Smalley, Barry; (1992) The atmospheric parameters of main sequence A and F stars. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The problem of the fundamental atmospheric parameters of main-sequence A and F stars has been investigated using several different techniques. This work was motivated by the fact that recent analyses based on fitting optical and ultraviolet energy distributions have yielded systematically lower effective temperatures and surface gravities for metallic-lined (Am) stars than have previously been obtained using photometric methods. Abundance analyses using these lower values implied that the atmospheric composition of Am stars is roughly solar, compared to the usually expected enhancement. The use of photometry and spectrophotometry in the determination of effective temperatures and surface gravities has been critically evaluated. It is found that all the calibrations and methods yield broadly the same values, but only provided that metal abundance is known or explicitly allowed for in the determinations. Some methods are highly sensitive to metal abundance. The hydrogen Balmer lines have been studied extensively and used to confirm that the effective temperatures of Am stars are consistent with those determined from photometry. It has been found that photometry is not adversely affected by an flux excess around 4785Å, as had previously been suggested. This flux excess is not apparent in high-quality spectra and appears to have originated due to deficiencies in the model atmosphere fluxes around that wavelength. A differential abundance analysis has confirmed that Am stars are metal-rich. The atmospheric abundances are enhanced by up to a factor of about five, which is consistent with the current models for the origin of such anomalies. The atmospheric parameters of Am stars indicate that their masses, luminosities and radii are not significantly different from those of normal A and F stars of the same age. The abundance anomalies are most definitely real and presumably caused by atmospheric processes, such as radiative diffusion and gravitational settling.

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