High-resolution stellar vidicon spectrophotometry. I - Variable mass loss from Arcturus and the hypothesis of giant convective elements

High-resolution spectrophotometry of the variable Ca II K line in the K2 IIIp star Alpha Boo was performed with the McMath Solar Telescope at Kitt Peak National Observatory and an experimental SEC vidicon camera. The results are compared with Copernicus observations of the Mg II h and k lines and with earlier Ca II data. It is found that either of two states may typically occur in the Arcturus chromosphere. From comparison with the results of model calculations for expanding chromospheres, it is concluded that these correspond respectively to a 'normal' state in which the mass loss is less than one billionth of a solar mass per year and an 'abnormal' state in which the mass loss is about 8 billionths of a solar mass per year. In the latter case, the expansion velocity is around 13 km/s at optical depth unity in the K-line, which exceeds the local sound speed. It is suggested that the abnormal state represents the rise to the photosphere of a very large convective element, as hypothesized for red giants by Schwarzschild (1975).