The Circumstellar Envelope of Rho Cassiopeiae.

From a study of high-dispersion plates obtained between 1955 and 1960, we find that p Cas is surrounded by a shell which is expanding at 40 km/sec relative to the photosphere Absorption lines of excitation potential up to 2.9 volts and emission lines of Fe I, Ni 1, and Ca I are formed in the shell. Variations in the strengths of these lines are attributed to variations in the mass of the shell. From galactic rotation, we find = -85. The radius is about 700 R0 and the mass about 25 M0 The emission and absorption lines of Ca I indicate a small geometrical dilution (0 1-0 01) in the shell. A curve-of-growth analysis shows it to be composed of neutral hydrogen having a density of lOis H atoms/cm3. The excitation temperature in the shell is 3500 the ionization temperature is 4000 . The turbulent velocities in the shell and in the photosphere are estimated both from the curves of growth and from the line profiles In the shell both methods lead to a microturbulent velocity of 7 km/sec In the reversing layer the macroturbulent velocity is 20 km/sec for ionized and 14 km/sec for neutral atoms. The microturbulent velocity is 10 km/sec for zero-volt neutral lines and decreases to 6 km/sec with increasing excitation. A pronounced weakness in the Balmer absorption lines is attributed to their being filled in by emission arising in a chromosphere which lies between the reversing layer and the cool shell. There is insufficient Lyman continuum radiation from p Cas to heat the chromosphere, which must therefore be heated by the dissipation of mechanical energy The flux of turbulent energy is probably sufficient. It is found that p Cas is losing mass at a rate of 10-6 stellar masses per year. The ejection is sporadic, although the rate quoted may be comparable with the mean rate