Deneb Is a Large-amplitude Polarimetric Variable

Abstract

We write to report the discovery that Deneb is a large-amplitude polarization variable. Over a ∼400 days time span from 2022 August Deneb’s polarization was typically around 3900 parts per million (ppm) in the SDSS g′ band. Yet, it varied by several hundred parts per million in an irregular way on a timescale of weeks. The largest polarization change, amounting to 2500 ppm, occurred shortly after the last pulsation “resumption” event identified by Abt et al. in TESS photometry. The relationship between the observed polarization—particularly corresponding to the resumption event—and its brightness and H α spectra suggests a mechanism involving density changes in its wind and/or extended atmosphere. Smaller effects due to pulsations are not ruled out, and further study is recommended.