Constraints on the structure of the core of subgiants via mixed modes: the case of HD49385

The solar-like pulsator HD49385 was observed with the CoRoT satellite during 137 days. The analysis of its oscillation spectrum yielded precise estimates of the mode frequencies over nine radial orders and pointed out some unusual characteristics: there exist some modes outside the identified ridges in the echelle diagram and the curvature of the l=1 ridge significantly differs from that of the l=0 ridge. We here propose a toy-model to describe avoided crossings with a strong coupling between the p-mode and g-mode cavities and we show that the peculiar features of the oscillation spectrum of HD49385 can be accounted for only by a low-frequency l=1 avoided crossing. This leads us to establish the post-main-sequence status of HD49385. We then investigate the information which is brought by the mixed modes about the structure of the core. Having shown that traditional optimization techniques are ill-suited for stars with mixed modes in avoided crossing, we propose a new approach to the computation of grids of models, which we apply to HD49385. As a result, we show that the amount of core overshooting in HD49385 is either very low (0<alpha_ov<0.05) or moderate (0.18<alpha_ov<0.20). The mixing length parameter is found to be significantly lower than the solar one (alpha_CGM=0.55\pm0.04 compared to the solar calibrated value alpha_sun=0.64). We also find that the revised solar abundances of Asplund give a better agreement than the classical ones of Grevesse & Noels. Finally, we show that the strength of the coupling between the cavities, which can be estimated through the properties of the observed avoided crossing, plays a dominant role in constraining the parameters of the models. At each step, we investigate the origin and meaning of our seismic diagnostics in terms of the physical structure of the star.