Observations of Non-Thermal Radiation from Late - Stars

We study the statistical properties of the x-ray and radio emission from the coronae of a sample of late -type stars. Our sample consists of a large, nearly volume -limited, population of K and M dwarfs. From these analyses, we draw conclusions regarding stellar dynamos, the evolution of low-mass binary systems, and the nature of the coronal heating mechanism in low mass stars. We first examine the statistical behavior of the x-ray emission from the complete Einstein survey 243 late -type stars within 25 pc, focusing on the stellar age and color dependences. Optically selected, nearly volume limited subsamples which are reasonably free of known biases are used to show that, relative to the young disk stars, old disk stars have levels of coronal emission almost an order of magnitude smaller. There is also a steady, but small, decrease of coronal activity with decreasing mass. Furthermore, though we find that stars later than M5 are strongly under -represented in our x-ray survey, we are able to show that this is a direct result of their lower activity levels and cannot be due to a bias in the sample. A subset of these stars present in the x-ray survey was selected for observations at radio wavelengths. Combined with previously reported radio observations of M dwarfs and the x-ray data, we show that these observations can provide a new characterization of the coronal activity of late-type stars. The basic result is that the quiescent x-ray and radio emissions are strongly correlated, and that both show similar dependences on the stellar population. The high brightness temperatures inferred for the quiescent radio emission argues that a non-maxwellian distribution of electrons is responsible for the radio emission.