The Hydrogen-Deficient Carbon Stars

Open Access
Publisher Website
Google Scholar
Abstract
Abundance analyses of the five known non-variable carbon stars with weak hydrogen lines are carried out. With the possible exception of Na all observable elements heavier than O have normal abundances relative to Fe. H is deficient in four of the stars by factors of more than 10 5 ; in the remaining star a deficiency of a factor of 50 is found. C is over-abundant by factors of 3 to 10. The similarity of the compositions, space and velocity distributions and absolute magnitudes of the above stars, the R Cr B variables and the helium stars is stressed. These stars are collectively called hydrogen-deficient carbon (Hd C) stars and their various properties are discussed. They appear to form a class of star entirely different from the normal R and N stars. The continuous opacity in the cool Hd C stars is shown to arise from contributions from He−, C−, electron scattering, Rayleigh scattering by He and C, and photoionization of carbon. It is shown that the number of Fe atoms per gram of stellar material lies in the range 2 to 0.3 times that in the Sun. Most indications are that the Hd C stars are slightly metal deficient, in agreement with their large space velocities. It is suggested that the peculiar abundances of H, C, N and O in these objects arise from loss of outer envelope at a stage shortly after He-burning has started in a star of 1 MO . The predicted abundances are then in reasonable agreement with observations, and it emerges that most of the O will be in the form of O 18 . The lifetime of the Hd C stage appears to be ∼ 10 3 years, in which case secular changes should be observable. The proposed scheme of evolution shows how these helium stars appear as red supergiants and evolve into the white dwarf region of the HR diagram.

This publication has 0 references indexed in Scilit: