Radio detection of chemically peculiar stars with LOFAR

Context. Chemically peculiar stars are upper main sequence stars that show anomalies in their optical spectra. These anomalies suggest peculiar chemical abundances of certain elements. Some chemically peculiar stars possess strong magnetic fields. Electrons originating from the ionising stellar wind travel in the magnetosphere of the star and become the source of non-thermal radio and X-ray emission. Several chemically peculiar radio stars have been detected at GHz frequencies. Aims. We used the Low-Frequency Array (LOFAR) to search for radio emission from chemically peculiar stars to constrain their emission in the frequency band 120-168 MHz. We aimed to use LOFAR observations to test the models for radio emission of chemically peculiar stars. Methods. We performed a targeted search of known chemically peculiar stars in the fields of the LOFAR Two Metre Survey (LoTSS) Data Release 2 in Stokes I and V. We matched positions of radio sources in the LoTSS-DR2 catalogue with positions of chemically peculiar stars. Results. We report non-thermal emission at 120-168 MHz from two chemically peculiar stars in Stokes I, BP Boo, and alpha(2) CVn. The ensuing incidence rate at these frequencies is significantly lower than for higher frequencies. This results from the turnover at low frequencies which was predicted from the theory of radio emission from chemically peculiar stars. BP Boo is detected for the first time at radio wavelengths, while alpha(2) CVn had already been detected at higher frequencies. The upper limit of V/I indicates a level of circular polarisation significantly below 60%. We combined data obtained at different frequencies to derive the radio spectrum of alpha(2) CVn. The spectrum is nearly flat beyond turnover at low frequencies. We modelled radio emission for a large magnetosphere and small local magnetic field strength. The amplitude of variation in radio emission with the rotational phase of the system decreases at low frequencies.