The Atmosphere of Gamma Pegasi.

The bright main-sequence B2.5 star, -y Pegasi, shows strong lines of hydrogen and helium and sharp, well-defined lines of other elements. Equivalent widths have been measured on coudé plates taken at the Mount Wilson Observatory and at the McDonald Observatory and on two-prism spectrograms taken at the Dominion Astrophysical Observatory. The material has been analyzed by the method employed by Unsold for r Scorpii, by a curve-of-growth procedure which takes the wave-length variation of the con- tinuous absorption coefficient into account, and by the calculation of theoretical intensities of selected lines of hydrogen, carbon, nitrogen, and oxygen with the aid of a model atmosphere. All methods agree in assigning a higher hydrogen/oxygen ratio than in the sun and gaseous nebulae. Possibly this ratio is spurious and arises from the circumstance that the resonance lines of C II, 0 ii, Nn, etc., fall in a spectral region where the continuous absorption of hydrogen and helium is strong. The calculated abundances are very sensitive to the excitation temperature adopted, and only orders of magnitude can now be given. The ratios of sulphur, argon, and possibly chlorine with respect to oxygen seem to agree with results previously found from the planetary nebulae, while the phosphorus/oxygen ratio appears about the same as in the earth's crust. A satisfactory determination of the relative abundances of the light elements in early-type B stars will require better information on thef-values and damping constants of the observed transitions, as well as a good determination of the excitation temperature and the influence of the continuous hydrogen and helium absorption upon the radiation field and therefore upon the population of excited levels of C ii, Nii, Oii, etc