Deneb is a Large Amplitude Polarimetric Variable

TL;DR

Deneb exhibits large amplitude polarization variability over hundreds of ppm on weekly timescales, likely linked to wind density changes or atmospheric phenomena, with potential pulsation influences.

Contribution

This study reports the first detection of large amplitude polarization variability in Deneb, revealing new insights into its atmospheric and wind dynamics.

Findings

Polarization varies irregularly by hundreds of ppm over weeks.

Largest polarization change occurred after a pulsation resumption event.

Polarization changes are linked to wind density and atmospheric variations.

Abstract

We write to report the discovery that Deneb is a large amplitude polarization variable. Over a ~400 d time span from August 2022 Deneb's polarization was typically around 3900 parts-per-million (ppm) in the SDSS g'-band. Yet, it varied by several hundred ppm 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. (2023) in TESS photometry. The relationship between the observed polarization -- particularly corresponding to the resumption event -- and its brightness and H-alpha 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.