The chemical composition of algol systems - III. Beta Lyrae-nucleosynthesis revealed

Abstract

We have used high signal-to-noise ratio Reticon observations of visible and near-infrared lines of He, C, N, O, Ne and Fe to determine the abundances of these elements in the B8 primary of the eclipsing binary β Lyr. A direct estimate of the He abundance, based on measurements of weak He I lines, shows a definite He enrichment: N(H) = 0.4, N(He) = 0.6. An indirect estimate of the He abundance, which assumes that the initial CNO abundances were cosmic and that the sum of the CNO abundances has not changed during the evolution of β Lyr, demands the same He enrichment. It is supported by similar indirect estimates based on Ne and Fe and the assumption that the Ne and Fe abundances have not altered during the evolution of β Lyr. Nitrogen is extremely overabundant. With a microturbulence of 10 kms−1, determined from the Fe II lines, and the He-rich model atmosphere, we find that it is 20 times more abundant than in the Sun. Carbon and O are very underabundant relative to N; we find |$\text {C/N}$|≼0.011 and |$\text {O/N}=0.025$| (factor of 2 uncertainly for both ratios). These ratios, which are drastically different from the solar C/N and O/N ratios of 5 and 8, respectively, are sufficiently close to the equilibrium ratios of the CNO cycle as to leave little doubt that the material has been fully processed by the CNO cycle. In single stars these changes of composition associated with the burning of H to He are hidden from view by the unprocessed outer layers; even during red giant evolution they are only partially revealed. However, in β Lyr, which is unusual in that the phase of rapid mass transfer has been very recent, the original primary is still the primary in an observational sense, even though it is now much less massive than the secondary (⁠|$\sim2m_\odot$| versus |$\sim12m_\odot$|⁠), and its processed core is exposed to view. The spectroscopic evidence of He enrichment and extreme CNO cycling confirms the major compositional changes demanded by the theory of nuclear burning and, thereby, supports the modern picture of βLyr. From a comparison of the observed H, He, C, N, and O abundances with the calculated interior abundances of evolved stellar models, we conclude that the |$2m_\odot$| primary is the remnant of a |$12m_\odot$| star that has lost |$10m_\odot$| by Roche lobe overflow.