X-Ray and Optical Studies of Late-Type Stellar Coronae and Chromospheres.

I examine the nature of the coronae of late-type stars through X-ray observations of RS CVn systems, rapidly rotating F, G, and K dwarfs, a small sample of G giants, and some T Tauri and other pre-main sequence stars. X -ray observations of RS CVn systems with HEAO-1 and the EINSTEIN observatory show that these close binary systems exhibit extremely bright coronal soft X-ray surface fluxes which scale linearly with the stellar angular velocity (OMEGA). This linear relation between L(,x)/L(,bol) and (OMEGA) is found to hold as well for late-type dwarfs with rotation periods in excess of (TURN)12('d), and for the G giants. Slowly rotating dwarfs have coronal X-ray surface fluxes two orders of magnitude less than predicted from the linear relation. The break in the relation occurs for the same (OMEGA) as does the gap in the bimodal G dwarf CaII K index distribution. I interpret this within the framework of a simple (alpha)(omega) dynamo model, and hypothesize that this is evidence that the convective dynamo changes modes at a crucial (OMEGA). I discuss X-ray observations of pre-main sequence stars, including T Tauri stars. The T Tauri stars exhibit a striking anticorrelation between the H(alpha) equivalent width and the existence of detectable X-ray flux, which is likely attributable to X-ray absorption by the large extended atmospheres of the most active T Tauri stars. Post T Tauri pre-main sequence stars are also found to be strong X-ray sources, as might be expected from rapidly rotating convective stars. I investigate the role of the stellar magnetic field in the corona by considering the correlated variability of the coronal X-ray luminosity and temperature and the magnetic field strength. I conclude with an investigation into the extraordinarily active chromosphere of FK Comae. Optical spectroscopy of the peculiar, variable H(alpha) emission profile leads to the development of a model involving a close binary system of large mass ratio. The unseen secondary is likely to be transferring mass to the primary on a thermal timescale; asymmetric accretion heating of the chromosphere and photosphere produces the broad H(alpha) emission and the photospheric variability.