THE DISTANT FUTURE OF SOLAR-ACTIVITY - A CASE-STUDY OF BETA-HYDRI .3. TRANSITION REGION, CORONA, AND STELLAR WIND

The secular decay of solar-type activity with age is studied through a detailed comparison of the present (G2 V) with the very old (9-10 Gyr) solar-type star beta Hyi (G2 IV), taken as a proxy of the future Sun. Analyses of successive atmospheric layers are made, and this Paper III in the series treats the outermost parts. The far-UV emission lines from the transition zone are among the faintest so far seen in any solar-type star. The significance of the deduced fluxes for the weak and marginally detected lines is tested through two independent reductions of IUE data. The coronal soft X-ray spectrum was measured through different filters on EXOSAT and compared to simulated X-ray observations of the Sun seen as a star. The flux from beta Hyi is weaker than that from the solar corona and has a different spectrum. The complex filter transmission functions preclude a unique determination of the coronal temperature: an emission measure analysis based on an isothermal plasma emissivity model yields either congruent-to 5 x 10(5) or 4 x 10(6) K. A corresponding analysis for the Sun ''observed'' by EXOSAT gives 3 x 10(6) K. The ''cool'' solution for beta Hyi might be physically stable due to a local minimum in the coronal radiative loss function. It is conjectured that this could be an intermediate evolutionary step to losing an observable corona altogether, as is the case in more evolved cool giants. If the ''cool'' coronal solution is appropriate, it could imply that a thermally driven stellar wind can no longer be supported, removing the mechanism for further rotational braking of the star through a magnetic stellar wind.