2023ApJ...957...53D

We report ultra-wideband (0.4-4.0 GHz) observation of coherent radio emission via electron cyclotron maser emission (ECME) produced by the hot magnetic star HD 142990. With nearly perpendicular rotation and magnetic dipole axes, it represents an extreme case of oblique rotators. The large obliquity is predicted to cause a complex distribution of stellar wind plasma in the magnetosphere. It has been proposed that such a distribution will give rise to a nontrivial frequency dependence of ECME. Indeed we discovered strong frequency dependence of different pulse properties, such as the appearance of secondary pulses, different cutoff frequencies for pulses observed at different rotational phases, etc. But the unique feature that we observed is that while at sub-gigahertz frequencies, the star appears to produce ECME in the extraordinary mode, at gigahertz frequencies, the mode indicated by the pulse property is the ordinary mode. By considering the physical condition needed by such a scenario, we conclude that the required transition of the magnetoionic mode with frequency is unlikely to occur, and the most promising scenario is refraction caused by the complex plasma distribution surrounding the star. This suggests that the conventional way to deduce the magnetoionic mode based on ECME observed at a given frequency is not a reliable method for stars with large misalignment between their rotation and magnetic axes. We also find that ECME exhibits an upper cutoff at ≲3.3 GHz, which is much smaller than the frequency corresponding to the maximum stellar magnetic field strength.