The coronae of moderate-mass giants in the Hertzsprung gap and the clump

We have used the Roentgensatellit (ROSAT), the Extreme Ultraviolet Explorer (EUVE), and the Hubble Space Telescope (HST) to measure X-ray and ultraviolet emissions of moderate-mass (similar to 2-3 M-.) giants in the Hertzsprung gap (spectral types early F to mid-G) and the post-helium flash "clump" (similar to G8-K0). Our motivation was to document the evolution of hot coronae (T > 10(6) K) along the post-main-sequence trajectories traveled by such stars in order to gain insight concerning the "X-ray deficiency" of the F-G0 giants and the strong braking of stellar rotation at the red edge of the Hertzsprung gap. With few exceptions, Hertzsprung gap and clump giants observed by ROSAT PSPC show hot (T similar to 10(7) K) coronal energy distributions, regardless of any X-ray deficiency. EUVE spectra of gap star 31 Com (G0 III) indicate a broad coronal emission measure hump at similar to 10(7.2) K, while the active clump giant beta Ceti (K0 III) displays a sharp peak at similar to 10(6.8) K, as seen previously in the mixed clump/gap binary Capella (alpha Aur: G8 III + G0 III). The gap giants v Peg (F8 III) and 24 UMa (G4 III) have EUV emissions of intermediate temperature (similar to 10(7.0) K). The stars 31 Com, psi(3) Psc (G0 III), and beta Cet exhibit redshifted transition zone (TZ: similar to 10(5) K) lines in HST GHRS spectra, as reported earlier in Procyon (alpha CMi: F5 IV-V) and Capella G0. Such redshifts on the Sun are thought to signify flows in magnetic loops. beta Cas (F2 III)-a rare soft coronal source among the gap stars-displays blueshifts of C IV and O IV], although emissions at cooler and hotter temperatures are near the photospheric velocity. The remarkably broad line profiles of the fastest rotating gap giants suggest that the 10(5) K "subcoronal" emission zones extend to h similar to R-star above the photosphere, about 50 scale heights. In contrast to the TZ line redshifts, the upper chromospheric emissions (e.g., Mg II and Si III) of 31 Com and psi(3) Psc have blueshifted cores. Blue-asymmetric peaks in the solar Mg II lines are thought to indicate dynamical heating in the chromosphere. Observations of the H I Ly alpha feature of 31 Com taken 9 months apart reveal striking profile changes, reminiscent of those noted previously in the Ly alpha blue peak of the Capella G0 star. We used the far-ultraviolet diagnostics, in combination with ROSAT X-ray photometry and EUVE high-excitation line strengths, to constrain physical models of the stellar outer atmospheres. Quasi-static magnetic loops can simulate the empirical coronal emission measures of the giant stars, but the inferred pressures for sensible loop lengths conflict with direct measurements of subcoronal densities. Furthermore, the high rate of emission at similar to 10(5) K cannot be explained by thermal conduction down the legs of hot quasi-static loops. On the other hand, the possible existence of elongated (l similar to R-star) emission structures on the gap giants leads to a speculative scenario to explain the X-ray deficiency. It is based on the increased importance of the dynamical filling phase ("explosive evaporation") of the loop life cycle; conductive cooling, yielding TZ emissions at the footpoints, when the heating is interrupted; and the possibility for transitions between "hot" and "cool" energy balance solutions owing to dynamical suspension and centrifugal trapping of the cooling gas. The long loops might represent a vestigial global "magnetosphere" inherited from the main-sequence phase, which ultimately is disrupted near similar to G0 by the deepening convective envelope and growth of a more solar-like dynamo. Coronal emissions might be boosted temporarily as the X-ray deficiency is removed but soon would be quenched by wind braking previously inhibited by the magnetospheric "dead zone.".