Simón-Díaz, S.; Aerts, C.; Urbaneja, M. A.; Camacho, I.; Antoci, V.; Fredslund Andersen, M.; Grundahl, F.; Pallé, P. L.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 612, id.A40, 11 pp.
Fecha de publicación:
4
2018
Número de autores del IAC
3
Número de citas referidas
29
Descripción
Context. Despite important advances in space asteroseismology during the
last decade, the early phases of evolution of stars with masses above
15 M⊙ (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. Aim. Our goal is to detect,
analyze, and interpret variability in the early-B-type supergiant HD
2905 (κ 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
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: HD 2905 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 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 Hα,
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.
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