An episodically variable stellar wind in the planetary nebula IC 4997

Abstract

IC 4997 is a planetary nebula well known by its variability. We present high-resolution spectra of IC 4997 obtained in 1993, 2019, and 2020 that reveal changes in the Hα and [N II] emission line profiles, which had never been reported for this object. The Hα P Cygni emission profile observed in 1993 changed to a single-peaked profile in 2019−2020, implying that the stellar wind has largely weakened. The very broad Hα emission wings narrowed by a factor of ∼2 between 1993 and 2019−2020, indicating that the efficiency of the Rayleigh–Raman scattering has noticeably decreased. A high-velocity [N II] nebular component detected in 1993 is missing in 2019 and 2020, probably due to a decrease in its electron density. A correlation exists between the strength of the stellar wind and the episodic (∼50−60 yr) variation in the [O III]λ4363/Hγ line intensity ratio, suggesting that an episodic, smoothly variable stellar wind is the main cause of the variability of IC 4997. Monitoring of that intensity ratio and of the Hα emission line profile in the coming years and new multiwavelength observations are key to unveiling the ongoing processes in IC 4997 and constraining the origin of the wind variability.