Cyclic phenomena in the circumstellar envelope of the A0e/B9e Herbig star AB Aur

An analysis of spectra of the well known Herbig Ae/Be star AB Aur obtained with a high resolution Coude spectrograph (R similar to 20000-25000) installed on the 2.6-m Shejn telescope at the Crimean Astrophysical Observatory (Ukraine) and a medium resolution echelle spectrometer (R similar to 14000) on the 2-m Zeiss telescope at the Shemakha Astrophysical Observatory of the National Academy of Sciences of Azerbaijan yielded an unexpected result. It turns out the the variations in the equivalent widths EW of the HeI 5876 and D NaI emission lines have an inverse correlation (r=-0.67 +/- 0.14 and -0.73 +/- 0.10 according to the data from the CrAO and the ShAO, respectively), while the correlation is direct for the H alpha and HeI 5876 lines (r=+0.68 +/- 0.14 and +0.59 +/- 0.14 according to the data from the CrAO and the ShAO, respectively). The Lafler-Kinman method gives a period P=123.7 days for the variation in the EW of these lines. On phase diagrams plotted for this period, the variations in the EW are sinusoidal, with the variations in EW(HeI) and EW(DNaI) in counterphase, while EW(HeI) and EW(H alpha) are in phase. A possible interpretation of this phenomenon is examined. The need for new spectral observations of AB Aur in order to confirm the observed effects on a larger statistical sample is emphasized.