LESIA - Observatoire de Paris

Photospheric velocities of Cepheids from high-resolution spectroscopy

lundi 16 juillet 2018, par Simon Borgniet (LESIA)

Jeudi 19 juillet 2018 à 16h00 , Lieu : Salle de conférence du bâtiment 17

Cepheids are primary extragalactic distance indicators as their pulsation period correlates with their luminosity — the so-called Period-Luminosity relation. Therefore, their contribution to precision cosmology is tremendous. However, the Cepheid distance scale needs to be accurately calibrated due to intrinsic biases in the P-L relation. The Baade-Wesselink (BW, or Parallax of Pulsation) method allows to derive Cepheid distances in a quasi-geometrical way, by computing the ratio of the linear radius variation (through the integration of the velocity curve) over the angular diameter variation during the Cepheid pulsation phase.

Still, the BW technique is affected by significant uncertainties due to the so-called projection (p-) factor that lies between the real pulsation velocities (VP) and the measured, disc-integrated radial velocities (RV). Classical BW-derived distances are thus fully degenerate with this p-factor.

I will present here the first results of a new spectroscopic approach, that aims at deriving directly the Cepheid photospheric (pulsation) velocities instead of the RV through the comparison of observed Cepheid line profiles with a library of synthetic line profiles generated for different VP, effective temperatures and other stellar parameters. Such an approach should allow a new insight inside the p-factor and its different components, and should lead to more accurate BW Cepheid distances.

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