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± 0.07 . The distance has been found by analyzing the interstellar extinction maps to be D≈ 2400 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(λ 4959+λ 5007)/F(λ 4363) by a factor of ∼ 4 from 1945 to the present time. The [O III]/[O II] line ratio F(λ 4363)/F(λ 3727+λ 3729) 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≈ 41 000 K. We conclude that a decrease in the electron temperature and, possibly, electron density in the [O III] line formation region is mainly responsible for the spectroscopic variability.