The Excitation of Absorption Lines in Outer Atmospheric Shells of Stars.

Abstract

I. The He i lines show abnormal intensities in the outer shells of peculiar B stars. In ~ Tauri and co Persei He I X 3965 (21S_41P) is narrow and sharp, while all lines arising from the levels 2'P and 23P are broad and diffuse. This is attributed to the dilu- tion of the radiation from the B star in the shell. The anomalous intensities of He i are investigated in the entire sequence of B stars having outer shells. II. The theory of the excitation of absorption lines in a field of diluted radiation is developed and applied to six levels of He I: i'S, 2'S, 2'P, 2~S, 2~P, and the state of ionization. The populations in the various states have been computed for dilution factors o.i, 0.02, and o.oi, and for temperatures of i~4 and 2.5X io4 degrees. III. The theoretical populations for a dilution factor of the order of o.oi agree with the observations. Accordingly, the radius of the shell is approximately five times the radius of the star. The entire He i anomaly is accounted for by the theory. IV. The absence of rotational broadening in lines produced in the shells shows that these shells do not rotate as solid bodies with the photospheres. The latter are rotating rapidly in most of the stars investigated. Some lines, such as Si ii in ~ Tauri, show broadening intermediate between the normal He i lines in the deepest layers and the X 3965 line of He i in the shell. Hence, the elements are stratified. The ionization de- creases outward in all stars investigated except j3 Lyrae. The strength of the H cores in the shells is explained by the metastability of the 2S-level. The weakness of Mg ii ~8i is explained by the fact that the lower level has strong transitions downward. V. The He i anomaly in ordinary giants and dwarfs of class B bears a strong resem- blance to the anomaly in the shells. The explanation is doubtless similar, but there are complications due to Stark effect, coffisional damping, etc