Correlation and variability of the He I 4471 and Mg II 4481 absorption lines in Be stars: A possible diagnostic tool for nonradial pulsations

In a total of 181 photographic spectrograms, obtained at the Manuel Foster Observatory of the Catholic University in Santiago (Chile), the equivalent widths of the absorption lines He I 4471 and Mg II 4481 have been determined for 10 selected southern Be stars and 4 normal B-type stars. The mean equivalent widths of each star are compared to published mean values from largers samples of B stars. On the average, Be stars coincide with normal B-type stars in their equivalent width. However, a striking difference between rapidly and slowly rotating stars in their line ratios (He I/Mg II) was found, which is probably an artifact due to a systematic misclassification of spectral types of rapidly rotating B stars (v sin i>200 km s^-1). Six of the ten Be stars show evidence for variability in He I and/or Mg II with time-scales shorter than a few days and amplitudes up to a factor 2 in equivalent width. Line variability occurs in the entire range of projected rotation velocity (70 km s^-1<v sin i<350 km s^-1), but seems to be restricted to spectral types B2-B4. In addition, variations at longer time-scales (years) were observed in two cases. Three of the variable stars (HR 4074, HR 4537, and α Ara) present correlated variations in both lines, one (μ Cen) anti-correlated variability. We interpret our results in terms of nonradial pulsations and stress the importance to study the He I/Mg II lines and their variability in a larger sample of Be, Bn, and normal B stars, including archival material in order to follow-up line variability for several decades in the past.