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Title: Rotation and chemical composition of early-type stars
Author: Guthrie, Bruce N. G.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1968
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Some early-type stars with abnormal line strengths for their intrinsic colours are investigated in order to determine their structure and the chemical composition of their atmospheres. The abnormal line strengths in several B-type stars (αScl, 36 Lyn, 20 Tau and HD 175156) are shown to be probably due to large ranges of surface temperature such as would be observed in rapidly rotating stars of normal chemical composition, if viewed pole-on. These stars differ from the peculiar A stars, which rotate slowly and have abnormal chemical composition. Members of the Mn group of peculiar A stars rotate more slowly than those of the Si-4200 group, although the mean intrinsic colours of the two groups are similar. Previous work on abundance determination in peculiar A stars is reviewed. The chemical composition of the manganese star 53 Tau is studied by a refined differential curve-of-growth method with αLyr as the comparison star. Groups of lines of similar mean excitation and ionisation potentials and wavelengths are used so that the results are independent of the structure of 53 Tau, which is shown to be abnormal. A new double-line peculiar A spectroscopic binary HR 4072 was discovered, and it was found that excesses of Sr and Y are common to both components. Spectral variations in some peculiar A stars are described. Well established abundance abnormalities in peculiar A stars may be explained on a modified form of van den Heuvel's theory that peculiar A stars have been secondaries of binary systems with initial separations in the range 10 a.u. to 100 a.u. approximately. The primaries, initially earlier than spectral type B8, have evolved and exploded as type II supernovae. During the explosions material enriched in heavy elements by interior nuclear reactions was transferred to the surfaces of the secondaries, which are now observed as peculiar A stars and are still on the main sequence. Excesses of Be and Si and deficiencies of 0 are caused by surface nuclear reactions on the secondaries after the explosions of the primaries. Ln stars differ from other peculiar A stars in many respects; the differences may be due to the primaries expanding beyond the Roche limits before they explode. On the oblique rotator theory, spectrum variables result from irregular distributions of surface nuclear reactions and ion migration on the secondaries. Some other theories of peculiar A stars are reviewed and shown to be in conflict with observation.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available