The remarkable light and colour variability of Small Magellanic Cloud Be stars

Abstract

(Abridged) Some 240 blue stars in the Small Magellanic Cloud are investigated on their fantastic irregular continuum variability. We report here two results regarding these stars. First, their optical flux excess is correlated to their near-IR flux excess, as determined from optical EROSII light curves and 2MASS measurements. Second, the relation between optical colour and magnitude is observed to be bi-valued in 40% of the cases, resulting in a "loop" when the light curve is presented in a colour-magnitude diagram. We argue that optical variability for a large fraction of the variable stars is due to variations in the amount of bound-free and free-free radiation. We do simple model calculations that allow us to interpret the observed colour-magnitude variability as due to an outflowing ionized circumstellar disk. The mass loss of the central star is variable, i.e. on or off. Once the star stops losing mass, the disk evolves naturally into a ring. The observed bi-valued colour-magnitude relation is the transition of a partially optically thick to a fully optically thin disk. Significantly, the loop is traversed clockwise by outflowing matter, but anti-clockwise by infalling matter. It is observed that the material is generally outflowing, but few cases of inflow are also observed.