RADIO-EMISSION FROM CHEMICALLY PECULIAR STARS

We have extended the initial survey of radio emission from magnetic Bp-Ap stars by Drake et al. in five subsequent VLA2 observing runs. A total of 16 sources have been detected at 6 cm out of 61 observed, giving a detection rate of 26%. Of these stars, three are also detected at 2 cm, four at 3.6 cm, and five at 20 cm. In addition to the three He-strong and two He-weak/Si-strong stars previously reported as radio sources, we have detected 11 new stars as radio sources with spectral types B5-A0 that are He-weak and Si-strong. We have not yet detected any of the classical (SrCrEu-type) Ap stars despite many attempts. We find a wide range of high 6 cm radio luminosities for the detected stars, with log L6 = 14.7-17.9. The early-B He-strong stars are on average 20 times more radio luminous than the late-B He-weak stars and 1000 times more luminous than theta-Aurigae, the star with both the lowest radio luminosity and the lowest effective temperature. Multifrequency observations indicate flat spectra in all cases. Four stars have a detectable degree of circular polarization at one or more frequencies. We believe that the radio-emitting CP (chemically peculiar) stars form a distinct class of radio stars that differs from both the hot star wind sources and the active late-type stars. For the detected CP stars we find that L6 is-proportional-to M0.5 B(rms), where M is the mass-loss rate and B(rms) is the rms value of the observed longitudinal field strength. We show that all of the observed properties of radio emission from these stars may be understood in terms of optically thick gyrosynchrotron emission from a nonthermal distribution of electrons. We propose a model in which the electrons are produced in current sheets forming 10-20 radii from the star. In this model the electrons travel along magnetic fields to smaller radii and higher magnetic latitudes where they mirror and radiate microwave radiation.