We have undertaken a study designed to determine, as accurately and as consistently as possible, the physical properties of a group of metallic-line stars. We have obtained observational data that are of higher resolution and that cover a wider spectral range than have any previous authors. Our analysis has been carried out with the most sophisticated methods available, including line-blanketed model atmospheres of differing composition and spectral synthesis techniques. We observed the Am star 81 Tauri to undergo a transient change in its energy distribution over a period of five days. It appears that several other Am stars have displayed this sort of phenomenon in the past and it may be more common than has previously been believed. We have determined effective temperatures and surface gravities for our programme stars by comparing our observed energy distributions to fluxes calculated from model atmospheres that have line-blanketing which reflect solar, three times solar and ten times solar abundances. The theoretical fluxes calculated with the three times solar models provide the best fit to the observed energy distributions and are consistent with the surface gravities determined from the ionization equilibrium of iron. We have used these models to synthesize the Ca K line profiles in our stars and for suitable calcium abundances, and match between theory and observation is excellent. We have calculated the radii of our programme stars and find them to be similar to those of normal A -type stars of the same effective temperature. The radii are thus consistent with normal main sequence gravities. We have determined the rotational and microturbulent velocities of these stars using spectral synthesis techniques. The rotational velocities are small and similar to those determined by other authors. The microturbulent velocities are much smaller than those determined by the classical curve-of -growth techniques. This results in a microturbulent scale for Am stars similar to that for normal A dwarfs. We have used spectral synthesis techniques in combination with high resolution International Ultraviolet Explorer Satellite spectra to investigate the abundances of the lighter elements, many of which cannot be studied in the visual. We find nitrogen to range from slightly overabundant in the cooler Am stars to slightly underabundant in the hotter Am stars. Carbon is underabundant by almost a factor of two, aluminium is solar, phosphorus is fifteen times overabundant and silicon ranges from solar to ten times overabundant. We have investigated the helium abundance in Am stars and find some evidence that it may be temperature dependant. If this is true, helium may be far more deficient, by up to a factor of thirty, in the coolest Am stars than has previously been believed. When our abundance calculations are carried out with helium deficient models, there are small changes in the abundance of some of the lighter elements in the sense that they become more solar. We have compared the results of our abundance determinations to the published results of diffusion theory. The process is somewhat confusing and our results are not conclusive. Many of the tests we have performed support diffusion theory, some do not. Finally, we have combined our data with those of Heacox (1979) for the Hg-Mn stars. We have found substantial observational evidence for a connection between these classes. We believe that they form a continuous sequence of non-magnetic peculiar stars.
- Pub Date:
- Physics: Astronomy and Astrophysics