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

TL;DR

This study reveals episodic variability in the stellar wind of planetary nebula IC 4997 through high-resolution spectral analysis over decades, showing changes in emission profiles and linking wind activity to nebular line variations.

Contribution

It provides the first detailed temporal analysis of IC 4997's spectral line profile changes and links stellar wind variability to episodic nebular emission fluctuations.

Findings

Hα P Cygni profile changed to single-peaked, indicating wind weakening

Broad Hα wings narrowed, suggesting decreased Raman scattering efficiency

High-velocity [N II] component disappeared, likely due to lower electron density

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 H$\alpha$ and [N\,{\sc ii}] emission line profiles, which had never been reported for this object. The H$\alpha$ 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$\alpha$ 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$50--60\,yr) variation in the [O\,{\sc iii}]$\lambda$4363/H$\gamma$ 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$\alpha$ 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.