The domain of γ Doradus variables in the Hertzsprung-Russell diagram

Abstract

70 new γ Doradus candidates were identified from Hipparcos photometry, which represents more than a doubling of the presently known number. Selecting the objects with good evidence for multiperiodicity, it is found that these stars, together with bona fide members of the class, occupy a well-defined region in a colour-magnitude diagram. This domain corresponds to a range of 7200-7700 K on the zero-age main sequence (ZAMS) and 6900-7500 K one magnitude above it, which partly overlaps with the instability strip of δ Scuti stars. For the first time, γ Doradus stars can be discussed as a group. They can be found over a significant fraction of the main sequence lifetimes for objects in the relevant temperature range. An upper limit on the surface metallicity of γ Doradus stars is apparent, which may guide the search for their pulsation driving mechanism. The importance of possible objects exhibiting both γ Doradus and δ Scuti-type pulsations is discussed.