Abstract
Aims. Classical Cepheids provide the foundation for the empirical extragalactic distance ladder. Milky Way Cepheids are the only stars in this class accessible to trigonometric parallax measurements. However, the parallaxes of Cepheids from the second Gaia data release (GDR2) are affected by systematics because of the absence of chromaticity correction, and occasionally by saturation. Methods. As a proxy for the parallaxes of 36 Galactic Cepheids, we adopt either the GDR2 parallaxes of their spatially resolved companions or the GDR2 parallax of their host open cluster. This novel approach allows us to bypass the systematics on the GDR2 Cepheids parallaxes that is induced by saturation and variability. We adopt a GDR2 parallax zero-point (ZP) of −0.046 mas with an uncertainty of 0.015 mas that covers most of the recent estimates. Results. We present new Galactic calibrations of the Leavitt law in the V, J, H, KS, and Wesenheit WH bands. We compare our results with previous calibrations based on non-Gaia measurements and compute a revised value for the Hubble constant anchored to Milky Way Cepheids. Conclusions. From an initial Hubble constant of 76.18 ± 2.37 km s−1 Mpc−1 based on parallax measurements without Gaia, we derive a revised value by adopting companion and average cluster parallaxes in place of direct Cepheid parallaxes, and we find H0 = 72.8 ± 1.9 (statistical + systematics) ±1.9 (ZP) km s−1 Mpc−1 when all Cepheids are considered and H0 = 73.0 ± 1.9 (statistical + systematics) ±1.9 (ZP) km s−1 Mpc−1 for fundamental mode pulsators only.
Highlights
Classical Cepheids (CCs) have a historical major importance among variable stars because of the simple correlation between the pulsation period and intrinsic luminosity, called the Leavitt law or the period–luminosity (PL) relation (Leavitt 1908; Leavitt & Pickering 1912)
The determination of the Hubble constant by Planck Collaboration VI (2020) exhibit a tension at the ∼5σ level with the latest empirical estimate by Riess et al (2019b) based on Large Magellanic Cloud (LMC) Cepheids combined with masers in NGC 4258 and Milky Way parallaxes measured by the Hubble Space Telescope (HST)/Fine Guidance Sensor (FGS), HST/WFC3, and Hipparcos
We presented an original calibration of the Milky Way Leavitt law based on GDR2 parallaxes of resolved Cepheid companions and on GDR2 parallaxes of open clusters hosting Cepheids
Summary
Classical Cepheids (CCs) have a historical major importance among variable stars because of the simple correlation between the pulsation period and intrinsic luminosity, called the Leavitt law or the period–luminosity (PL) relation (Leavitt 1908; Leavitt & Pickering 1912). The determination of H0 from the cosmic microwave background (CMB) based on the standard Λ cold dark matter (ΛCDM) model (Planck Collaboration VI 2020) is currently found to be in ∼5σ tension with the empirical or direct distance ladder measurements (Riess 2019) This tension may have important implications in cosmology, and may even point toward new physics beyond ΛCDM (Verde et al 2019). Calibrating the Leavitt law requires independent and accurate distance measurement for a sample of CCs. Gaia’s second data release (hereafter GDR2) contains a number of systematic effects that may reduce the precision of the parallaxes of CCs (Gaia Collaboration 2018). In the present paper our aim is to calibrate the Milky Way (MW) Cepheid Leavitt law using stars that are not affected by these issues and to benefit from the gain in precision afforded by cluster average parallaxes.
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