VARIABILITY OF LUMINOUS BLUE VARIABLES .2. PARAMETER STUDY OF THE TYPICAL LBV VARIATIONS

We present predicted photometric and spectroscopic variations for Lumi nous Blue Variables (LBVs) based on a computed grid of Non-LTE extende d and expanding atmosphere models. The effects of (i) mass loss, (ii) terminal dow velocity, (iii) rate of acceleration of the flow, (iv) mi nimum wind temperature, (v) effective gravity, and (vi) underlying ste llar radius are investigated. We show that the characteristic variatio ns in visual magnitude for LBVs of Delta V similar or equal to 1 to 2 mag, which occur at constant luminosity and are therefore associated w ith a change in photospheric radius and temperature, cannot be due to the formation of a 'pseudo' photosphere in the wind induced by a drama tic increase in mass loss rate. This implies that the origin of the me chanism responsible for the variations is located below the atmosphere . The change in photospheric radius that can be achieved by changing t he wind parameters (i) to (iv), using observed mass loss rates, is sim ilar to 40%. However, a change of a factor of 4 to 12, depending on lu minosity, is needed to reach the observed minimum effective temperatur e of LBVs of T-eff approximate to 8000 K. We conclude that the observe d increase in photospheric radius is due to the combined effect of an increase in the underlying stellar radius (vi) and a reduced effective gravity (v). Our calculations further show that mass loss and velocit y structure variations may in principle cause visual magnitude changes of Delta V less than or similar to 0.2m, however only if the optical depth of the wind is at the outset of the order of unity. To estimate the optical depth of the wind, we provide a simple formula. We also sh ow the sensitivity of the profiles of several H and He I lines to chan ges in the photospheric and wind parameters and explain the behaviour of Ha in simple physical terms.