Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993hst..prop.4492a&link_type=abstract
HST Proposal ID #4492
Computer Science
Hst Proposal Id #4492
Scientific paper
Post-main-sequence evolution of the high-excitation chromospheric and coronal activity of cool stars may profitably be studied through detailed comparisons of the current Sun with old sun-like stars of known age, whose parameters can be interpreted in terms of perturbed solar models. Magnetically-inspired activity subsides with age, and in very old solar-mass K-giants might attain the `basal' limit set by residual acoustic heating. In the solar neighborhood, the nearest normal single sun-like star, with an advanced age (~9 Gyr) known from evolutionary tracks, is Beta Hydri (G2 IV). The closest subgiant, it presents a tempting target for detailed scrutiny across the electromagnetic spectrum. We, and our collaborators, have carried out such a study based on optical spectroscopy of traditional chromospheric tracers, and satellite (IUE & EXOSAT) measurements of high-excitation species that form in the subcoronal ``transition zone'', and in the corona itself. The historical data reveal -- through evidence for a `cool' corona (500,000 K) and a low-excitation wind akin to those of the red giants -- that Beta Hyi is a natural link between the ostensibly disparate properties of activity on the MS and in the giant branch. Nevertheless, a number of puzzles remain concerning the future fate of solar activity. The answers lie beyond the limits of the historical missions, but are easily addressed by the HST/GHRS, through high-S/N spectroscopy in its medium resolution and echelle modes.
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