Mass loss from P Cygni - The evidence of the Balmer lines

Envelopes of stars which are losing mass are divided into two groups according to their velocity distribution. 'Rapid accelerators' are driven to nearly terminal velocity near the photosphere and then coast outward at nearly constant speed. 'Gradual accelerators' experience a steady increase in velocity over a sizable fraction of the envelope. The Balmer-line profiles from P Cygni are shown to be consistent with the latter class of envelope model. Very good agreement between theory and observation is shown by a model with v(r) proportional to r throughout and with a mass-loss rate of 3 hundred-thousandths solar mass per year. A model in which the envelope decelerates is not required to account for the Balmer spectrum. This brings into accord the various indicators of envelope velocity: the line-width-excitation-energy correlation, the Balmer progression, and the free-free continuum.