Unlocking the potential of Cepheids as primary distance calibrators – UnlockCepheids

Our approach is based on the combination of multiple observing techniques (angular sizes measured by interferometry, spectroscopy, radial velocities, Gaia parallaxes) to constrain a coherent model of the pulsation of Cepheids through the SPIPS algorithm. This method presents the advantage of a high degree of robustness to biases.

We have applied the SPIPS technique to several Cepheid stars, the latest example being the long period pulsator RS Puppis (Kervella et al. 2017). We are working also on the fine analysis of the spectra of the Cepheids of our sample, thanks to several ongoing observing programs at different large telescopes (in particular at ESO). The development of a second version of the SPIPS algorithm modeling directly the line profile of the high resolution spectra (in place of radial velocity measurements) is currently being developed (HR-SPIPS). Thanks to the publication of the second Gaia data release (Gaia DR2), we have determined the binary fraction of the Milky Way Cepheids, and we have discovered resolved companions of about thirty of these stars.

The cornerstone of our program is the Gaia parallaxes. The second data release published in April 2018 (second «data release«) was however disappointing as the Cepheid parallaxes were generally unreliable. We have therefore revised our strategy. These parallaxes are the key to the calibration of all the parameters (in particular the projection factor) of all the Cepheids of our sample. We are actively preparing the immediate exploitation of the Gaia parallaxes from the day they are published. Thanks to our original approach based on the analysis of the resolved companions of Cepheids in binary systems, we have obtained a first calibration of the period-luminosity relation in the Milky Way.

We have published 18 articles bearing the ANR acknowledgements in refereed journals (Nature, Astronomy & Astrophysics, MNRAS, ApJ) as of April 2019. We have also presented the results of our work in 21 oral or poster communications at international conferences.

Cepheids are the backbone of the extragalactic distance ladder thanks to their tight period-luminosity (PL) relation. Although they contributed tremendously to most major discoveries of observational cosmology, the accuracy of their distance scale is now insufficiently competitive due to systematic uncertainties.

Through an innovative combination of state-of-the-art observations and modelling, we propose to assemble a comprehensive understanding of the pulsation and close environment of Cepheids (companions, circumstellar envelopes). We will take advantage of the upcoming Gaia data releases that will provide the parallaxes of several hundred Galactic Cepheids with an accuracy better than 5%. Our research project is designed to transform the exquisite Gaia parallaxes into a highly accurate Cepheid distance scale, as soon as they are released.

There is currently no competitive substitute for Cepheids as primary distance indicators. Reliable and accurate distances are mandatory to establish cosmological models on solid ground, and our project will bring a strong and lasting contribution in this context.