Abstract
We have used time-series spectra of the sodium doublet at 589.0 and 589.6 nm in AB Doradus to investigate the effects of a chromospheric temperature rise on Doppler images derived from strong lines whose cores are formed in the upper photosphere and lower chromosphere. On solar-type stars, the Na D lines probe a range of heights and tend to be much more sensitive to the chromospheric temperature structure than the weaker photospheric lines that are commonly used for Doppler imaging. Contrary to suggestions that polar spots may be an artefact arising from chromospheric emission reversals, we find that the images obtained from the NaD lines show less high-latitude structure, and give more reliable light-curve predictions, than images derived previously from fits to several weaker photospheric lines.
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