T.R Ayres, T. Simon, R.A. Stern, S.A. Drake, B.E. Wood, and A. Brown
We have used ROSAT, EUVE, and HST to measure X-ray and ultraviolet emissions of moderate mass (~2-3 M_Sun) giants in the Hertzsprung gap (spectral types earlyF-midG) and the post-helium-flash "clump" (~G8-K0). Our motivation was to document the evolution of hot coronae (T > 1E+06 K) along the post-main-sequence trajectories traveled by such stars; 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 ~ 1E+07 K) coronal energy distributions, regardless of any X-ray deficiency. EUVE spectra of gap star 31 Com (G0III) indicate a broad coronal emission measure hump at ~1E+07.2 K, while the active clump giant beta Ceti (K0III) displays a sharp peak at ~1E+06.8 K, as seen previously in the mixed clump/gap binary Capella (alpha Aur; G8III + G0III). The gap giants upsilon Peg (F8III) and 24 UMa (G4III) have EUV emissions of intermediate temperature (~1E+07.0 K).
31 Com, psi^3 Psc (G0III), and beta Cet exhibit redshifted transition zone (TZ; ~1E+05 K) lines in HST GHRS spectra, as reported earlier in Procyon (alpha CMi; F5IV-V) and Capella G0. Such redshifts on the Sun are thought to signify flows in magnetic loops. beta Cas (F2III) -- a rare soft coronal source among the gap stars -- displays blueshifts of CIV and OIV], 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 1E+05 K "subcoronal" emission zones extend to h ~ R_star, about fifty scale heights.
In contrast to the TZ line redshifts, the upper chromospheric emissions (e.g., MgII and SiIII) of 31 Com and psi^3 Psc have blueshifted cores. Blue-asymmetric peaks in the solar MgII lines are thought to indicate dynamical heating in the chromosphere. Observations of the HI Ly-alpha feature of 31 Com taken nine 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. Quasistatic 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 ~1E+05 K cannot be explained by thermal conduction down the legs of hot quasistatic loops.
On the other hand, the possible existence of elongated (l ~ 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 lifecycle; 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 MS phase, which ultimately is disrupted near ~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."