OBSERVING STELLAR CORONAE WITH THE GODDARD HIGH RESOLUTION SPECTROGRAPH. Ⅱ. THE RS CVn BINARY SYSTEM HR 1099

摘要

We report time series observations of the RS CVn star HR 1099 taken with the Goddard High Resolution Spectrograph onboard the Hubble Space Telescope. The data cover a wavelength range from 1342 to 1375 A and show a measurable continuum, as well as emission lines of O Ⅰ, C Ⅰ, Cl Ⅰ, Fe Ⅱ, O Ⅴ, and Fe XXI. The chromospheric and transition region features are seen only in the active K1 Ⅳ component of the binary system, while the Fe XXI (10~7 K) flux may come from both components, with the active component having the stronger flux. There is no indication of Fe Ⅻ emission, formed at 1.3 x 10~6 K. The width of the Fe XXI profile indicates that the corona of the primary is unlikely to extend to heights greater than 2.3R_★, while other indicators suggest that the average loops are really much smaller, having a length of ～3 x 10~(10) cm with an electron density on the order of 10~(10) cm~(-3). Some evidence for atmospheric turbulence is detected in all of the observed emission lines. This turbulence initially increases with height, going from less than 30 km s~(-1) in the chromosphere to as much as 150 km s~(-1) in the transition region. The turbulence then decreases in the corona, where velocities of less than 65 km s~(-1) are indicated. Theoretical fits to the O Ⅴ profile also suggest that this turbulence is anisotropically distributed, with motions directed primarily along or perpendicular to the radial direction. While admitting the possibility that the atmosphere is heated by microflare events, we examine an alternative heating process that involves the damping of MHD turbulence, which might be generated by nonlinear Alfven waves or by shocks. Simple calculations indicate that the observed turbulence is sufficient to account for the transition region and coronal heating.