Improving Milky Way Cepheid distance estimations with high-resolution spectroscopy

Séminaire IPAG de Simon Borgniet (LESIA), jeudi 27 juin 2019 à 11h00, salle Manuel Forestini

Classical Cepheid pulsators have been primary galactic and extragalactic distance indicators for a century, thanks to their Period-Luminosity (P-L) relationships. They have thus tremendously contributed to precision cosmology. However, the accuracy of the Cepheid P-L relations is now inadequate, and an independent calibration of Cepheid distances is required. An elegant and quasi-geometrical way of measuring distances of bright, nearby Milky Way (MW) Cepheids is the Parallax-of-Pulsation (PoP) technique. This method relies on computing the ratio of the Cepheid linear radius variation (measured by integrating the radial velocity curve over the pulsation phase) over the angular diameter variation (measured directly through long-base interferometry).

Still, the PoP method is affected by systematic errors (at a 5 to 10 % level) induced by the so-called projection factor, i.e. the conversion factor between the measured radial velocities and the true, physical Cepheid pulsation velocities. This projection factor is fully degenerate with the derived distance. So far, the projection factor has only been either approximated as a constant, or calibrated over a sample of Cepheids with measured parallaxes, that cannot yet bring the level of accuracy and precision needed.

In my presentation, I will detail how an in-depth analysis of high-resolution spectroscopy of Milky Way Cepheids can contribute to improving the Parallax-of-Pulsation method and Cepheid distance estimations, by providing : (a) consistent radial velocities and corresponding projection factors ; (b) precise effective temperatures ; and (c) potentially allowing to estimate directly the true Cepheid pulsation photospheric velocities, thus "bypassing" the projection factor.