标题：VEGA/CHARA interferometric observations of Cepheids - I. A resolved structure around the prototype classical Cepheid δ Cep in the visible spectral range
其他标题：I. A resolved structure around the prototype classical Cepheid
δ Cep in
the visible spectral range⋆
摘要：Context. The B-W method is used to determine the distance of Cepheids
and consists in combining the angular size variations of the star, as derived from
infrared surface-brightness relations or interferometry, with its linear size variation,
as deduced from visible spectroscopy using the projection factor. The underlying
assumption is that the photospheres probed in the infrared and in the visible are located
at the same layer in the star whatever the pulsation phase. While many Cepheids have been
intensively observed by infrared beam combiners, only a few have been observed in the
visible.
Aims. This paper is part of a project to observe Cepheids in the visible
with interferometry as a counterpart to infrared observations already in hand.
Methods. Observations of δ Cep itself were secured with the VEGA/CHARA
instrument over the full pulsation cycle of the star.
Results. These visible interferometric data are consistent in first
approximation with a quasi-hydrostatic model of pulsation surrounded by a static
circumstellar environment (CSE) with a size of θCSE = 8.9 ± 3.0 mas and a relative flux
contribution of fCSE = 0.07 ± 0.01. A model of visible
nebula (a background source filling the field of view of the interferometer) with the same
relative flux contribution is also consistent with our data at small spatial frequencies.
However, in both cases, we find discrepancies in the squared visibilities at high spatial
frequencies (maximum 2σ) with two different regimes over the pulsation
cycle of the star, φ = 0.0 −
0.8 and φ =
0.8−1.0. We provide several hypotheses to explain these
discrepancies, but more observations and theoretical investigations are necessary before a
firm conclusion can be drawn.
Conclusions. For the first time we have been able to detect in the
visible domain a resolved structure around δ Cep. We have also shown that a simple model
cannot explain the observations, and more work will be necessary in the future, both on
observations and modelling.
