap.

This unique time-series of Doppler images shows that the surface of XX Tri is covered with large high-latitude and even polar spots and with occasional small equatorial spots. Just over the course of six years, we see a systematically changing spot distribution and morphology such as spot fragmentation and spot merging as well as spot decay and formation. To quantify starspot area decay and growth, we match the observed images with simplified spot models based on a Monte-Carlo approach. Furthermore, we looked for active longitudes and differential rotation.

We infer an average linear decay of D = -0.022 ± 0.002 SH/day. From the decay rate, we determine a turbulent diffusivity of ηT = (6.3 ± 0.5) x 1014 cm2/s and predict a magnetic-activity cycle of approx. 26 ± 6 years. This value fits very well into the range of so far estimated solar and stellar activity cycle lengths and agrees with results from decades-long photometric monitoring.

The most spotted longitude appear on average in phase towards the unseen companion star. Furthermore, we detect a weak solar-like differential rotation with a surface shear of α = 0.016 ± 0.003. The surface shear on XX Tri is therefore only around a tenth of the solar value.

All 36 Doppler images are merged into an animated gif-file (xxtri-di-anim.gif) and may be used for general demonstration purposes. Each image is shown in four spherical projections separated by 90 degrees. The rotational shift between consecutive images is corrected, i.e. the stellar orientation remains the same from map to map and from season to season. The time-difference between each Doppler image is indicated in units of rotational phase phi. The movie shows all 36 Doppler maps as a function of time, including a time-delay of 250 ms between each image.

Link to the published article: Künstler et al. 2015, A&A, 578, 101