Low-frequency photospheric and wind variability in the early-B supergiant HD2905

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    S. Simon-Diaz, Univ La Laguna, Universidad de la Laguna, Dept Astrofis, C. Aerts, Univ Calif Santa Barbara, University of California Santa Barbara, University of California System, Kavli Inst Theoret Phys, M. A. Urbaneja, Univ Innsbruck, University of Innsbruck, Inst Astro & Teilchenphys, I. Camacho, Univ La Laguna, Universidad de la Laguna, Dept Astrofis,
  • V. Antoci
  • M. Fredslund Andersen
  • F. Grundahl
  • P. L. Palle, Univ La Laguna, Universidad de la Laguna, Dept Astrofis

Context. Despite important advances in space asteroseismology during the last decade, the early phases of evolution of stars with masses above similar to 15 M circle dot (including the O stars and their evolved descendants, the B supergiants) have been only vaguely explored up to now. This is due to the lack of adequate observations for a proper characterization of the complex spectroscopic and photometric variability occurring in these stars.

Aims. Our goal is to detect, analyze, and interpret variability in the early-B-type supergiant HD2905 (kappa Cas, B1 Ia) using long-term, ground-based, high-resolution spectroscopy.

Methods. We gather a total of 1141 high-resolution spectra covering some 2900 days with three different high-performance spectrographs attached to 1-2.6m telescopes at the Canary Islands observatories. We complement these observations with the HIPPARCOS light curve, which includes 160 data points obtained during a time span of similar to 1200 days. We investigate spectroscopic variability of up to 12 diagnostic lines by using the zero and first moments of the line profiles. We perform a frequency analysis of both the spectroscopic and photometric dataset using Scargle periodograms. We obtain single snapshot and time-dependent information about the stellar parameters and abundances by means of the FASTWIND stellar atmosphere code.

Results. HD2905 is a spectroscopic variable with peak-to-peak amplitudes in the zero and first moments of the photospheric lines of up to 15% and 30 km s(-1), respectively. The amplitude of the line-profile variability is correlated with the line formation depth in the photosphere and wind. All investigated lines present complex temporal behavior indicative of multi-periodic variability with timescales of a few days to several weeks. No short-period (hourly) variations are detected. The Scargle periodograms of the HIPPARCOS light curve and the first moment of purely photospheric lines reveal a low-frequency amplitude excess and a clear dominant frequency at similar to 0.37 d(-1). In the spectroscopy, several additional frequencies are present in the range 0.1-0.4 d(-1). These may be associated with heat-driven gravity modes, convectively driven gravity waves, or sub-surface convective motions. Additional frequencies are detected below 0.1 d(-1). In the particular case of H alpha, these are produced by rotational modulation of a non-spherically symmetric stellar wind.

Conclusions. Combined long-term uninterrupted space photometry with high-precision spectroscopy is the best strategy to unravel the complex low-frequency photospheric and wind variability of B supergiants. Three-dimensional (3D) simulations of waves and of convective motions in the sub-surface layers can shed light on a unique interpretation of the variability.

Original languageEnglish
Article number40
JournalAstronomy & Astrophysics
Number of pages11
StatePublished - 17 Apr 2018

    Research areas

  • stars: early-type, rotation, stars: fundamental parameters, stars: oscillations, techniques: spectroscopic, LINE-PROFILE VARIATIONS, ATMOSPHERIC NLTE-MODELS, SPACE-BASED PHOTOMETRY, H-ALPHA, O-STARS, SPECTROSCOPIC ANALYSIS, BLUE STARS, ASTEROSEISMOLOGY, PARAMETERS, SPECTRA

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