Discrepancy between infrared and optical emission in some Be stars

An expanding ring model is put forward to investigate the original regions of Hα line emission and infrared excess emission in Be stars, by taking optical depth into account. We find that the two regions depend strongly on the physical properties of the envelope of Be stars such as the initial density and density structure. This model can be used to qualitatively interpret the disagreement between the near infrared excess and the equivalent width (EW) of Hα emissionline, as observed in some Be stars.