WIND VARIABILITY OF B-SUPERGIANTS .3. COROTATING SPIRAL STRUCTURES INTHE STELLAR WIND OF HD-64760

Fourier analysis of two spectroscopic time series obtained with the IU E observatory confirm that the ultraviolet stellar wind profiles of HD 64760 (B0.5 Ib) are periodically variable. The periodic component con sists of modulations that extend over most of the P Cygni absorption t rough, and can frequently be traced through the emission lobe. The mod ulations coexist with variations due to the propagation of discrete ab sorption components, but there does not seem to be a direct link betwe en these two types of variability. In a long time series obtained in 1 995 January during the IUE MEGA Campaign, the modulations in the P Cyg ni profiles of the Si III, Si IV, C IV, and N V resonance lines were d ominated by two sinusoidal variations with semi-amplitudes between sim ilar to 5-10% of the continuum flux and periods of 1.202 +/- 0.004 and 2.44 +/- 0.04 days. The weak emission-lobe variability was predominan tly due to the 2.4-day modulation. In the absorption trough, the ratio of the amplitude of the 1.2-day modulation to the amplitude of the 2. 4-day modulation increased systematically as a function of ionization potential. For both periods, the distribution of the phase constant wi th position in the absorption trough exhibited a maximum near -710 km s(-1), and decreased symmetrically toward larger and smaller velocitie s. There was a systematic decrease in the value of the maximum phase b etween Si Iv and N v. Only the 2.4-day period was present in a shorter time series obtained in 1993 March, when its amplitude was nearly twi ce its 1995 value and it was more concentrated toward smaller velociti es in the absorption trough. There is no clear evidence for phase bowi ng in the 1993 data. Since the 2.4- and 1.2-day periods are approximat ely a half and a quarter of the estimated rotational period of HD 6476 0, respectively, we interpret the modulations in terms of 2 (1993) and 4 (1995) broad, corotating circumstellar structures that modulate the optical depth of the stellar wind. The bowed distribution of phase im plies that the structures are azimuthally extended, probably spiral-sh aped arms, and we develop a kinematic interpretation of the projected velocity associated with the phase turnover in terms of the degree of bending of the spirals. We derive a value for the exponent governing t he radial expansion of the wind of beta approximate to 1, which is in good agreement with the canonical value for smooth, spherically symmet ric winds and suggests that the spiral structures are long-lived pertu rbations through which material flows. The systematic phase lag associ ated with higher ions suggests that they are preferentially located al ong the inner, trailing edge of the spiral, as expected if the structu res are formed by the collision of fast and slow winds originating fro m equally-spaced longitudinal sectors of the stellar surface. Although a photospheric process is implicated in the origin of these structure s, it is not clear that magnetic fields or nonradial pulsations could readily account for the switch between 2- and 4-equally spaced surface patches that evidently occurred between 1993 and 1995.