Surprising Variability of the Planetary Nebula IC 4997 = QV Sge

Arkhipova, V.P.; Burlak, M.A.; Ikonnikova, N.P.; Komissarova, G.V.; Esipov, V.F.; Shenavrin, V.I.

Авторы: Arkhipova V.P., Burlak M.A., Ikonnikova N.P., Komissarova G.V., Esipov V.F., Shenavrin V.I.
Журнал: Astronomy Letters
Том: 46
Номер: 2
Год издания: 2020
Издательство: Maik Nauka/Interperiodica Publishing
Местоположение издательства: Russian Federation
Первая страница: 100
Последняя страница: 119
DOI: 10.1134/S1063773720020012
Аннотация: 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 0m. 15 in V , a slight rise (<0m. 1) in B, and constancy in U. The B−V color has got redder from 0m. 4 in 2000 to 0.7 mag in 2019, whereas the U−B color has not changed significantly at that time. We carried out near-infrared (IR) JHKLphotometry in 2019, and comparing it to the data obtained in 1999–2006 we found the source to be fainter by 0.4 mag in L and bluer by 0.2 mag 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] λ4363 to Hγ intensity ratio: it decreased by a factor of ∼3 in 30 years and reached the level of 1960–1970. We discovered that the absolute intensities of [O III] λ4959 and λ5007 nebular lines had increased by a factor of >2 from 1990 to 2019, whereas the [O III] λ4363 auroral line had weakened by a factor of 2 comparing to the maximum value observed in 2000. The variation of Hβ absolute intensity in 1960–2019 was shown to be similar to that of [OIII] λ4959 (and λ5007), but of smaller amplitude. The electron density in the outer part of the nebula was estimated from the [SII] and [Cl III] lines. Basing on the data on absolute intensities for the Hβ, [OIII] λ4363, 4959 lines and their ratios we propose a possible scenario describing the change of physical conditions (Ne, Te) 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.
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Surprising Variability of the Planetary Nebula IC 4997 = QV Sgeстатья

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