Indications of stellar prominence oscillations on fast rotating stars: the cases of HK Aqr and PZ Tel

We present the analysis of six nights of spectroscopic monitoring of two young and fast rotating late-type stars, namely the dMe star HK Aqr and the dG/dK star PZ Tel. On both stars, we detect absorption features reminiscent of signatures of corotating cool clouds or prominences visible in H alpha. Several prominences on HK Aqr show periodic variability in the prominence tracks which follow a sinusoidal motion (indication of prominence oscillations). On PZ Tel, we could not find any periodic variability in the prominence tracks. By fitting sinusoidal functions to the prominence tracks, we derive amplitudes and periods which are similar to those of large-amplitude oscillations seen in solar prominences. In one specific event, we also derive a periodic variation of the prominence track in the H beta spectral line which shows an anti-phase variation with the one derived for the H alpha spectral line. Using these parameters and estimated mass density of a prominence on HK Aqr, we derive a minimum magnetic field strength of similar to 2 G. The relatively low strength of the magnetic field is explained by the large height of this stellar prominence (a parts per thousand yen 0.67 stellar radii above the surface).