Unlocking the projection factor of Cepheids – Unlock-pfactor

We will use a novel generation of interferometer, dedicated photometric and spectroscopic observations as well as state-of-the-art models of Cepheids,

We expect to better understand the projection factor of Cepheids.and open a new road toward Ho.

The BW method, if robust, has the potential in the next decade to test the Hubble tension, by providing the distance of Cepheids in the Local Group (LG) and beyond, individually, using the spectroscopic capabilities of ANDES and MOSAIC instruments at the focus of the ELT.

The publications will be listed here (https://lagrange.oca.eu/fr/unlockpfactor-publications) and on HAL

Cepheids are the backbone of the extragalactic distance ladder. For instance, the discovery of the accelerated expansion of the Universe (Nobel Prize 2011) is largely based on the Cepheids. However, there is currently a 5 sigma tension between the acceleration rate of the universe derived from the cosmic microwave background and the one derived from the distance ladders. If confirmed, the tension would mean that the lambda-CDM model of the universe need refinements. The goal of this ANR project is to open a new route toward Ho using the Baade-Wesselink method (BW) of Cepheid distance determination. The concept of the method is simple: The variation of the angular diameter (from surface brightness-color relations or interferometry) is compared to the variation of the linear diameter (from the integration of the radial velocity). The distance of the Cepheid is then obtained by a simple division of the linear and angular amplitudes. The major weakness of the BW technique is that it uses a numerical factor to convert disk-integrated radial velocities into photospheric velocities, the projection-factor. This factor, whose value is typically around 1.3, characterises simultaneously the spherical geometry of the pulsating star, the limb darkening, and the difference in velocity between the photosphere and the line-forming regions ! Due to this intrinsic complexity, the p-factor is currently uncertain to ~7%, and accounts for almost all the systematic uncertainties of the Galactic Cepheid BW distances. Using a novel generation of interferometer, dedicated photometric and spectroscopic observations as well as state-of-the-art models of Cepheids, we aim to Unlock the projection factor. The BW method, if robust, has the potential in the next decade to test the Hubble tension, by providing the distance of Cepheids in the Local Group (LG) and beyond, individually, using the spectroscopic capabilities of ANDES and MOSAIC instruments at the focus of the ELT.