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 $\lambda$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$\beta$)=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] $\lambda$4959 and $\lambda$5007 lines and a decrease in the relative
intensity of the auroral [O III] $\lambda$4363 line, which has led to an
increase in the observed flux ratio $F(\lambda 4959+\lambda 5007)/F(\lambda
4363)$ by a factor of $\sim$4 from 1945 to the present time. The [O III]/[O II]
line ratio $F(\lambda 4363)/F(\lambda 3727+ \lambda3729)$ 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.
