Spectra of M Supergiant Stars.

High-dispersion spectral plates of the late supergiant stars a Orionis and a Herculis have been reduced in the region XX 4000-4300, and the line profiles measured for five multiplets of neutral iron and one of manganese. Approximate determinations of the energy distribution in the continuous background over this range and of the appropriate correction for finite resolving-power allow a reduction of the apparent relative intensi- ties to true absolute intensities. A determination of the variation of ax, the atomic ab- sorption coefficient, with increasing distance from the line center A0, shows that ax is given by the usual radiation-damping formula with a dampi~ig constant some five hundred times the classical value. For the greater widths shown by the Na lines, how- ever, ax falls off much more rapklly than (X - X0)_2. These results are attributed to a turbulent distribution of velocities. Different properties of the atmosphere of a Orionis correspond to temperatures which range from 2 ioo° for the excitation temperature, to 17,000° derived from the energy density of La radiation, and up to 200,000° for the value derived from the magnitude of the observed velocities. The evidence suggests that the atmospheres of these two stars are characterized by an almost complete lack of thermodynamic equilibrium. Examination of these and other spectral plates of the stars a Orionis, a Scorpii, ci. Herculis, ~ Pegasi, ~ Andromedae, a Boötis, and a Tauri reveals asymmetries in the strong ultimate lines for the first three stars, the most extended of the seven. The gen- eral appearance is that of a wide line at the expected position, blended with a narrow, deep component a tenth of an angstrom or so to the violet. The asymmetry is at- tributed to a thin shell of outward-moving atoms; these absorb radiation from the violet side of the normal symmetrical lines which are formed by the underlying stationary reversing layer. The measurements indicate velocities for this shell ranging from 4 to 40 km/sec. An ionization cycle is suggested in which neutral atoms kave the surface, possibly accelerated by radiation pressure, and then become ionized and return to the star under the force of gravity. It is shown that such a cycle will operate only if the number of free electrons in the shell is less than 5* io6 per cm3 and if the intensity of the ionizing radiation is also restricted-for Na atoms this must be less than the equilibrium intensity at 3100°