In order to observe spots on the surfaces of other stars, astronomers need to "resolve" the stellar disk. This can not be done with even the largest (existing and planned) telescopes but Strassmeier and collaborators apply an indirect imaging technique called Doppler imaging whose principle is very similar to medical brain tomography. Instead of a scanner rotating around a fixed human brain, one uses a rotating star observed with a fixed telescope. When a cool starspot rotates into view at the preceding limb of the star it will cause a blue-shifted asymmetry in each spectral line profile. This asymmetry will move into the line center at the time of its meridian passage, turn into a red-shifted asymmetry after meridian passage, and 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 be even circumpolar if the stellar rotation axis is inclined. All this information is hidden in the variation of the spectral line profiles and is reconstructed with a complex mathematical inversion procedure to create a true picture of the stellar surface.
For a succesfull application, the telescope needs to "see" the entire stellar surface during at least one stellar rotation. For the Sun, the rotation period is approximately 25 days. For XX Triangulum, it is 24 days, meaning that astronomers need 24 consecutive (clear) nights of telescope time at a telescope with an excellent high-resolution optical spectrograph. One can not come back later and do, say, the other half of the star. That is because starspots vary on the same (short) time scales as Sunspots do (they are stable for about one stellar rotation). At the moment, NSF's Kitt Peak National Observatory is the only institution worldwide that offers this capability with the 0.9-m coude feed telescope.
Klaus Strassmeier said, "The feed is our true workhorse and the long-term stability of the coude spectrograph is just outstanding. I remember that there were rumors that it could be closed because there are now 8-m and 10-m telescopes. No other telescope schedule in the world allows such observations. Thanks to the KPNO staff this 0.9-m 'baby' telescope is producing lots of matured science."
Still, the KPNO observations involved a little bit of luck. 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. This was surprising because a single cool spot could not have produced the observed huge photometric amplitude; there is simply not enough room for a completely dark spot on the visible surface. The puzzle was solved when the Doppler-imaging inversion algoritm recovered a second, but not quite as large but warm, spot in the opposite hemisphere of XX Triangulum. Surprisingly, the fact that the star was brighter at a time of high spot activity is in agreement with solar analogy despite that the gigantic spot dimensions are very "unsolar".
Author: K. G. Strassmeier
Institut für Astronomie, Universität Wien,
Türkenschanzstraße 17, A-1180 Wien, Austria;
Sponsored by the Austrian Science Foundation FWF S-7301AST and S-7302AST
Accepted by A&A; May 5, 1999