Radial and Pulsation Velocities of Cepheids from High-Resolution Spectra

Simon Borgniet, Pierre Kervella, Antoine Mérand, Boris Trahin and Alexandre Gallenne

The calibration of the Period-Luminosity (P-L) relation of Cepheids through the Baade-Wesselink (BW) technique is affected by systematic errors due to uncertainties on the projection (p-) factor. The p-factor is the conversion parameter between the observed radial velocities and the actual pulsational velocities of Cepheids and depends on the limb-darkening and the stellar atmosphere dynamics. Thus, it is the main obstacle to an accurate enough distance determination with the BW method.
We have recently developed the Spectro-Photo-Interferometry of Pulsating Stars (SPIPS) tool, that models the pulsation of Cepheids and estimates their physical parameters (including the p-factor) based on the combination of atmosphere models and all observables. The next step in this approach is to estimate directly the pulsational velocities (v_puls) of Cepheids by comparing observed high-resolution (HR) spectra with synthetic Cepheid spectra built at different v_puls and temperatures. This HR-SPIPS approach will allow us to compute and characterize the p-factor in a new, spectroscopic way.

Proceedings of the Polish Astronomical Society, vol. 6, 208-212 (2018)

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