A geometrical 1% distance to the short-period binary Cepheid V1334 Cygni

TL;DR

This study precisely measures the distance to the binary Cepheid V1334 Cyg, revealing the impact of binarity on the Period-Luminosity relation and providing highly accurate stellar masses, which enhances calibration of cosmic distance scales.

Contribution

The paper presents the most accurate geometric distance to a Cepheid using interferometry and spectroscopy, and highlights the significance of binarity in calibrating the Period-Luminosity relation.

Findings

Distance to V1334 Cyg is 720.35 +/- 7.84 pc with 1% accuracy.

Binarity significantly affects the star's integrated magnitude, especially in visible wavelengths.

Dynamical masses of the components are precisely determined: 4.288 and 4.040 solar masses.

Abstract

Cepheid stars play a considerable role as extragalactic distances indicators, thanks to the simple empirical relation between their pulsation period and their luminosity. They overlap with that of secondary distance indicators, such as Type Ia supernovae, whose distance scale is tied to Cepheid luminosities. However, the Period-Luminosity (P-L) relation still lacks a calibration to better than 5%. Using an original combination of interferometric astrometry with optical and ultraviolet spectroscopy, we measured the geometrical distance d = 720.35+/-7.84 pc of the 3.33 d period Cepheid V1334 Cyg with an unprecedented accuracy of +/-1 %, providing the most accurate distance for a Cepheid. Placing this star in the P-L diagram provides an independent test of existing period-luminosity relations. We show that the secondary star has a significant impact on the integrated magnitude, particularly at visible wavelengths. Binarity in future high precision calibrations of the P-L relations is not negligible, at least in the short-period regime. Subtracting the companion flux leaves V1334 Cyg in marginal agreement with existing photometric-based P-L relations, indicating either an overall calibration bias or a significant intrinsic dispersion at a few percent level. Our work also enabled us to determine the dynamical masses of both components, M1 = 4.288 +/- 0.133 Msun (Cepheid) and M2 = 4.040 +/- 0.048 Msun (companion), providing the most accurate masses for a Galactic binary Cepheid system.