An in-depth study of HD 174966 with CoRoT photometry and HARPS spectroscopy. Large separation as a new observable for delta Sct stars

The aim of this work was to use a multi-approach technique to derive the most accurate values possible of the physical parameters of the delta Sct star HD174966. In addition, we searched for a periodic pattern in the frequency spectra with the goal of using it to determine the mean density of the star. First, we extracted the frequency content from the CoRoT light curve. Then, we derived the physical parameters of HD174966 and carried a mode identification out from the spectroscopic and photometric observations. We used this information to look for the models fulfilling all the conditions and discussed the inaccuracies of the method because of the rotation effects. In a final step, we searched for patterns in the frequency set using a Fourier transform, discussed its origin and studied the possibility of using the periodicity to obtain information about the physical parameters of the star. A total of 185 peaks were obtained from the Fourier analysis of the CoRoT light curve, being almost all reliable pulsating frequencies. From the spectroscopic observations, 18 oscillation modes were detected and identified, and the inclination angle ($62.5^{circ}$$^{+7.5}_{-17.5}$) and the rotational velocity of the star (142 km/s) were estimated. From the multi-colour photometric observations, 3 frequencies were detected, which correspond to the main ones in the CoRoT light curve. We looked for periodicities within the 185 frequencies and found a periodic pattern ~64 mu Hz. Using the inclination angle, the rotational velocity and an Echelle diagram, showing a double comb outside the asymptotic regime, we concluded that the periodicity corresponds to a large separation structure. The periodic pattern allowed us to discriminate models from a grid, finding that the value of the mean density is achieved with a 6% uncertainty. So, the pattern could be used as a new observable for A-F type stars.