Spectroscopic and photometric variability of the O9.5 Vp star HD 93521

Aims: The line profile variability and photometric variability of the O9.5 Vp star HD 93521 are examined in order to establish the properties of the non-radial pulsations in this star.
Methods: Fourier techniques are used to characterize the modulations of the He i λλ 5876, 6678 and H α lines in several spectroscopic time series and to search for variations in a photometric time series.
Results: Our spectroscopic data confirm the existence of two periods of 1.75 and 2.89 h. The line profiles, especially those affected by emission wings, exhibit also modulations on longer time scales, but these are epoch-dependent and change from line to line. Unlike previous claims, we find no unambiguous signature of the rotational period in our data, nor of a third pulsation period (corresponding to a frequency of 2.66 d^-1).
Conclusions: HD 93521 very likely exhibits non-radial pulsations with periods of 1.75 and 2.89 h with l ≃ 8 ± 1 and l ≃ 4 ± 1 respectively. No significant signal is found in the first harmonics of these two periods. The 2.89 h mode is seen at all epochs and in all lines investigated, while the visibility of the 1.75 h mode is clearly epoch-dependent. Whilst light variations are detected, their connection to these periodicities is not straightforward.

Based on observations collected at the Observatoire de Haute Provence (France), the Flemish 1.2 m Mercator telescope at the Roque de los Muchachos observatory (La Palma, Spain) and the Observatorio Astronómico Nacional of San Pedro Mártir (Mexico).