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NOAO Newsletter - NOAO Highlights - December 1999 - Number 60


XX Marks the Spot

Caty Pilachowski

Klaus Strassmeier (Vienna) used the technique of Doppler imaging on observations obtained with the KPNO 0.9-m coudé feed telescope to discover the largest starspot ever seen. Images reconstructed from a series of spectra obtained of the K0 giant XX Triangulum (HD 12545) reveal a starspot with dimensions of 12×20 solar radii—60 times the extension of the largest sunspot group ever observed or 10,000 times its areal surface coverage. This spot is by far the largest ever observed.

Figure 1. A Doppler image of the K0 giant star HD 12545 (= XX Triangulum) and its super starspot (the dark area in the upper hemisphere). The Doppler image in this picture is shown in a spherical projection at a particular rotational phase with the stellar rotation axis in the plane of the paper. An image of the solar disk is shown to scale as a comparison.

To observe spots on the surfaces of other stars, astronomers need to "resolve" the stellar disk. This cannot be done directly with the largest telescopes even planned, but Doppler imaging can be used to obtain a map of inhomogeneities on a star's surface. The principle is similar to medical tomography, but instead of a scanner rotating around a fixed object, a rotating star is observed with a fixed telescope. A cool starspot rotating into view at the preceding limb of the star causes a blue-shifted asymmetry in each spectral line profile. This asymmetry moves into the line center at the time of meridian passage, and turns into a red-shifted asymmetry after meridian passage. The asymmetry fades away when the spot disappears at the receding limb. The higher the latitude of the spot, the shorter will be its visible path across the projected disk of the star, or the spot may even be circumpolar if the stellar rotation axis is inclined. All this information is hidden in the variation of the spectral line profiles and is reconstructed by mathematical inversion to create a true picture of the stellar surface. For a successful application, the telescope needs to "see" the entire stellar surface during at least one stellar rotation.

XX Triangulum is an active K0 giant binary star, approximately 10 times larger and twice as massive as the Sun. Its rotation period is 24 days, so that 24 consecutive (clear) nights of telescope time with an excellent high-resolution optical spectrograph are needed to obtain a good Doppler image. Because starspots vary on the same (short) time scales as Sunspots do (they are stable for about one stellar rotation), all the observations must be made on one rotation cycle. NSF's Kitt Peak National Observatory is one of the few facilities worldwide that offers this capability with the 0.9-m coudé feed telescope.

During the observations, XX Triangulum had its brightest magnitude since the discovery of its light variability in 1985 and showed the largest photometric amplitude so far (0.63 magnitudes in V). The large photometric amplitude was explained when the Doppler-imaging inversion algorithm also recovered a not-quite-as-large equatorial warm spot (350 K above the photospheric temperature) in the hemisphere opposite the dark spot. Strassmeier speculates that the warm spot harbors the same magnetic field as the cool spot but opposite polarity. Surprisingly, the fact that the star was brighter at a time of high spot activity is in agreement with the solar analogy despite the "unsolar" dimension of the gigantic spot.


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