Hipparcos parallaxes and the nature of delta Scuti stars

Hipparcos parallaxes give new tests of the nature of delta Sct variables. For many individual stars accurate distances are now available, i.e. directly determined luminosities and radii can be used to test theoretical models. Of particular interest are the at present very unclear relations between high-amplitude delta Sct stars and the much more abundant low-amplitude delta Sct variables, and the relations between field variables and the SX Phe stars in globular clusters. Only a few high-amplitude variables have Hipparcos parallaxes sufficiently accurate to provide precise model tests. Here we give a discussion of this group, considering SX Phoenicis and AI Velorum, which have the best parallaxes among the high-amplitude stars, in some detail. It is shown that two new tests based on the improved parallaxes are in good agreement with the (generally accepted) assumption that the high-amplitude variables are normal stars following standard evolution. AD Canis Minoris may be an interesting exception with strongly deviating properties. We briefly comment on globular cluster variables and period-luminosity relations.