An episodically variable stellar wind in the planetary nebula IC\,4997

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 H$α$ and [N\,{\sc ii}] emission line profiles, which had never been reported for this object. The H$α$ 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 H$α$ emission wings narrowed by a factor of $\sim$2 between 1993 and 2019--2020, indicating that the efficiency of the Rayleigh--Raman scattering has noticeably decreased. A high-velocity [N\,{\sc ii}] nebular component detected in 1993 is missing in 2019 and 2020, probably due to a decrease in its electron density. A correlation exists between the strength of the stellar wind and the episodic ($\sim$50--60\,yr) variation in the [O\,{\sc iii}]$λ$4363/H$γ$ 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 H$α$ 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.

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