Spectroscopic Variability of the Compact Planetary Nebula Hb 12

We present the results of our new low-resolution spectroscopic observations of the young compact planetary nebula Hb 12 performed in 2011-2020 with SAI MSU telescopes. We have measured the intensities of more than 50 nebular emission lines in the spectral range $λ$3687-9532, detected interstellar absorption features, and conducted a search for absorptions belonging to the possible secondary component of the central star. The extinction coefficient has been estimated from the Balmer decrement to be $c$(H$β$)=1.15$\pm$0.07. The distance has been found by analyzing the interstellar extinction maps to be $D\approx2400$ pc. We have traced the history of the spectroscopic observations of Hb 12, beginning with the first spectra taken by Aller (1951) in 1945. We have detected a systematic increase in the relative intensities of the nebular [O III] $λ$4959 and $λ$5007 lines and a decrease in the relative intensity of the auroral [O III] $λ$4363 line, which has led to an increase in the observed flux ratio $F(λ4959+λ5007)/F(λ4363)$ by a factor of $\sim$4 from 1945 to the present time. The [O III]/[O II] line ratio $F(λ4363)/F(λ3727+ \lambda3729)$ remains constant, suggesting that the degree of ionization, on average, for the nebula is invariable. The temperature of the exciting star has been estimated to be $T\approx41~000$ K. We conclude that a decrease in the electron temperature and, possibly, the electron density in the [O III] line formation region is mainly responsible for the spectroscopic variability.