Dating young open clusters using δ Scuti stars

Abstract

Aims. The main goal of this work is to date young open clusters using δ Sct stars. Seismic indices such as the large separation and the frequency at maximum power can help constrain the models to better characterise the stars. We propose a reliable method to identify some radial modes, which would give us greater confidence in the constraints placed on the models. Methods. We extracted the frequency content of a sample of δ Sct stars belonging to the same open cluster. We estimated the low-order large separation by means of different techniques and the frequency at maximum power for each member of the sample. We used a grid of models built with the typical parameters of δ Sct stars, including mass, metallicity, and rotation as independent variables, and we determined the oscillation modes. We selected the observed frequencies whose ratios match those of the models. Once we established a range of radial modes matching the observed frequencies, mainly the fundamental mode, we added it to the other seismic parameters to derive the stellar ages. Assuming star groups have a similar chemistry and age, we estimated their mean ages by computing a weighted probability density function fit to the age distribution of the seismically constrained models. Results. We estimated the age of Trumpler 10 to be 30−20+30 Myr and that of Praesepe to be 580 ± 230 Myr. In this latter case, we find two apparent populations of δ Sct stars in the same cluster, one at 510 ± 140 Myr and another at 890 ± 140 Myr. This may be due to two different stellar formation events, a variety of rotational velocities among the members in our sample of stars (as rapid rotation may modify the observed large separation) or membership to unresolved binary systems.