A SPECTROSCOPIC ANALYSIS OF DAO AND HOT DA WHITE-DWARFS - THE IMPLICATIONS OF THE PRESENCE OF HELIUM AND THE NATURE OF DAO STARS

A comprehensive study of optical spectrophotometry of DAO white dwarfs is presented. A detailed analysis of the He II lambda4686 line profil es demonstrates conclusively that the chemical composition in the atmo spheres of DAO stars is not stratified, except perhaps in one object ( PG 1305-017). On the other hand, line profiles calculated from models with homogeneous compositions are in excellent agreement with the obse rved profiles. This result indicates that the helium abundance distrib ution, in the line-forming region, is homogeneous and implies, in turn , that some physical mechanism is competing with gravitational settlin g to support helium in these layers. A similar analysis is then carrie d out for a sample of hot DA stars. In particular, the sensitivity of the atmospheric parameter determination to the presence of unobservabl e traces of helium in the photosphere is investigated in detail. The r esults show that, above approximately 40,000 K, the effective temperat ures of DA stars can be overestimated by up to several thousand degree s if spectroscopically invisible traces of helium [N(He)/N(H) less tha n or similar to 10(-4)] are present in their atmospheres. For instance , the optically determined effective temperature of a DA white dwarf w ith T(eff) = 60,000 K and N(He)/N(H) = 10(-4) would be overestimated b y approximately 20,000 K if analyzed with pure hydrogen model atmosphe res. In the light of these findings, we present a critical analysis of past and current studies of extreme ultraviolet and X-ray observation s of hot DA stars. A comparison of the atmospheric parameters of the h ot DA stars with those of the DAO stars reveals that the presence of h elium in most DAO white dwarfs is directly related to their unusually low surface gravities (log g less than or similar to 7.5), although se veral exceptions are found and considered individually. The nature and evolution of DAO white dwarfs is discussed at length. We identify sev eral subclasses of DAO stars, a result which indicates that the DAO sp ectral class is a rather inhomogeneous collection of objects, with onl y a few DAO stars in our sample being consistent with post-asymptotic giant branch evolution. We present supporting evidence that the progen itors of most DAO stars are post-extended horizontal branch stars (M a pproximately 0.48 M.) which have evolved directly from the extended ho rizontal branch to the white dwarf state. Those DAO stars which are me mbers of white dwarf + M dwarf composite systems represent an independ ent subclass with unique proper-ties. PG 1305 - 017 is the only DAO st ar that shows evidence for stratification, while the variable spectrum of PG 1210 + 533 cannot be reproduced successfully with any of the mo dels explored in this analysis. Finally, we offer observational eviden ce that the presence of heavy elements is the most plausible explanati on for the origin of the so-called Bahner-line problem in DAO stars.