| Issue |
A&A
Volume 471, Number 2, August IV 2007
|
|
|---|---|---|
| Page(s) | 661 - 669 | |
| Section | Stellar atmospheres | |
| DOI | https://doi.org/10.1051/0004-6361:20066853 | |
| Published online | 18 June 2007 | |
High-resolution spectroscopy for Cepheids distance determination*
II. A period-projection factor relation
1
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: nardetto@mpifr-bonn.mpg.de
2
Observatoire de la Côte d'Azur, Dpt. Gemini, UMR 6203, 06130 Grasse, France
3
Institute of Astronomy of the Russian Academy of Sciences, 48 Pjatnitskaya Str., Moscow 109017, Russia
4
Observatoire de Haute Provence, 04870 Saint-Michel l'Observatoire, France
Received:
1
December
2006
Accepted:
5
May
2007
Context.The projection factor is a key quantity for the interferometric Baade-Wesselink (hereafter IBW) and surface-brightness (hereafter SB) methods of determining the distance of Cepheids. Indeed, it allows a consistent combination of angular and linear diameters of the star.
Aims.We aim to determine consistent projection factors that include the dynamical structure of the Cepheids' atmosphere.
Methods.Hydrodynamical models of δ Cep
and 
Car have been used to validate a spectroscopic method of
determining the projection factor. This method, based on the
amplitude of the radial velocity curve, is applied to eight stars
observed with the HARPS spectrometer. The projection factor is
divided into three sub-concepts : (1) a geometrical effect, (2) the
velocity gradient within the atmosphere, and (3) the relative motion
of the “optical” pulsating photosphere compared to the
corresponding mass elements (hereafter 
). Both, (1) and (3)
are deduced from geometrical and hydrodynamical models,
respectively, while (2) is derived directly from observations.
Results.The
4896.439 Å line is found to be the best one to use
in the context of IBW and SB methods. A coherent and consistent
period-projection factor relation (hereafter Pp relation) is
derived for this specific spectral line: ![$p = [-0.064 \pm 0.020] \log P + [1.376 \pm 0.023]$](/articles/aa/abs/2007/32/aa6853-06/aa6853-06_tex_eq4.png)
. This procedure is then extended to
derive dynamic projection factors for any spectral line of any
Cepheid.
Conclusions.This Pp relation is an important tool for removing
bias in the calibration of the period-luminosity relation of
Cepheids. Moreover, it reveals a new physical quantity to
investigate in the near future.
Key words: techniques: spectroscopic / stars: atmospheres / stars: oscillations (including pulsations) / stars: variables: Cepheids / stars: distances
© ESO, 2007
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