Abstract
Context. As primary anchors of the distance scale, Cepheid stars play a crucial role in our understanding of the distance scale of the Universe because of their period-luminosity relation. Determining precise and consistent parameters (radius, temperature, color excess, and projection factor) of Cepheid pulsating stars is therefore very important. Aims. With the high-precision parallaxes delivered by the early third Gaia data release (EDR3), we aim to derive various parameters of Cepheid stars in order to calibrate the period-luminosity and period-radius relations and to investigate the relation of period to p-factor. Methods. We applied an implementation of the parallax-of-pulsation method through the algorithm called spectro-photo-interferometry of pulsating stars (SPIPS), which combines all types of available data for a variable star (multiband and multicolor photometry, radial velocity, effective temperature, and interferometry measurements) in a global modeling of its pulsation. Results. We present the SPIPS modeling of a sample of 63 Galactic Cepheids. Adopting Gaia EDR3 parallaxes as an input associated with the best available dataset, we derive consistent values of parameters for these stars such as the radius, multiband apparent magnitudes, effective temperatures, color excesses, period changes, Fourier parameters, and the projection factor. Conclusions. Using the best set of data and the most precise distances for Milky Way Cepheids, we derive new calibrations of the period-luminosity and period-radius relations: MKS = −5.529±0.015 − 3.141±0.050(log P − 0.9) and log R = 1.763±0.003 + 0.653±0.012(log P − 0.9). After investigating the dependences of the projection factor on the parameters of the stars, we find a high dispersion of its values and no evidence of its correlation with the period or with any other parameters such as radial velocity, temperature, or metallicity. Statistically, the p-factor has an average value of p = 1.26 ± 0.07, but with an unsatisfactory agreement (σ = 0.15). In absence of any clear correlation between the p-factor and other quantities, the best agreement is obtained under the assumption that the p-factor can take any value in a band with a width of 0.15. This result highlights the need for a further examination of the physics behind the p-factor.
Highlights
IntroductionThey link the distance scale in the Local Group with type Ia supernova host galaxies
We applied an implementation of the parallax-of-pulsation method through the algorithm called spectro-photointerferometry of pulsating stars (SPIPS), which combines all types of available data for a variable star in a global modeling of its pulsation
We present the application of the SPIPS method to a sample of classical Cepheids (CCs) for which we used the best and most complete data, in combination with the new Gaia EDR3 parallaxes, and we derive various precise and consistent parameters and investigate their dependences
Summary
They link the distance scale in the Local Group with type Ia supernova host galaxies. A thorough understanding of the pulsation of these stars is required to obtain the best accuracy on the Hubble constant H0 (Breuval et al 2020; Riess et al 2021). Obtaining accurate distances to Cepheid stars is still a nontrivial issue. Cepheid distances may be derived through main-sequence fitting for Cepheids in clusters or through the measurement of their parallax. Very precise geometric parallaxes for about 9500 Cepheids were measured by the Gaia satellite (Gaia Collaboration 2020), which is the first competitive alternative to Hubble Space Telescope (HST) parallaxes (Benedict et al 2007; Riess et al 2018)
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