The Solar–Stellar Connection

The discovery that radio emission from stars is quite common and readily detectable was one of the unexpected advances produced by the Very Large Array [35,11]. This discovery could not be predicted based on what we know of the Sun’s radio emission, even though similar physical mechanisms are operating in stellar radio sources and so knowledge of the Sun’s emission is essential for understanding stellar radio emission: this is the solar–stellar connection in radio astronomy. Most of the obvious classes of star have been surveyed, and many of them have been detected, including the expected thermal wind sources amongst both cool and hot mass–losing giant stars and symbiotic binaries, but also solar–like activity from magnetically active stars in the giant, subgiant, main–sequence and (more recently) brown dwarf classes, as well as in chemically peculiar B stars and pre–main–sequence stars. However, the total number of stars detected is still relatively small, and selection effects mean that in all cases we detect only the most luminous tail of the most nearby members of the populations of these objects, representing a strongly biassed sample. For no populations of stars do we have a complete radio sample, and particularly a completely detected sample, that can be used to study the relationship of radio emission, and by extension atmospheric structure, with other stellar properties such as evolutionary state. Radio emission from stars comes from the atmosphere, and hence can be used to study the nature of the atmosphere. Two types of atmospheric emission are most common in the sources detected so far: emission from an extended outflowing envelope or wind, or emission from confined parts of the atmosphere such as the corona (where the temperature exceeds 10 K) or chromosphere (the lower transition region between the stellar photosphere and the hot corona, with temperatures in the range 5–20 × 10 K). Stellar outflows and chromospheres are generally detected via the mechanism known variously as thermal free–free or bremsstrahlung emission. This is responsible for the classic stellar–wind radio emission. The opacity for this mechanism varies as ne T −1.5 ν, where ne is the electron density, T the electron temperature and ν the radio frequency. Since the temperature of a stellar wind or a chromosphere is generally around 10 K, large radio fluxes require very large optically thick surface areas: a 1 milliarcsecond disk, which is the order of magnitude for the photosphere of nearby giant stars, with a brightness temperature of 10 K produces of order 1 microJy of radio flux at 10 GHz. For the currently detected objects, the source size is much larger than 1 milliarcsecond due to high densities or very low outflow speeds for the stellar winds, or very distended atmospheres in the case of supergiants such as Betelgeuse [20,16]. Nonthermal synchrotron emission is the basic microwave emission process operating in solar flares and in stars which show solar–like magnetic activity. This mechanism requires nonthermal distributions of mildly (“gyrosynchrotron”) or highly (“synchrotron”) relativistic