Seismic constraints on open clusters

¹ Institut d'Astronomie et d'Astrophysique, ULB, CP226, 1050 Brussels, Belgium e-mail: piau@astro.ulb.ac.be
² CEA/DSM/DAPNIA/Service d'Astrophysique, CE Saclay, 91191 Gif-sur-Yvette Cedex 01, France e-mail: [jballot;cturck]@cea.fr

Received: 3 June 2004
Accepted: 27 August 2004

Abstract

The aim of this theoretical and modelling paper is to derive knowledge on the global and structural parameters of low-mass stars using asteroseismology and taking advantage of the stellar collective behavior within open clusters. We build stellar models and compute the seismic signal expected from main sequence objects in the range. We first evaluate apparent magnitudes and oscillations-induced luminosity fluctuations expected in the Hyades, the Pleiades and the α Persei clusters. The closest cluster presents a feasible challenge to observational asteroseismology in the present and near future. The remainder of the work therefore focuses on the Hyades. 
We combine seismological and classical computations to address three questions: what can be inferred about 1) mass; 2) composition; and 3) extension of outer convection zones of solar analogs in the Hyades. The first issue relies on the strong sensitivity of the large separation to mass. We show that seismic constraints provide masses to a precision level () that is competitive with the actual mass estimations from binary systems. Then large separations () and second differences () are used to respectively constrain metal and helium fractions in the Hyades. When plotted for several masses, the relation of effective temperature () vs. large separation () is found to be strongly dependent on the metal content. Besides this the second difference main modulation is related to the second ionization of helium. An accuracy in the helium mass fraction of 0.02 to 0.01 can be achieved provided mass and age are accurately known, which is the case for a few Hyades binary systems. The second difference modulations are also partly due to the discontinuity in stellar stratification at the convective envelope/radiative core transition. They permit direct insight in the stellar structure. We compute acoustic radii of the convective bases for different values of the mixing length theory parameter in convection modelling, i.e. different convective efficiency in the superadiabatic layers. For a given effective temperature we show that the acoustic radius changes with convection efficiency. This suggests that seismology can provide constraints on the extension of outer convection and also more generally on the direct approaches of convection and dynamical phenomena being currently developed.