Multi-periodic photospheric pulsations and connected wind structures in HD64760

A.Kaufer(1), O.Stahl(2), R.K.Prinja(3), and D.Witherick(3)

(1) European Southern Observatory,
(2) Landessternwarte Heidelberg,
(3) University College London

We report on the results of an extended optical spectroscopic
monitoring campaign on the early-type B supergiant HD64760 (B0.5Ib)
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
4.810hrs and a splitting of +/-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 0.012-0.020mag 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 162.8hrs (6.8days). 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 Halpha 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 Halpha line-profile variations.

Reference: Accepted for A&A on October 17, 2005.
Status: Manuscript has been accepted