The modelling of the interior of stars has made a lot of progress due to helioseismology. The unusually rich set of data available for the Sun has enabled extensive testing of physical models. For other stars the situation is less favorable. Even with the best solar models, the effect of rotation and the physics of a convective core are still poorly understood. The Sun is a slow rotator and has no convective core. Both type of physics have important effects on the mixing in the stellar interior. In order to derive the age of stars we must understand the physics of stellar interiors better than we do at present.
Normally the observables, of interest for the interior of a star, constitute a very small set and do not provide strong constraints on the physics, nor on the model parameters like mass and radius. Moreover, stellar oscillations in solar type stars are difficult to observe due to their low amplitudes (Frandsen 1997, Kjeldsen & Bedding 1997). To extend the range of observables we consider Scuti stars, which also exhibit a rich oscillation spectrum. For example, recent results for FG Vir (Breger et al. 1997, 1998) show the presence of 24 modes. Unfortunately, the modes in this class of pulsators are low order modes that are more difficult to analyse than the high order overtones observed in the Sun, so that it will be necessary to have a large data base spanning a long time scale and a large number of stars. This requirement can only be met by observing many pulsators simultaneously. It should also be emphasized, that additional high quality observations, photometric as well as spectroscopic, are needed to determine abundances (Z), rough initial stellar parameters and rotational velocities. Without these parameters it is impossible to use the observed frequencies to test the models.
The magnitude of the work to be carried out is clearly too large for a single institute. This led to the formation of STACC (Small Telescope Array with CCD Cameras) to search for suitable targets and subsequently to make multi-site observations (Frandsen, 1992).
This article begins with a section on the status of the search. New CM diagrams are needed for most clusters, and several such diagrams, obtained using CCD photometry, are presented. A first example of a good northern target for time series CCD photometry is presented: the open cluster NGC 1817. A revised strategy for the search for targets preceeds the conclusion.
© European Southern Observatory (ESO) 1998
Online publication: April 20, 1998