Surprising Variability of the Planetary Nebula IC 4997 $=$ QV Sge

Abstract

We present the results of a new epoch (2009–2019) of a long-term (50 years) photometric monitoring of the variable planetary nebula IC 4997 (QV Sge). The integral (star + nebula) $UBV$ light curves display a continuing brightening of $0\overset{m}{.}15$ in $V$, a slight rise (${<}0\overset{m}{.}1$) in $B$, and constancy in $U$. The $B-V$ color has got redder from $0\overset{m}{.}4$ in 2000 to $0\overset{m}{.}7$ in 2019, whereas the $U-B$ color has not changed significantly at that time. We carried out near infrared (IR) $JHKL$ photometry in 2019, and comparing it to the data obtained in 1999–2006 we found the source to be fainter by $0\overset{m}{.}4$ in $L$ and bluer by $0\overset{m}{.}2$ in the $K-L$ color. The long-term brightness variations in the optical and IR regions are shown to be due mostly to the changing input of emission lines to the integral light. Low-dispersion spectroscopic observations carried out in 2010–2019 revealed a continuing decrease in the [O III] $\lambda$4363 to H$\gamma$ intensity ratio: it decreased by a factor of ${\sim}3$ in 30 years and reached the level of 1960–1970. We discovered that the absolute intensities of [O III] $\lambda$4959 and $\lambda$5007 nebular lines had increased by a factor of ${>}2$ from 1990 to 2019, whereas the [O III] $\lambda$4363 auroral line had weakened by a factor of 2 comparing to the maximum value observed in 2000. The variation of H$\beta$ absolute intensity in 1960–2019 was shown to be similar to that of [O III] $\lambda$4959 (and $\lambda$5007), but of smaller amplitude. The electron density in the outer part of the nebula was estimated from the [S II] and [Cl III] lines. Basing on the data on absolute intensities for the H$\beta$, [O III] $\lambda 4363,4959$ lines and their ratios we propose a possible scenario describing the change of physical conditions ($N_{e}$, $T_{e}$) in IC 4997 in 1970–2019. The main features of spectral variability of IC 4997 could be explained by a variation of electron temperature in the nebula caused by not so much the change in ionizing flux from the central star as the variable stellar wind and related processes. The photometric and spectral changes observed for IC 4997 in 1960–2019 may be interpreted as an observable consequence of a single episode of enhanced mass loss from the variable central star.