We report on the results of an extended optical spectroscopic monitoring campaign on the early-type B supergiant HD 64760 ( B0.5 Ib ) designed to probe the deep-seated origin of spatial wind structure in massive stars .
This new study is based on high-resolution echelle spectra obtained with the Feros instrument at ESO La Silla .
279 spectra were collected over 10 nights between February 14 and 24 , 2003 .
From the period analysis of the line-profile variability of the photospheric lines we identify three closely spaced periods around 4.810 hrs and a splitting of \pm 3 % .
The velocity – phase diagrams of the line-profile variations for the distinct periods reveal characteristic prograde non-radial pulsation patterns of high order corresponding to pulsation modes with l and m in the range 6 - 10 .
A detailed modeling of the multi-periodic non-radial pulsations with the Bruce and Kylie pulsation-model codes ( ) favors either three modes with l = - m and l = 8 , 6 , 8 or m = -6 and l = 8 , 6 , 10 with the second case maintaining the closely spaced periods in the co-rotating frame .
The pulsation models predict photometric variations of 0.012 - 0.020 mag consistent with the non-detection of any of the spectroscopic periods by photometry .
The three pulsation modes have periods clearly shorter than the characteristic pulsation time scale and show small horizontal velocity fields and hence are identified as p-modes .
The beating of the three pulsation modes leads to a retrograde beat pattern with two regions of constructive interference diametrically opposite on the stellar surface and a beat period of 162.8 hrs ( 6.8 days ) .
This beat pattern is directly observed in the spectroscopic time series of the photospheric lines .
The wind-sensitive lines display features of enhanced emission , which appear to follow the maxima of the photospheric beat pattern .
The pulsation models predict for the two regions normalized flux amplitudes of A = +0.33 , -0.28 , sufficiently large to raise spiral co-rotating interaction regions ( ) .
We further investigate the observed H \alpha wind-profile variations with a simple rotating wind model with wind-density modulations to simulate the effect of possible streak lines originating from the localized surface spots created by the NRP beat pattern .
It is found that such a simple scenario can explain the time scales and some but not all characteristics of the observed H \alpha line-profile variations .