NEAR-IR VARIABILITY OF BE STARS

Near-IR observations of 125 Be stars from twelve epochs in the period 1972 to 1991 are combined to identify near-IR-variable Be stars. This study represents the first time that IR-variable Be stars have been studied in a statistically robust manner. A sample of non-variable Be stars is identified and used to ensure that a consistent photometric system is applied to all epochs of data, and that quoted uncertainties are consistent with observational scatter at each epoch. The percentage of variable stars increases with wavelength. In the K band 18 per cent of the sample (21 stars) are identified as variables. The amplitude of variations also increases with wavelength. The variations are shown to be due to variations in the circumstellar plasma as opposed to photospheric variations. The amplitude of variations ranges from 0.2 mag up to 1.2 mag. Attributing the variations to circumstellar plasma density fluctuations, it is shown that in an optically thin or partially optically thick circumstellar envelope a variation of 0.5 mag requires a change in plasma density DELTArho/rho approximately 2 - 3. The temporal behaviour of the variations is presented. The observed time-scale of variations ranges from 73 d to 18.5 yr. The minimum observed time-scale of variation is consistent with the travel time through the emitting volume.