Large-scale wind structures in OB supergiants: a search for rotationally modulated Hα variability

Abstract

We present the results of a long-term monitoring campaign of the Hα line in a sample of bright OB supergiants (O7.5-B9) which aims at detecting rotationally modulated changes potentially related to the existence of large-scale wind structures. A total of 22 objects were monitored during 36 nights spread over six months in 2001-2002. Coordinated broad-band photometric observations were also obtained for some targets. Conspicuous evidence for variability in Hα is found for the stars displaying a feature contaminated by wind emission. Most changes take place on a daily time-scale, although hourly variations are also occasionally detected. Convincing evidence for a cyclical pattern of variability in Hα has been found in two stars: HD 14134 and HD 42087. Periodic signals are also detected in other stars, but independent confirmation is required. Rotational modulation is suggested from the similarity between the observed recurrence time-scales (in the range 13-25 d) and estimated periods of stellar rotation. We call attention to the atypical case of HD 14134, which exhibits a clear 12.8-d periodicity, both in the photometric and in the spectroscopic data sets. This places this object among a handful of early-type stars where one may observe a clear link between extended wind structures and photospheric disturbances. Further modelling may test the hypothesis that azimuthally-extended wind streams are responsible for the patterns of spectral variability in our target stars.