Variability of luminous blue variables. II. Parameter study of the typical LBV variations.

We present predicted photometric and spectroscopic variations for Luminous Blue Variables (LBVs) based on a computed grid of Non-LTE extended and expanding atmosphere models. The effects of (i) mass loss, (ii) terminal flow velocity, (iii) rate of acceleration of the flow, (iv) minimum wind temperature, (v) effective gravity, and (vi) underlying stellar radius are investigated. We show that the characteristic variations in visual magnitude for LBVs of {DELTA}V =~ 1 to 2 mag, which occur at constant luminosity and are therefore associated with a change in photospheric radius and temperature, cannot be due to the formation of a `pseudo' photosphere in the wind induced by a dramatic increase in mass loss rate. This implies that the origin of the mechanism responsible for the variations is located below the atmosphere. The change in photospheric radius that can be achieved by changing the wind parameters (i) to (iv), using observed mass loss rates, is ~40 %. However, a change of a factor of 4 to 12, depending on luminosity, is needed to reach the observed minimum effective temperature of LBVs of T_eff_=~8000 K. We conclude that the observed 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 velocity structure variations may in principle cause visual magnitude changes of {DELTA}V<~0.2mag , 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 show the sensitivity of the profiles of several H and He i lines to changes in the photospheric and wind parameters and explain the behaviour of Hα in simple physical terms.