# Toward a renewed Galactic Cepheid distance scale from Gaia and optical interferometry

## TL;DR

This paper introduces the SPIPS modeling tool that combines various observational data to improve the calibration of the Cepheid distance scale, leveraging Gaia data for enhanced accuracy.

## Contribution

The development and application of the SPIPS tool to Cepheids, providing new insights into the projection factor and its relation to pulsation period.

## Key findings

- The p-factor for long-period Cepheids is approximately constant at 1.29.
- SPIPS effectively combines radial velocity, photometry, and interferometry data.
- Gaia DR2 will significantly improve Cepheid distance measurements.

## Abstract

Through an innovative combination of multiple observing techniques and mod- eling, we are assembling a comprehensive understanding of the pulsation and close environment of Cepheids. We developed the SPIPS modeling tool that combines all observables (radial velocimetry, photometry, angular diameters from interferometry) to derive the relevant physical parameters of the star (effective temperature, infrared ex- cess, reddening,...) and the ratio of the distance and the projection factor d/p. We present the application of SPIPS to the long-period Cepheid RS Pup, for which we derive p = 1.25 +/- 0.06. The addition of this massive Cepheid consolidates the existing sample of p-factor measurements towards long-period pulsators. This allows us to conclude that p is constant or mildly variable around p = 1.29 +/- 0.04 (+/-3%) as a function of the pulsation period. The forthcoming Gaia DR2 will provide a considerable improvement in quantity and accuracy of the trigonometric parallaxes of Cepheids. From this sample, the SPIPS modeling tool will enable a robust calibration of the Cepheid distance scale.