FG Sge: New Multicolor Photometry and Short-Term Dust Shell Clearing in 2019

Abstract

We present the results of a new stage of the long-term photometric study of FG Sge which is a quickly evolving central star of the planetary nebula Hen 1-5. Our new observations carried out on the SAI MSU telescopes in the optical ($BVR_CI_C$) and infrared (IR) ($JHKLM$) regions in 2008-2021 and 2013-2021, respectively, allowed us to trace the evolution of the star's brightness in recent years. The most significant observations were performed in 2019 when the star suffered a short clearing of the dust shell and became visible in $BVR_C$. Based on the spectral energy distribution of FG Sge in the 0.4-5 $\mu$m wavelength range we derived the dust shell parameters: the size of dust grains $a=0.01\mu$m, the inner radius temperature $T_{\text{dust}}=900$ K, optical depth $\tau (K)=0.5$ ($\tau (V)=4.5$), the total mass of dust $M_{\text{dust}}=7\cdot10^{-5} M_{\odot}$. After the short-term clearing of the dust shell in 2019, another dust structure was ejected that resulted in the star fading in all the observed bands. Based on the IR brightness and color curves, we estimated the dust depth growth in 2019-2020.