Neutral-Helium Line Strengths.VI. the Variations of the Helium Spectrum Variable alpha Centauri

Spectra (XX38006700) and continuum measures ( ) have been obtained of a Cen during the years 1967-1969. These have been used to determine the variations of several observational parameters: hydrogen line profiles, helium and metallic line strengths, continuum magnitudes and colors, and radial velocities. The period of the helium-line variations is found to be 8.814 days-in disagreement with the period of 8.8172 days obtained from spectral-type data stretching back to 1895. This suggests that the variation is not strictly periodic. An analysis of the hydrogen line profiles, the continuum measures, and the helium line strengths shows that while the helium abundance apparently changes from (He) [=N(He)/N(H)] = 0.004 to 0.23, only small changes in effective temperature and gravity are allowed. Whiie the helium line strengths show no evidence for departures from LTE, and may be explained in terms of an uneven element distribution over the surface of a rotating star ("rotator" hypothesis), the metallic-line variations defy explanation in terms of such a model. In particular, the lines of Si II show no variation, while those of Si III show an antiphase variation with respect to the helium lines. All of the metallic lines either show no variation or vary in antiphase with those of helium. The radialvelocity measures, however, exhibit the same behavior for both the helium and the metallic lines. This leads to the rejection of the "rotator" hypothesis. It is suggested that a Cen may be related to the peculiar A and B stars. This is made feasible by the existence of a robable helium spectrum variable, I'D 162374, among the weak-helium-line stars, and the position of a en in the (0,, log g)-plane where it coincides with the region found to be occupied by the weak-helium-line stars.