A team of astronomers at the Leibniz Institute for Astrophysics Potsdam, Germany, has created a video that shows the evolution of stellar spots on the red giant star HD 12545.
HD 12545, also known as XX Tri, is an active K0 giant star located about 1,500 light-years away toward the constellation Triangulum. This star is approximately 10 times larger and twice as massive as the Sun.
“The star is famous for its detected super-spot with a linear extension of 12 x 20 solar radii,” said team member Dr Andreas Künstler, lead author of a paper reporting the results in the journal Astronomy & Astrophysics (arXiv.org preprint).
To observe spots on the surfaces of stars other than the Sun, astronomers need to resolve the stellar disk.
This cannot be done directly with the largest telescopes even planned, but the so-called Doppler imaging technique can be used to obtain a map of inhomogeneities on a star’s surface.
The rotation period of HD 12545 is 24 days, so that 24 consecutive nights of telescope time with an excellent high-resolution optical spectrograph are needed to obtain a good Doppler image.
“We obtained continuous high-resolution and phase-resolved spectroscopy with the 1.2-m robotic STELLA (STELLar Activity) telescope at the Teide Observatory in Tenerife, Spain, over six years, and these observations are ongoing,” Dr Künstler and co-authors explained.
“For each observing season, we obtained between 5 to 7 independent Doppler images, one per stellar rotation, making up a total of 36 maps.”
This image shows HD 12545 (center), a red giant star about 1,500 light-years away. Image credit: NASA / IPAC / 2MASS / Centre de Données astronomiques de Strasbourg / SIMBAD.
Altogether 667 usable spectra between July 2006 and April 2012 were inverted into a movie of the stellar surface, which cover altogether 86 rotational periods of HD 12545.
“Shown is the stellar surface in three different projection styles; a spherical projection like if we would see the star by eye (real view), a Mercator projection where the entire surface can be seen at once (Mercator map), and a pole-on view like if the star could be viewed down onto the one visible rotation pole,” the astronomers explained.
“The movie shows a star-spot distribution with ever changing morphology, such as spot fragmentation and spot merging, and with apparently a large range of variability timescales.”
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A. Künstler et al. 2015. Spot evolution on the red giant star ‘HD 12545.’ A starspot-decay analysis based on time-series Doppler imaging. A&A 578, A101; doi: 10.1051/0004-6361/201525687
