Doppler imaging : the polar spot controversy

Doppler imaging studies have revealed that most rapidly rotating cool stars have high-latitude spots, which in many cases cover the stellar poles. The spectroscopic signature of polar spots is a filling in of the cores of spectral lines, which become flat-bottomed and may show bumps. Although the existence of polar spots is corroborated by spectroscopic and photometric measurements, and although theoretical models predict polar spots, they remain controversial. Most notably, it has been proposed that the line core filling in might also be caused by chromospheric activity. We present a NLTE radiative transfer analysis of 14 of the most-used Doppler-imaging lines which demonstrates that chromospheric activity can produce filling in of the observed line profiles only in a few of these lines. Moreover, such filling in is in general not of the type observed in the spectra of active stars. We are able to produce a flat-bottomed line core by concentrating the chromospheric activity near the poles, but only for two of the strongest lines, Fe i 5497 Å and Fe i 6430 Å. In the observations, however, also the weaker lines have flat-bottomed cores. Therefore, it is unlikely that polar spots are an artifact due to misinterpretation of the spectral signature of chromospheric activity. Nevertheless, we cannot exclude that chromospheric activity provides part of the filling in of the cores of some stronger lines; we present a diagnostic that may help to separate the contributions of chromospheric activity and spots.