The Milky Way Cepheid Leavitt law based on Gaia DR2 parallaxes of companion stars and host open cluster populations

¹ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
e-mail: louise.breuval@obspm.fr
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
⁵ GEPI, Observatoire de Paris, Université PSL, CNRS, 5 place Jules Janssen, 92190 Meudon, France
⁶ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
⁷ Universidad de Concepción, Departamento de Astronomía, Casilla 160-C, Concepción, Chile
⁸ Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland
⁹ Unidad Mixta Internacional Franco-Chilena de Astronomía (CNRS UMI 3386), Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
¹⁰ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹¹ Department of Physics, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
¹² INAF-Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone, Italy

Received: 10 June 2020
Accepted: 9 September 2020

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, K_S, and Wesenheit W_H 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.