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Paper: The photosphere of P Cygni
Volume: 233, P Cygni 2000: 400 Years of Progress
Page: 215
Authors: de Jager, C.
Abstract: Investigations of P Cygni during the last few decades have yielded a fairly consistent set of data for the photospheric parameters. I use T eff = 18600; log(L/Lo) = 5.8; vturb = 20 km/s; log(Mdot)= -4.7; X = 0.35. The resulting photospheric model has negative values of geff in the higher parts of the photosphere (optical depths below 0.7). In addition, the sonic point is reached already at a Rosseland optical depth of 0.18. These properties explain the outward motions in the atmosphere and the observed microturbulent velocities. While the body of the star has a <Gamma1>-value of about 4/3, the average atmospheric <Gamma1> = 1.338 is just above this limit. Hence the star is marginally dynamically stable. The strong stellar wind is a consequence of the outward-directed radiation pressure. The luminosity variations of relative short periods (smaller than about 500 d) are pressure waves with wavelengths of tens of millions of kilometers. The brightness variations of longer periods, between 1500 and 3700 d, are ascribed to large-scale gravity waves (nonradial pulsations) with a quasi recurrence time of that order. For the evolution I suggest the sequence Blue supergiant - Red supergiant - Yellow hypergiant - S Dor star (LBV) - WR star or supernova.
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