High-Dispersion Observations of Alpha Bootis (K1 III) with the International Ultraviolet Explorer

Abstract

The authors have obtained very deeply exposed IUE echelle spectrograms of the bright, early-K giant Arcturus. They did not detect significant flux in the most prominent high-excitation species of a solar-like transition zone, C IV λ1548.2. The presence of a weak feature of Si III] λ1892.0, and possibly also Si IV λ1393.8, indicates the existence of a small amount of plasma at temperatures as hot as 6×10⁴K. Measurements of C II] multiplet UV0.01 near 2325 Å provide a lower limit of about 5×10⁹cm^-3 for the electron density in the chromospheric layers. C II UV1 (1335 Å) emission is very weak. These results confirm that the "coronal" activity of the old red giants is considerably diminished from that of even the "quietest" of their main-sequence predecessors, stars like the Sun.