A need to revise stellar opacities from asteroseismology of $δ$ Scuti stars
We construct seismic models of the four double-mode radial $δ$ Scuti stars adopting opacities from three databases: OPAL, OP and OPLIB. The aim is to find the models that fit the observed frequencies of the two radial modes and have the effective temperature and luminosity consistent with the observed values. Using the Bayesian analysis based on Monte Carlo simulations, we obtain that only the OPAL seismic models are caught within the observed error box in the HR diagram. Seismic models computed with the OP and OPLIB data are much cooler and less luminous. By including the relative amplitude of the bolometric flux variations (the so-called parameter $f$) into these simulations, we constrain the efficiency of convection in the envelopes, described by the mixing length parameter $α_{\rm MLT}$. We get $α_{\rm MLT}\approx 0.5$ for BP Peg, AE UMa and RV Ari (Population I stars) and $α_{\rm MLT}\approx 1.0$ for SX Phe (Population II star). For all the stars, overshooting from the convective core seems inefficient. A similar effect of opacity should occur also for classical Cepheids or RR Lyr stars that are used as standard candles to measure the universe.