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Small Magellanic Cloud Be stars : colour-magnitude relations and mass-loss W

We present an analysis of optical lightcurves of Small Magellanic Cloud (SMC) Be-type stars. Observations show that (1) optical excess flux is correlated with near-IR excess flux indicating a similar mechanism and (2) the lightcurves can trace out “loops” in a colour-magnitude diagram. A simple model for the time dependence of bound-free and free-free (bf-ff) emission produced by an outflowing circumstellar disk gives reasonable fits to the observations. Mennickent et al. (2002) present and describe optical OGLE lightcurves for a large number of blue stars in the SMC. A subset of these stars is found to be irregular variable on timescales of weeks to months and amplitudes up to 1. An example is given in the left panel of Fig. 1. A cross-correlation with 2MASS shows that there exists a correlation between same epoch optical excess and near-IR excess (Fig. 1, right panel). The optical excess at 2MASS epoch is with respect to the brightness minimum, which should then correspond to the stellar photosphere. To assure this, we plot only those stars that have actual B-type optical colours at brightness minimum. Some 40% of our sample stars that have an amplitude of variation > 0.2m (101 objects) trace out a “loop” in a colour-magnitude diagram (CMD). A good example is given in Fig. 2, other examples can be found in de Wit et al. (2006). It is noteworthy that for 90% the loops are traced out in a clock-wise sense. Only a small minority makes the loop in an anti clock-wise sense. We develop a simple model to describe the observed variability. The model consists of a mass losing star and an outflowing, geometrically thin CS disk producing bf-ff emission. The disk is described by powerlaws in temperature, opening angle and outflow velocity. For an exact model description, see de Wit et al. (2006). Using this model, one can interpret the observed CMD behaviour in the following way. Once the B star starts losing mass a CS disk is built up. At small radii the disk is optically thick, and is the dominant source of the total flux (and colour) excesses. Once the mass-loss halts, the optically thick part of the disk is removed (this depends on wavelength) and the predominantly optically thick disk makes the transition to an optically thin