Is the magnetic activity of Classical Cepheids modulated by pulsation? The case of eta Aquilae

Classical Cepheids are evolved descendants of massive B stars with complex evolutionary histories. They exhibit large-amplitude radial pulsations, and their Period-Luminosity relation (Leavitt Law) makes them a foundational element in the Cosmic Distance Ladder and crucial to the recent increase in the significance of the "Hubble tension". In addition, it has been shown that Cepheids exhibit a number of intriguing phenomenon including infrared excess, cycle-to-cycle line variability, UV variability and phase-dependent X-ray "flashes". There are also open questions regarding the rotational history of Cepheids, mass loss through stellar winds and a discrepancy in stellar masses derived from evolution models versus pulsation calculations. Despite the cosmological and evolutionary importance of Cepheids, virtually nothing is known about their magnetic properties and what role magnetic fields may play in these observed phenomena. To date, the only Cepheid with a confirmed magnetic field detection is eta Aquilae. Here we present preliminary results of magnetic measurements from high-resolution spectropolarimetry of eta Aquilae across its 7.2 d pulsation cycle. The Least-Squares Deconvolution (LSD) Stokes V signatures exhibit an unusual shape across all phases which may be attributable to velocity gradients due to pulsation. We also discuss a newly proposed magnitude limited systematic survey of the brightest Galactic Cepheids. The aim of this survey is to provide the first characterization of magnetic fields across the Cepheid instability strip and identify suitable targets for follow-up phase-resolved spectropolarimetric observations.