HD203608, a quiet asteroseismic target in the old galactic disk

Context. We conducted a 5-night observing run with the spectrometer harps at the ESO 3.6- m telescope in August 2006, to continue exploring the asteroseismic properties of F-type stars. In fact, Doppler observations of F- type on the main sequence are demanding and remain currently limited to a single case (HD 49933). Comparison with photometric results obtained with the CoRoT space mission (Convection, Rotation and planetary Transits) on similar stars will be possible with an enhanced set of observations. Aims. We selected the 4th magnitude F8V star HD203608 to investigate the oscillating properties of a low- metallicity star of the old galactic disk. Methods. We reduced the spectra with the on- line data reduction software provided by the instrument. We developed a new statistical approach for extracting the significant peaks in the Fourier domain. Results. The oscillation spectrum shows a significant excess power in the frequency range [1.5, 3.0 mu Hz]. It exhibits a large spacing of about 120.4 mu Hz at 2.5 mu Hz. Variations of the large spacing with frequency are clearly identified, which require an adapted asymptotic development. The modes identification is based on the unambiguous signature of 15 modes with l = 0 and 1. Conclusions. These observations show the potential diagnostic of asteroseismic constraints. Including them in the stellar modeling significantly enhances the precision on the physical parameters of HD203608, resulting in a much more precise position in the HR diagram. The age of the star is now determined in the range 7.25 +/- 0.07Gyr.