5.6 Latitudes and butterfly diagrams
Spot latitudes can be directly recovered from Doppler images. Interesting and still controversial features
in stellar Doppler images are large polar spots, cool active regions over the stellar poles. Out of 65 stars,
whose surfaces were mapped with the Doppler imaging technique, 36 showed prominent polar
spots, independently on the rotational rate or effective temperature (Strassmeier, 2002). Earlier
studies, however, suggested that latitudinal distribution of spot activity may depend on the
rotational period, since faster rotators apparently show a tendency for polar spots, while slower
rotators more often have high-latitude spots not covering the pole (Hatzes, 1998). Such a feature
appears to be a convenient compensation for a uniformly distributed spot area which cannot
be recovered from rotational modulation of spectral lines and of light curves. On the other
hand, flat-bottomed spectral line profiles in rapid rotators may indicate a lack of the continuum
flux at the polar region, and this kind of profiles appears to be common for stars with a lower
inclination of the rotational axis, implying changes of spot visibility near the poles (Hatzes
et al., 1996).
The reliability of polar spots has been thoroughly discussed (Strassmeier et al., 1991; Schüssler and
Solanki, 1992
; Piskunov and Wehlau, 1994; Strassmeier and Rice, 1998). For instance, the chromospheric
activity could reduce absorption in cores of strong spectral lines which can be interpreted as presence of cool
polar caps. To investigate such a bias in Doppler imaging Unruh and Collier Cameron (1997) used Na D
lines which are sensitive to the chromospheric temperature structure and found that the images obtained
from the Na D lines show less high-latitude structure and give more reliable light-curve predictions than
images derived previously from fits to several weaker photospheric lines. Also, Bruls et al. (1998) studied
effects of the chromospheric temperature increase on spectral line profiles, including non-LTE effects. They
concluded that it is unlikely that polar spots are an artefact due to unaccounted chromospheric activity.
It appears that Doppler imaging using simultaneous inversions of atomic and molecular lines
can limit the amount of non-modulating spot area and resolve the controversy of polar spots
(Berdyugina, 2002).
In very active stars spots are distributed over a large range of latitudes, including low- and high-latitude
spots, as shown by Doppler imaging and eclipse mapping models (Donati et al., 1999; Strassmeier
et al., 1999; Berdyugina et al., 1998a, 1999a; Rodonò et al., 1995). A long-term Doppler imaging
monitoring of the active component of HR 1099 suggests that starspots form at low or intermediate
latitudes and then slowly migrate towards the pole on time scales of a few years (Vogt et al., 1999). Further
evidence of poleward spot migration, based on more densely-sampled time-series imaging, was provided by
Strassmeier and Bartus (2000).
The time span of Doppler images for most studied stars is not long enough to see the latitude changes
over the spot cycle, i.e., to recover stellar butterfly diagrams. Light curve modelling offers a longer time
scale but spot latitudes obtained from the model are usually not unique and, thus, are less
reliable.
A new approach for recovering stellar butterfly diagrams was suggested by Berdyugina (2005). As was
emphasised in Section 5.4, phase migration of the active longitudes bears the information on both
differential rotation and mean spot latitudes. Thus, knowing the surface differential rotation and phase
migration of active longitudes one can recover mean spot latitudes during the course of sunspot-like cycles.
For young dwarfs the butterfly diagrams are reminiscent of the solar case, although the limited amount of
collected results does not yet allow for any conclusions.
