Classical Cepheids have served as a fundamental tool for the calibration of the cosmic distance scale and as the result for determination of the Hubble constant ever since the discovery of the Cepheid period-luminosity (PL) relation by Miss Leavitt about one hundred years ago. Since Milky Way (MW) Cepheids are the only stars of this class accessible to trigonometric parallax measurements they are particularly important for calibration of this method.
Unfortunately in spite of a huge efforts to measure parallaxes for MW Cepheids the achieved precision was not good enough for precision determination of the Hubble constant. In particular the parallaxes from the second Gaia data release (GDR2) are affected by systematics due to the absence of chromaticity correction and occasionally by saturation.
An international team of astronomers in the course of the Araucaria project conducted in CAMK PAN (Grzegorz Pietrzyński) announced the most precise calibration of the PL relations of MW Cepheids. They adopted as a proxy for the parallaxes of 36 Galactic Cepheids, either the GDR2 parallaxes of their spatially resolved companions or the GDR2 parallax of their host open cluster. This allowed to bypass the systematics on the GDR2 parallaxes and provide very precise calibration of the fiducial PL relations. Based on this relations they determined H0 = 73.51 ± 2.53 km s-1Mpc-1
Fig. Period-luminosity diagram for Milky Way Cepheids in the Ks band calibrated with GDR2 parallaxes of Cepheids companions (blue) and open clusters (red).
