Recent observations of AB Dor and interpretation

We use minimal empirical modelling techniques to interpret recent (2006-2007) photometry and spectroscopy of AB Dor. We compare, in particular, broadband (B and V) maculation effects with emission features in high-resolution Ca II K-line spectroscopy. We also compare emission effects in the Ca II K and H alpha lines observed at different rotational phases. We refer to a broader multiwavelength campaign, of which these optical data were a part, involving X-ray and microwave observations to be published later. The broadband light curves are characterized by one outstanding macula, whereas the emission lines suggest 4 possible main chromospheric activity sites ('faculae'). These appear at a similar latitude and with comparable size to the main umbra, but there are significant displacements in longitude. However, one strong facular concentration near phase zero may have a physical relationship to the main macula. The derived longitudes of these features would have been affected by differential rotation operating over the several months between the spectroscopic and photometric observations, but the difference of at least similar to 30 degrees between facula and umbra appears too great to allow their coincidence. The possibility of a large bipolar surface structure is considered, keeping in mind the bipolar character of solar activity centres: the activity of rapidly rotating cool stars being generally compared with that of the Sun, scaled up by a few orders of magnitude. Observed microwave activity may link to this same main photospheric and chromospheric centre picked up by the optical analysis. Characterization of macular and facular contributions in stellar activity sites would be improved with a closer timing of observations and higher signal to noise ratios in emission line data. (C) 2009 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim