Interpretation of the core-wing anomaly of Balmer line profiles of cool Ap stars

A number of cool magnetic chemically peculiar stars exhibit abnormal profiles of hydrogen Balmer lines. This anomaly, which is most clearly visible in Halpha, consists of a sharp transition between broad Stark wings and an unusually narrow Doppler core. Although the core-wing anomaly is a clear indication of an abnormal structure of the atmospheres of cool Ap stars, it has so far eluded even qualitative interpretation. In this Letter we report results of an attempt to reproduce the core-wing anomaly of Balmer lines by empirical modification of the thermal atmospheric structure. We find that it is possible to obtain a very good fit to the inner and outer wings as well as to reproduce the abrupt core-wing transition and widths of both Halpha and Hbeta by increasing the temperature by 500-1000 K at intermediate atmospheric layers (-4 less than or equal to log tau(5000) less than or equal to -1). Thus, detailed analysis of hydrogen lines provides a very useful method for revealing the atmospheric structure of cool Ap stars and should serve as a crucial test for future self-consistent model atmospheres of peculiar stars.