Coronal Structures in Cool Stars

Surprisingly, cool stars have coronae that differ substantially from the solar prototype. A continuous distribution of temperature can extend to temperatures of tens of millions of degrees. Such high values have previously been observed only in solar flares. Moreover, a totally new coronal feature appears in some stars, marked by a narrow enhancement in the emission measure distribution near ~10⁷K (the ``bump''). This bump is a small scale, and stable feature of coronae that appears to be located at high latitudes. Its presence demands high magnetic fields and a continuous input of energy. Relatively high densities (10¹²--10¹³ cm^-3) are found near temperatures of 10⁷K. Coronal abundances appear to deviate from the simple First Ionization Potential (FIP) dependence that occurs in regions of the solar corona. Analyses of far ultraviolet, extreme ultraviolet, and X-ray spectra of coronae as obtained with Chandra, XMM-Newton, FUSE, and EUVE will be presented that place hard constraints on their energy requirements, magnetic field strengths, coronal presence, and evolution. Simple theoretical models are not adequate. A review of the current understanding of coronal structure on the cool side of the HR diagram will identify problems to be addressed in the future. This research is supported in part by NASA Grant NAG5-11093.