Mathematics – Logic
Scientific paper
Sep 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986phdt........14g&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF TORONTO (CANADA), 1986.Source: Dissertation Abstracts International, Volume: 48-11, Section: B, pa
Mathematics
Logic
12
Scientific paper
The A-type stars have been studied spectroscopically to (1) refine the MK-classification system for the A-type stars (B8-F2), (2) to investigate in detail the relationships between the uvbybeta photometric system and the refined classification system, and (3) to explore the effect rotation has on MK classification, the uvbybeta system and the relationships between these two systems. The MK-classification system for the A-type stars has been refined in two ways: (1) the classification criteria (which are used as guidelines in the process of comparing program spectra with standard spectra) have been critically evaluated and modifications have been made which help to improve the consistency of the spectral classifications. (2) Parallel systems of narrow-lined and broad-lined standards have been set up. This refinement greatly facilitates the process of the comparison of program stars with the standards and helps to remove systematic effects with rotation in the classification system. Detailed comparisons of the resulting precise classifications (of over 1300 stars from the field and 17 open clusters) have been made with uvbybeta photometry. The relationships between these two systems are found to be strongly dependent on the presence of rotation; some theoretical effects of rotation on the uvbybeta indices (especially the beta and delta{c }_1 indices) have been confirmed qualitatively. The lambda Bootis stars have been studied in some detail and they have been found to fall into two distinct classes distinguished by their hydrogen-line profiles. A shell-like mechanism has been proposed to explain the second class which is characterized by peculiar hydrogen-line profiles. The delta Delphini stars have also been critically examined and have been found to be an inhomogeneous class of stars. It is recommended that the classification "delta Delphini" be dropped and that these stars be classified morphologically. However, attention is drawn to a class of stars with spectra similar to rho Puppis (formerly classified as delta Delphini stars) whose study may give us insights into the relationship between convection and metallicity. An MK-Mv map (absolute-magnitude calibration of the spectral classes) has been derived for the A-type stars. Vega is over-luminous for its spectral type; it is proposed that this might be understood if Vega is a rapid rotator seen pole-on.
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