Time-Series Observations of O Stars. II. Optical Observations of zeta Ophiuchi

Abstract

We present photopolarimetry and extensive high-resolution, high-signal-to-noise optical spectroscopy of the rapidly rotating late O-type star zeta Ophiuchi (HD 149757). The polarimetric data show no substantial variability over 1 week (sigma(p) < 0.04%), while the spectroscopic data show characteristic line-profile variations in the form of ''bumps'' migrating from blue to red on typical time scales of several hours. These variations are relatively strong (amplitude approximately 1% of the continuum level) in He I lambda4471, and Si III lambdalambda4552, 4567, 4575. They are seen at lower amplitudes in Mg II lambda4481, but are almost undetectable (amplitude less than or similar to 0.3%) in He II lambdalambda4541, 4686 and N III lambdalambda4511, 4515, 4518. We attribute this to a combination of equatorial gravity darkening and a latitudinally confined origin of the variations.Using a Fourier CLEAN technique, it is shown that the line-profile variations can be adequately represented as a set of sinusoids. The periods, combined with an estimate of the rotational period, lead us to rule out rotational modulation as the origin of the variability. The phase changes of the sinusoids across the line-profiles indicate a repetitive pattern, consistent with sectorial (l = -m) nonradial pulsation. We find four periods: P = 3.339 hr (Absolute value of m = 4), 2.435 hr (Absolute value of m = 5 or 6), 1.859 hr (Absolute value of m = 9 +/- 1), and either 1.366 or 1.292 hr (Absolute value of m = 11 +/- 1). The ''superperiods,'' P Absolute value of m, may be nearly commensurate (at approximately 13-14 hr), but that commensurability is not exact.