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., 1991Schüssler and Solanki, 1992Jump To The Next Citation PointPiskunov and Wehlau, 1994Strassmeier and Rice, 1998Jump To The Next Citation Point). 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., 1999Strassmeier et al., 1999Berdyugina et al., 1998aJump To The Next Citation Point1999aJump To The Next Citation PointRodonò et al., 1995Jump To The Next Citation Point). 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.

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