Session 87 - Star Parameters.
Oral session, Thursday, June 13
Our analysis of high-resolution GHRS spectra of active late-type stars (e.g. AU Mic and HR 1099) shows that transition region (TR) emission lines formed near 10^5 K possess broad, non-Gaussian wings. These profiles can be modeled as the sum of two Gaussians --- a narrow component and a very broad component. GHRS spectra of relatively inactive stars such as \beta Gem and \alpha Cen A+B show weak or no detectable broad components. For the main sequence stars, the widths of the narrow components suggest subsonic nonthermal velocities, and there appears to be a correlation between these nonthermal velocities and stellar surface gravity. For the evolved stars with lower surface gravities, the nonthermal velocities suggested by the narrow component are at or just above the sound speed. Nonthermal velocities computed from the widths of the broad components are always supersonic. We find that the fractional contribution of the broad component to the total line flux is well correlated with both the C IV surface flux, a good measure of the TR heating rate, and the X-ray surface flux, a good measure of the coronal heating rate. We propose that the narrow components are diagnostics for shock wave heating, most likely due to MHD waves, and that the broad components are diagnostics for microflare heating. A solar analog for the broad components might be the explosive events'' detected by the HRTS experiment.