Issue |
A&A
Volume 489, Number 3, October III 2008
|
|
---|---|---|
Page(s) | 1263 - 1269 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361:200809589 | |
Published online | 06 May 2008 |
High-resolution spectroscopy for Cepheids distance determination*
IV. Time series of H
line profiles
1
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: nardetto@mpifr-bonn.mpg.de
2
Observatoire de Haute-Provence, 04870 Saint-Michel l'Observatoire, France
Received:
15
February
2008
Accepted:
1
April
2008
Context. In recent years, infrared interferometry has revealed the presence of faint dusty circumstellar envelopes (CSE) around Cepheids. However the size, shape, chemical nature, and the interaction of the CSE with the star itself are still under investigation. The presence of a CSE might have an effect on the angular diameter estimates used in the interferometric Baade-Wesselink and surface-brightness methods of determining the distance of Cepheids.
Aims. By studying Hα profiles as a function of the period, we investigate the permanent mass loss and the CSE around Cepheids. Our high spectral- and time-resolution data, combined with a very good S/N, will be useful in constraining future hydrodynamical models of Cepheids atmosphere and their close environment.
Methods. We present HARPS (High Accuracy Radial velocity
Planetary Search project developed by the European Southern
Observatory.) high-resolution spectroscopy (R = 120 000) of eight
galactic Cepheids: R Tra, S Cru, Y Sgr, β Dor, ζ Gem,
RZ Vel, Car, and RS Pup, providing a good period sampling
(
d to
d). The Hα line profiles are
described for all stars using a 2D (wavelength versus pulsation
phase) representation. For each star, an average spectral line
profile is derived, together with its first moment
(γ-velocity) and its asymmetry (γ-asymmetry).
Results. Short-period Cepheids show Hα line profiles following the
pulsating envelope of the star, while long-period Cepheids show very
complex line profiles and, in particular, large asymmetries. We find
a new relationship between the period of Cepheids and their
γ-velocities and -asymmetries. These results may be related
to the dynamical structure of the atmosphere and to a permanent mass
loss of Cepheids. In particular, we confirm for Car a
dominant absorption component whose velocity is constant and nearly
of zero km s-1 in the stellar rest frame. This component is
attributed to the presence of circumstellar envelope.
Conclusions. To understand these very subtle γ effects, fully consistent hydrodynamical models are required, including pulsating and evolutionary theories, convective energy transport, adaptive numerical meshes, and a refined calculation of the radiative transfer.
Key words: techniques: spectroscopic / stars: atmospheres / stars: oscillations / stars: variables: Cepheids / stars: distances
© ESO, 2008
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