The ejection of shells in the stellar wind of the hypergiant P Cygni (B1Ia+).

Abstract

Radial velocity variations of six Fe II and six Fe III lines in the UV spectrum of the hypergiant star P Cygni (HD 193237) are studied in 33 high resolution IUE spectra, obtained over an interval of more than 5 yr. The Fe II lines have a stable component at -206±2 km s^-1 which did not vary in velocity nor in strength in 5 yr. There are two possible explanations for this component. Either it is due to material in the quiescent wind, moving at the terminal wind velocity at a large distance (r ⪆ 10³R_*). Alternatively, the stable component could be due to a shell which was ejected in 1969 or earlier and has a mass of at least 6×10^-5M_sun;. The authors consider the first possibility the most probable one. In either case, the lack of strong Fe III lines in the quiescent wind indicates that the wind is ionized to at least Fe IV up to large distances from the star (r ⪆ 10³R_*) In addition to the stable Fe II components the authors found variable shell components in Fe II and Fe III. These variable components show an increasing radial velocity within each of the three observing periods of 1978/1979, 1980, and 1983 and can be explained in terms of thick shells, which are ejected with a frequency of about once per year.