Current Science at NOAO
XX Marks the Spot
Klaus Strassmeier, University of Vienna
An image of the K0 giant star XX Triangulum
and its super starspot (the dark area in the upper hemisphere).
An image of the solar disk is shown to scale as a comparison.
December, 1999 | The largest starspot ever seen was discovered recently on the Coude
Feed telescope. Astronomer Klaus Strassmeier from the University of
Vienna used a technique similar to a CAT scan to map the surface of
the star XX Triangulum, revealing a starspot about 8 times larger than
the diameter of the Sun - 60 times larger than the largest sunspot group
every observed on the Sun.
To observe spots on the surfaces of 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
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 on one side of the star causes a change in the profile of
absorption lines in the spectrum of the star. These changes move from
the short-wavelength to the long-wavelength side of the absorption
line profile as the spot rotates across the face of the star and
disappears around the other side. The map of the star 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 full stellar rotation.
XX Triangulum is a cool, 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 were 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. The Coude Feed telescope at Kitt Peak is one
of the few facilities worldwide that offers this capability.
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