Stellar seismology by mean of multisite coordinated observations

Scientific context:

Our purpose is to improve our understanding and description of the physical processes at work in the interior of stars. These processes ( here, principally transport mechanisms of chemical species, angular momentum, heat ...) induce important uncertainties (over 50 %) on the age determination for intermediate mass stars. This affects several connex field of research. These stars are, for instance, used as " traceurs" of the Galactic dynamical evolution. The study of the chemical evolution of the universe is also based on the understanding of the stellar interiors in which the different species are formed through nucleosynthesis.

Beyond the strict astrophysical interest, stellar interiors constitute laboratories to test several aspects of fondamental physics (eg opacities, nuclear reaction rates, equation of state,...) under conditions which cannot be reproduced in laboratories.

Delta Scuti stars are pulsating stars representative of normal A-type stars on the main sequence and in the following rapid phase of shell Hydrogen burning. Their oscillation spectra are rich (more than 20 oscillations detected in some of them) and their structure makes possible the existence of mixed modes,  carrying information on the very internal parts of the star. They thus are considered as very good candidates for seismological study of the physical processes at work in intermediate mass stars on the main sequence.
 

The STEPHI campaigns: (an overview)

For more than ten years now, STEPHI is harvesting multisite rapid photometry data of delta Scuti stars on the main sequence. The STEPHI campaigns are three-weeks long, which corresponds to a 0.5 microHz resolution in the Fourier spectra. With three-observational sites involved, we currently obtain a 40% effective observational coverage and the ambiguity due to daily aliases in the analysis is removed. and the  In several occasions, we found close peaks in the oscillations spectra of objects, justifying the necessity of such a high resolution.

Nine campaigns have already been realized, during which 12 delta scuti stars have been observed (overview). This represents a significant amount of the data available on delta Scuti stars for seismological study. For more informations about these observational results, see publications.
 
 

Modelling and theoretical interpretation:

The STEPHI data already were the base of several theoretical interpretations (see publications). Among them, Goupil et al. 93 illustrates the strong potential of delta Scuti seismology to estimate rotation rates inside the stars and study transfer of angular momentum along their life. This interest was confirmed by Goupil et al. 96 .
Fast rotation rapidely came out as a key factor in the modelling and the analysis of delta Scuti observed spectra. This is true for the calculation of oscillation frequencies, even for radial modes as revealed by Perez Hernandez et al. 95 and Soufi et al 95. This is also true for the estimation of global parameters as illustrated in Michel et al 99 and Hernandez et al 98  and Perez Hernandez et al. 99. Recently, our work on delta Scuti stars in the Praesepe cluster revealed that we can obtain a satisfying agreement between the
predicted ranges of unstable modes and the range derived from observations. This is only possible
for a restricted range of values of the mixing-length parameter (Michel et al 99). These results constitute only
a modest part of what will be achieved with seismology of delta Scuti stars. They however
suggest that we are now able to adress the physics at work in A-type stars on the main sequence.
While the interpretation of data from Praesepe is going on, we are extending this work to other clusters.