Interplay between pulsations and mass loss in the blue supergiant 55 Cygnus = HD 198478

Abstract

Blue supergiant stars are known to display photometric and spectroscopic variability that is suggested to be linked to stellar pulsations. Pulsational activity in massive stars strongly depends on the star's evolutionary stage and is assumed to be connected with mass-loss episodes, the appearance of macroturbulent line broadening, and the formation of clumps in the wind. To investigate a possible interplay between pulsations and mass-loss, we carried out an observational campaign of the supergiant 55 Cyg over a period of five years to search for photospheric activity and cyclic mass-loss variability in the stellar wind. We modeled the H, He I, Si II and Si III lines using the nonlocal thermal equilibrium atmosphere code FASTWIND and derived the photospheric and wind parameters. In addition, we searched for variability in the intensity and radial velocity of photospheric lines and performed a moment analysis of the line profiles to derive frequencies and amplitudes of the variations. The Halpha line varies with time in both intensity and shape, displaying various types of profiles: P Cygni, pure emission, almost complete absence, and double or multiple peaked. The star undergoes episodes of variable mass-loss rates that change by a factor of 1.7-2 on different timescales. We also observe changes in the ionization rate of Si II and determine a multiperiodic oscillation in the He I absorption lines, with periods ranging from a few hours to 22.5 days. We interpret the photospheric line variations in terms of oscillations in p-, g-, and strange modes. We suggest that these pulsations can lead to phases of enhanced mass loss. Furthermore, they can mislead the determination of the stellar rotation. We classify the star as a post-red supergiant, belonging to the group of alpha Cyg variables.

20 pages, 18 figures, 3 tables, accepted to Astronomy & Astrophysics

References in corpus (11)

• The IACOB project: I. Rotational velocities in Northern Galactic O and early B-type stars revisited. The impact of other sources of line-broadening
• Bright OB stars in the Galaxy IV. Stellar and wind parameters of early to late B supergiants
• Quantitative studies of the optical and UV spectra of Galactic early B supergiants I. Fundamental parameters
• Statistical properties of a sample of periodically variable B-type supergiants - Evidence for opacity-driven gravity-mode oscillations
• Most Detects G- and P-Modes in the B Supergiant HD 163899 (B2Ib/II)
• On the use of the Fourier Transform to determine the projected rotational velocity of line-profile variable B stars
• Mass loss from inhomogeneous hot star winds III. An effective-opacity formalism for line radiative transfer in accelerating, clumped two-component media, and first results on theory and diagnostics
• The puzzle of the CNO abundances of α Cygni variables resolved by the Ledoux criterion?
• On the H$\alpha$ Behaviour of Blue Supergiants: Rise and Fall over the Bi-stability Jump
• An Observational Evaluation of Magnetic Confinement in the Winds of BA Supergiants
• Wind structure of late B supergiants I. Multi-line analyses of near-surface and wind structure in HD 199 478 (B8 Iae)

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