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A 5-night asteroseismic observation of the F8V star HD 203608 was conductedin August 2006 with HARPS, followed by an analysis of the data, and apreliminary modeling of the star (Mosser et al. 2008). The stellar parameterswere significantly constrained, but the behavior of one of the seismic indexes(the small spacing d01) could not be fitted with the observed one, even withthe best considered models. We study the possibility of improving the agreementbetween models and observations by changing the physical properties of theinner parts of the star (to which d01 is sensitive). We show that, in spite ofits low mass, it is possible to produce models of HD 203608 with a convectivecore. No such model was considered in the preliminary modeling. In practice, weobtain these models here by assuming some extra mixing at the edge of the earlyconvective core. We optimize the model parameters using the Levenberg-Marquardtalgorithm. The agreement between the new best model with a convective core andthe observations is much better than for the models without. All theobservational parameters are fitted within 1-sigma observational error bars.This is the first observational evidence of a convective core in an old andlow-mass star such as HD 203608. In standard models of low-mass stars, the corewithdraws shortly after the ZAMS. The survival of the core until the presentage of HD 203608 provides very strong constraints on the size of the mixed zoneassociated to the convective core. Using overshooting as a proxy to model theprocesses of transport at the edge of the core, we find that to reproduce bothglobal and seismic observations, we must have alpha_ov = 0.17 +/- 0.03 Hp forHD 203608. We revisit the process of the extension of the core lifetime due toovershooting in the particular case of HD 203608.

Survival of a convective core in low-mass solar-like pulsator HD 203608