Radio Emission from Cool Stars

Recent developments in the study of radio emission from cool stars are reviewed. The eld of stellar radio astronomy would barely exist if active cool stars were similar to the Sun. However, many active stars appear to be fundamentally diierent from the Sun in that they possess a nonthermal corona which produces strong and steady radio emission. That is, in addition to the thermal populations at 10 6?7 K in stellar coronae which radiate X{rays, there are nonthermal populations of electrons, extending up to MeV energies, which are trapped on closed magnetic eld lines and produce strong radio emission. There are several pieces of evidence for this conclusion: the radio spectra of the steady emission from active stars tend to be very at even out to high frequencies, where thermal radio spectra would fall oo rapidly with frequency; and the brightness temperatures of the radio emission, either measured directly by Very Long Baseline Interferometry (VLBI) which can resolve the sources, or inferred from observed ux levels using reasonable source sizes, are often in excess of 10 9 K. Such temperatures can only be produced by very energetic electrons unlikely to be in a thermal distribution. Nonthermal coronae are widespread. Figure 1 shows an HR diagram for some of the stars detected as radio sources. A number of diierent classes of star are identiied by the labels. The gure only includes systems for which V and B ?V measurements were readily available; in particular, it underrepresents the number of radio sources in close binaries (RS CVns and Algols) because separate measurements are not available for both components in most cases. Open symbols denote stars which probably do not possess nonthermal coronae in the sense deened above: these include luminous O stars, thought to be nonthermal radio sources due to energetic electrons accelerated in shocks in the massive, inhomogeneous stellar winds in these systems (e.g., White 1985), and the hot primaries in close binary systems. The magnetic Bp/Ap stars, on the other hand, do seem to possess nonthermal coronae. Some of the later{type M dwarfs have not been detected as steady radio sources but have shown radio ares. Most of the stellar classes represented in Fig. 1 have been known to be strong radio sources for some years. However, radio emission of main{sequence dwarfs in classes F{K was largely unknown until the recent systematic surveys of X{ray{selected stars in these spectral classes …