Seismology of delta Scuti stars in the Praesepe cluster - I. Ranges of unstable modes as predicted by linear analysis versus observations

We consider here a representative set of delta Scuti stars observed in the Praesepe cluster, thanks to recent efforts of multi-site networks. We show that the effect of rotation on the determination of fundamental parameters M-bol and T-eff cannot be neglected when modelling fast rotators as delta S cuti stars. We propose a method to take this effect into account and gain s ome indication on the rotation rate of the stars. We then use this unique homogeneous set of observational data to investigat e the pulsational behaviour of delta Scuti stars for different masses, corr esponding to different evolutionary stages at the age of the Praesepe clust er. We propose here an alternative approach to seismology of delta Scuti st ars, trying to avoid specific but questionable mode identifications. Via a large set of models, we compare the observed modes with the unstable modes as obtained from a linear stability analysis including a nonlocal time-depe ndent treatment of convection. We show that, in the framework of the linear analysis adopted here, a satis fying agreement can be reached between the predicted ranges of unstable mod es and the ranges derived from observations. However we found that for "low -mass" delta Scuti stars (1.6-1.80 M.) the agreement is found only for a re stricted range of values of the mixing length parameter. These stars thus b ring a valuable observational constraint on the description of the convecti on in the outer layers of stars.