A study of twenty-one sharp-lined non-variable cool peculiar A stars

Abstract

Elemental abundance analyses have been performed for twenty-one cool Ap stars, also known as SrCrEu stars, which are not known to be spectrum variables and whose projected rotational velocity is not greater than 10 km/sec. Zeeman and thermal Doppler broadening are the basic line intensification mechanisms considered. Effective temperatures estimated initially from UBV photoelectric data are modified by requiring that the iron abundances derived from both neutral and singly-ionized lines be equal. Log g has been assumed to be 4.0. In addition, two normal stars have been included in this study for comparison. The cool Ap stars, in general, have normal or over-abundances with respect to hydrogen for every element studied. The iron abundance is found to increase with effective temperature. Similar dependences are found for chromium, manganese, neodymium, gadolinium, and, perhaps, several other elements. The abundances are not dependent on the apparent rotational velocity or magnetic field strength. The elemental abundances are similar to those of the SrCrEu spectrum variables and agree reasonably well with the results of other studies of cool Ap stars. Comparisons with Mn and Si stars are similar to but not identical with those of other investigators. Substantial differences in certain abundance ratios are found which distinguish Am and cool Ap stars. The results of low dispersion classification spectroscopy correlate better with the appearance than with the derived abundances. The agreement between photometric indices and abundances is poor. Several stars are found whose properties are similar to those of Si stars studied by Sargent, Searle, and Lungershausen except with regard to their [Cr/Fe] values. Thus, in addition to the fact that both types have magnetic fields, the abundances indicate that the Si and cool Ap stars are probably closely related. The theories to account for the Ap star elemental abundances have difficulties in explaining the results of this study. The diffusion mechanism and the magnetic accretion hypothesis of Havnes and Conti cannot explain several features, for example, the abundance ratios of certain adjacent elements, and are at best incomplete. On the basis of the latter theory, it is hard to understand why the abundances are not correlated with magnetic field strength. Some features of the abundance anomalies are similar to those produced by the r- and by the s-processes. Theories which require mixing of the Ap star or binary companions are excluded.