La dynamique atmosphérique des Céphéides et l'étalonnage des échelles de distance dans l'Univers

Abstract : Applying the Interferometric Baade-Wesselink method, hereafter IBW method, toVINCI/VLTI observations we determine the distance of seven Cepheids. For l Car, the more resolved star, we obtain an uncertainty on the derived distance of 5%. We also calibrate the zero-point of the Cepheid P-L relation with an uncertainty of 0.08 in magnitude. Interferometric measurements lead to angular diameter estimations over the whole pulsation period, while the stellar radius variations can be deduced from the integration of the pulsation velocity. The latter is linked to the observational velocity deduced from line profiles by the projection factor p. The spectral line profile, in particular its asymmetry, contains the whole physics present in the dynamical atmosphere of Cepheids. Firstly, using an hydrodynamical model of pulsating star, I define an interferometric version of the projection factor, associated to the photospheric pulsation velocity. This definition is of crucial importance in the IBW method, as it can induce a bias of up to 6% on the derived distance if the wrong projection factor is used. Secondly, by comparing the outputs of a geometric quasi-static model, and the observed spectral line profiles of HARPS (R=120000), I gain access to dynamical effects: limb-darkening variation with the pulsation phase, turbulences and velocity gradients. In particular, I found that long-period Cepheids with strong velocity gradients have a systematic shift in their asymmetry curve. This will be useful to future studies of dynamic projection factors. Thirdly, we note a correspondence between the line-forming region (in radius) and the amplitude of the visibility signature, which allows to obtain a geometrical view of the atmosphere of the star and in particular of velocity gradients. This is of crucial importance to constrain directly the projection factor from spectro-interferometric observations. The projection factor will be crucial in the context of AMBER/VLTI observations to determine the distance of about thirty Cepheids, with a uncertainty lower than 5%. This would lead to an uncertainty of about 0.01 magnitude on the zero-point of the P-L relation.
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Contributor : Nicolas Nardetto <>
Submitted on : Wednesday, May 17, 2006 - 10:03:43 AM
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Nicolas Nardetto. La dynamique atmosphérique des Céphéides et l'étalonnage des échelles de distance dans l'Univers. Astrophysique [astro-ph]. Université Nice Sophia Antipolis, 2005. Français. ⟨tel-00069295⟩

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