Wind variability of B supergiants
III. Corotating spiral structures in the stellar wind of HD 64760
A.W. Fullerton 1,
R.K. Prinja 3,
S.P. Owocki 4 and
S.R. Cranmer 4, 5
Received 17 March 1997 / Accepted 19 June 1997
Fourier analysis of two spectroscopic time series obtained with the IUE observatory confirm that the ultraviolet stellar wind profiles of HD 64760 (B0.5 Ib) are periodically variable. The periodic component consists of modulations that extend over most of the P Cygni absorption trough, and can frequently be traced through the emission lobe. The modulations coexist with variations due to the propagation of discrete absorption components, but there does not seem to be a direct link between these two types of variability.
In a long time series obtained in 1995 January during the IUE MEGA Campaign, the modulations in the P Cygni profiles of the Si III, Si IV, C IV, and N V resonance lines were dominated by two sinusoidal variations with semi-amplitudes between 5-10% of the continuum flux and periods of and days. The weak emission-lobe variability was predominantly 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 with position in the absorption trough exhibited a maximum near -710 km s-1, and decreased symmetrically toward larger and smaller velocities. There was a systematic decrease in the value of the maximum phase between 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 twice its 1995 value and it was more concentrated toward smaller velocities in the absorption trough. There is no clear evidence for phase bowing in the 1993 data.
Since the 2.4- and 1.2-day periods are approximately a half and a quarter of the estimated rotational period of HD 64760, 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 implies that the structures are azimuthally extended, probably spiral-shaped 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 the radial expansion of the wind of , which is in good agreement with the canonical value for smooth, spherically symmetric winds and suggests that the spiral structures are long-lived perturbations through which material flows. The systematic phase lag associated with higher ions suggests that they are preferentially located along the inner, trailing edge of the spiral, as expected if the structures are formed by the collision of fast and slow winds originating from equally-spaced longitudinal sectors of the stellar surface. Although a photospheric process is implicated in the origin of these structures, 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.
Key words: stars: early-type stars: mass-loss stars: oscillations stars: magnetic fields stars: individual: HD 64760 (HR 3090)
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© European Southern Observatory (ESO) 1997
Online publication: April 6, 1998