Multi-site continuous spectroscopy .5. Rapid photospheric variability in the Be star 48 Persei from the MUSICOS 1989 campaign

Rapid variability in the photospheric He I 6678 line of the Be star 48 Per (HD 25940, HR 1273) has been detected from 258 high S/N CCD spectra taken with four 1.5-2.0 meter telescopes over three consecutive nights during the multisite spectroscopic MUSICOS 1989 campaign. 48 Per is a rather moderate-V sin i star, known to have presented slight long-term variations in the intensity of Balmer emission lines and in the V/R ratio. It is shown that the MUSICOS 1989 observations preceded a new activity phase. Search for line-profile variations, hereafter Ipv, was performed with time-series analysis using two methods (TF+CLEAN and Least-Squares) and with analysis of residuals. Weak blue-to-red and red-to-blue moving subfeatures with the same acceleration have been detected in the residuals. Their presence confirms that this star is seen under a moderate angle of inclination, i similar to 40 degrees, in agreement with estimates based on fundamental stellar parameters. A 6.04 c/d frequency, associated with the moving subfeatures mentioned above, has been firmly established from time-series analysis and corresponds more closely, in the frame of non-radial pulsations (NRP), to a tesseral mode (\m\ = l - 1 = 9 +/- 2). Two other possible frequencies (0.85 and 2.77 c/d) have been detected but need to be confirmed with new observations obtained over a longer time span. Despite additional spectra obtained at Haute Provence Observatory, we could not confirm the previous value of the orbital period or the amplitude of the radial velocity,curve of 48 Per, and therefore it was premature to search for tidally-forced oscillations.