EVALUATING POSSIBLE HEATING MECHANISMS USING THE TRANSITION REGION LINE-PROFILES OF LATE-TYPE STARS

Our analysis of high-resolution Goddard High-Resolution Spectrograph ( GHRS) spectra of late-type stars shows that the Si IV and C IV lines f ormed near 10(5) K can be decomposed into the sum of two Gaussians, a broad component and a narrow component. We find that the flux contribu tion of the broad components is correlated with both the C IV and X-ra y surface fluxes. For main-sequence stars, the widths of the narrow co mponents suggest subsonic nonthermal velocities, and there appears to be a tight correlation between these nonthermal velocities and stellar surface gravity (xi(NC) proportional to g(-0.68 +/- 0.07)). For evolv ed stars with lower surface gravities, the nonthermal velocities sugge sted by the narrow components are at or just above the sound speed. No nthermal velocities computed from the widths of the broad components a re always highly supersonic. We propose that the broad components are diagnostics for microflare heating. Turbulent dissipation and Alfven w aves are both viable candidates for the narrow component heating mecha nism. A solar analog for the broad components might be the ''explosive events'' detected by the High-Resolution Telescope and Spectrograph ( HRTS) experiment. The broad component we observe for the Si IV lambda 1394 line of alpha Cen A, a star that is nearly identical to the Sun, has a FWHM of 109 +/- 10 km s(-1) and is blueshifted by 9 +/- 3 km s(- 1) relative to the narrow component. Both of these properties are cons istent with the properties of the solar explosive events. However, the alpha Cen A broad component accounts for 25% +/- 4% of the total Si I V line flux, while solar explosive events are currently thought to acc ount for no more than 5% of the Sun's total transition region emission . This discrepancy must be resolved before the connection between broa d components and explosive events can be positively established. In ad dition to our analysis of the Si IV and C IV lines of many stars, we a lso provide a more thorough analysis of all of the available GHRS data for alpha Cen A (G2 V) and alpha Cen B (K1 V). We find that the trans ition region lines of both stars have redshifts almost identical to th ose observed on the Sun: showing an increase with line formation tempe rature up to about log T = 5.2 and then a rapid decrease. Using the O IV] lines as density diagnostics, we compute electron densities of log n(e) = 9.65 +/- 0.20 and log n(e) = 9.50 +/- 0.30 for alpha Cen A and alpha Cen B, respectively.