A delta Scuti star in the post-MS contraction phase: 44 Tauri

Context. The evolutionary stage of the delta Scuti star 44 Tau has been unclear. Recent asteroseismic studies have examined models on the main sequence, as well as in the expansion phase of the post-main sequence evolution. However, these models could not reproduce all of the observed frequencies, the mode instability range, and the fundamental stellar parameters simultaneously. A recent photometric study has increased the number of detected independent modes in 44 Tau to 15, and a newly found gravity mode at 5.30 cd(-1) extends the observed frequency range. Aims. One of the possible evolutionary stages of 44 Tau has not yet been considered: the overall contraction phase after the main sequence. We computed asteroseismic models to examine whether models in this evolutionary stage provide a better fit of the observed frequency spectrum. Methods. We used Dziembowski's pulsation code to compute nonadiabatic frequencies of radial and nonradial modes. Observation of two radial modes and an avoided crossing of dipole modes put strong constraints on the models. A two-parameter overshooting routine is utilized to determine the efficiency of element mixing in the overshoot layer above the convective core. Results. We find that pulsation models in the post-MS contraction phase successfully reproduce the observed frequency range, as well as the frequency values of all individual radial and nonradial modes. The theoretical frequencies of the mixed modes at 7.79 cd(-1) and 9.58 cd(-1) are in better agreement with the observations if efficient element mixing in a small overshoot layer is assumed.