2006A&A...457..987P

This study seeks to provide empirical constraints on the different physical components that can yield temporal variability in predominantly or partially wind-formed optical lines of luminous OB stars, and thus potentially affect the reliable determination of fundamental parameters, including mass-loss rates via clumped winds. Using time-series spectroscopy from epochs spread over ∼4-years, we present a case study of the O9.5 supergiant α Cam. We demonstrate that the HeI λ5876 (2³P⁰-3³D) line is an important diagnostic for photospheric and wind variability in this star. The actions of large radial velocity shifts (up to ∼30km/s) in the photospheric absorption lines can also affect the morphology of the H_α line profile, which is commonly used for measuring mass-loss rates in massive stars. We identify a 0.36-day period in subtle absorption profile changes in HeI λ5876, which likely reveals the photospheric structure, perhaps due to low-order non-radial pulsations. This signal persists over ∼2 months, but it is not present 2 years later (November 2004); it is also not seen in the stellar wind components of the line profiles. Using a pure H_α line-synthesis code we interpret maximum changes in the redward and peak emission of α Cam in terms of mass-loss rate differences in the range ∼5.1x10^–6 to 6.5x10^–6M_☉/yr. However, the models generally fail to reproduce the morphology of blueward (possibly absorptive) regions of the profiles. The optical line profiles of α Cam are affected by (i) deep-seated fluctuations close to, or at, the photosphere, (ii) atmospheric velocity gradients, and (iii) large-scale stellar wind structure. This study provides new empirical perspectives on accurate line-synthesis modelling of stellar wind signatures in massive luminous stars.