The rotational and magnetic properties of Polaris from long-term spectropolarimetric monitoring

TL;DR

This study presents the first direct measurement of Polaris' rotation period and magnetic field stability over five years, revealing a likely spin-orbit misalignment and providing insights into its magnetic and evolutionary properties.

Contribution

It provides the first direct measurement of Polaris' rotation period and magnetic field stability, and constrains its inclination and obliquity, offering new insights into its magnetic and evolutionary state.

Findings

Magnetic field remains stable over five years.

Rotation period of approximately 100 days measured.

Evidence suggests a significant spin-orbit misalignment.

Abstract

Polaris is a highly unusual Cepheid with observed properties that are difficult to reconcile with stellar evolutionary models. Since the initial detection of Polaris' magnetic field in 2020, we have conducted a magnetic monitoring campaign with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope. We compute Stokes $V$ least-squares deconvolution profiles and measure the associated mean longitudinal magnetic field strengths $\langle B_{z}\rangle$. The surface magnetic field has remained remarkably stable over five years of observations, with $\langle B_{z}\rangle$ varying between approximately $-3$ G and $+0.6$ G. From the periodic modulation of $\langle B_{z}\rangle$ we infer a stellar rotation period of $P_{\mathrm{rot}}=100.29\pm0.19$ days. This is the first direct measurement of $P_{\mathrm{rot}}$ for a classical Cepheid. Previous interferometric radius measurements and $P_{\mathrm{rot}}$ imply an equatorial rotation velocity of $v_{\mathrm{eq}}=23.3\pm0.2$ km s$^{-1}$. We set a conservative upper bound on the projected equatorial rotational velocity of $v_{\mathrm{eq}}\sin i_{\star} < 13.5$ km s$^{-1}$ and constrain the stellar inclination angle to be $i_{\star}<37^{\circ}$. Using the previously determined orbital solution, we find a high likelihood of a strong spin-orbit misalignment. We determine the lower bound on the obliquity angle between the stellar rotation and orbital axes to be $\beta>18.7^{\circ}$ at 99% confidence. We discuss the challenges in interpreting the origin and properties of the surface magnetic field in the context of Polaris' uncertain evolutionary history and the merger hypothesis.