What leads to premature upper cut-off frequencies of auroral radio emission from hot magnetic stars?

Abstract

Recently, a large number of hot magnetic stars have been discovered to produce auroral radio emission by the process of electron cyclotron maser emission (ECME). Such stars have been given the name of main-sequence radio pulse emitters (MRPs). The phenomenon characterizing MRPs is very similar to that exhibited by planets like Jupiter. However, one important aspect in which the MRPs differ from aurorae exhibited by planets is the upper cut-off frequency of the ECME spectrum. While Jupiter’s upper cut-off frequency was found to correspond to its maximum surface magnetic field strength, the same for MRPs are always found to be much smaller than the frequencies corresponding to their maximum surface magnetic field strength. In this paper, we report the wideband observations (0.4–4.0 GHz) of the MRP HD 35298 that enabled us to locate the upper cut-off frequency of its ECME spectrum. This makes HD 35298 the sixth MRP with a known constraint on the upper cut-off frequency. With this information, for the first time, we investigate into what could lead to the premature cut-off. We review the existing scenarios attempting to explain this effect, and arrive at the conclusion that none of them can satisfactorily explain all the observations. We speculate that more than one physical processes might be in play to produce the observed characteristics of ECME cut-off for hot magnetic stars. Further observations, both for discovering more hot magnetic stars producing ECME and to precisely locate the upper cut-off, will be critical to solve this problem.