The Orbit and Dynamical Mass of Polaris: Observations with the CHARA Array

• DOI: 10.48550/arxiv.2407.09641
• Publication Date: 2024-08-01
• Authors: Nancy Remage Evans; Gail H. Schaefer; Alexandre Gallenne; Guillermo Torres; Elliott P. Horch; Richard I. Anderson; John D. Monnier; Rachael M. Roette...; Fabien Baron; Narsireddy Anugu; James W. Davidson; Pierre Kervella; Garance Bras; Charles Proffitt; Antoine Mérand; Margarita Karovska; Jeremy Jones; Cyprien Lanthermann; Stefan Kraus; Isabelle Codron; Howard E. Bond; Giordano Viviani
• Keywords: Cepheid variable stars; FOS: Physical sciences; Astrophysics; 01 natural sciences; 520; [SDU] Sciences of the Universe [physics]; QB460-466; Astrophysics - Solar and Stellar Astrophysics; [SDU]Sciences of the Universe [physics]; 0103 physical sciences; Stellar masses; Solar and Stellar Astrophysics (astro-ph.SR)

Abstract The 30 yr orbit of the Cepheid Polaris has been followed with observations by the Center for High Angular Resolution Astronomy (CHARA) Array from 2016 through 2021. An additional measurement has been made with speckle interferometry at the Apache Point Observatory. Detection of the companion is complicated by its comparative faintness—an extreme flux ratio. Angular diameter measurements appear to show some variation with pulsation phase. Astrometric positions of the companion were measured with a custom grid-based model-fitting procedure and confirmed with the CANDID software. These positions were combined with the extensive radial velocities (RVs) discussed by Torres to fit an orbit. Because of the imbalance of the sizes of the astrometry and RV data sets, several methods of weighting are discussed. The resulting mass of the Cepheid is 5.13 ± 0.28 M ⊙. Because of the comparatively large eccentricity of the orbit (0.63), the mass derived is sensitive to the value found for the eccentricity. The mass combined with the distance shows that the Cepheid is more luminous than predicted for this mass from evolutionary tracks. The identification of surface spots is discussed. This would give credence to the identification of a radial velocity variation with a period of approximately 120 days as a rotation period. Polaris has some unusual properties (rapid period change, a phase jump, variable amplitude, and unusual polarization). However, a pulsation scenario involving pulsation mode, orbital periastron passage, and low pulsation amplitude can explain these characteristics within the framework of pulsation seen in Cepheids.