Evaluating Possible Heating Mechanisms Using the Transition Region Line Profiles of Late‐Type Stars

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Abstract
Our analysis of high-resolution Goddard High-Resolution Spectrograph (GHRS) spectra of late-type stars shows that the Si IV and C IV lines formed near 105 K can be decomposed into the sum of two Gaussians, a broad component and a narrow component. We find that the flux contribution of the broad components is correlated with both the C IV and X-ray surface fluxes. For main-sequence stars, the widths of the narrow components suggest subsonic nonthermal velocities, and there appears to be a tight correlation between these nonthermal velocities and stellar surface gravity (ξNC ∝ g-0.68±0.07). For evolved stars with lower surface gravities, the nonthermal velocities suggested by the narrow components are at or just above the sound speed. Nonthermal velocities computed from the widths of the broad components are always highly supersonic. We propose that the broad components are diagnostics for microflare heating. Turbulent dissipation and Alfvén waves are both viable candidates for the narrow component heating mechanism. 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 λ1394 line of α 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 consistent with the properties of the solar explosive events. However, the α Cen A broad component accounts for 25% ± 4% of the total Si IV line flux, while solar explosive events are currently thought to account for no more than 5% of the Sun's total transition region emission. This discrepancy must be resolved before the connection between broad components and explosive events can be positively established. In addition to our analysis of the Si IV and C IV lines of many stars, we also provide a more thorough analysis of all of the available GHRS data for α Cen A (G2 V) and α Cen B (K1 V). We find that the transition region lines of both stars have redshifts almost identical to those observed on the Sun: showing an increase with line formation temperature 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 ne = 9.65 ± 0.20 and log ne = 9.50 ± 0.30 for α Cen A and α Cen B, respectively.