Simultaneous intensive photometry and high resolution spectroscopy of δ Scuti stars IV .

The δ Scuti star X Caeli has been the target of a simultaneous photometric (14 consecutive nights) and spectroscopic campaign (8 consecutive nigths). From the analysis of light curves we were able to pick up 17 frequency components, most of which were already detected in two previous campaigns. The comparison with the results of the previous campaigns shows that while some terms are rather stable (in particular the dominant mode at 7.39 cd ) others have conspicuous amplitude variations. 14 photometric terms have been also detected in the radial velocity curve or in the analysis of the line profile variations. There are no spectroscopic terms without photometric counterparts, and this means that there are no high–degree modes as observed in other δ Scuti stars. The simultaneous fit of light and line profile variations has allowed the estimation of the inclination of rotational axis (about 70) and the l,m identification for many modes. In particular there is clear evidence that the two shortest period modes are retrograde. Rather reliable results were found for the dominant mode which has l = 1, m = −1. The resulting physical parameters of its pulsation are in good agreement with the prediction of theoretical models and suggest for this star a mixing length parameter of about 0.5. Finally the fundamental stellar physical parameters and their refinement are discussed in the light of the identification of the 7.47 cd term as a radial mode.