Estimating the Structure and Geometry of Winds from Luminous Blue Variables via Fitting the Continuum Energy Distributions

Abstract

By combining the UV spectra from IUE with photometric data in the optical band, we present a quantitative study on the continuum energy distributions of LBVs to determine the structure and geometry of LBV winds. It is shown that the shape of continuum energy distributions around the Balmer jump is sensitive to the velocity law of LBV winds. A simple, spherically symmetric wind model including free-bound and free-free radiation is constructed to compute the continuum energy distributions of LBVs. By matching theoretical ones to the observed continuum energy distributions around the Balmer jump, we have obtained value of the exponent of the velocity law beta in both minimum and maximum state for five LBVs, i.e., AG Car, HR Car, R40, S Dor, and R127. We have found that beta is about 0.5-0.7 in the minimum state and larger than 1.5 in the maximum state. Transitions in the ionization states of metals between the minimum and maximum state of LBVs, which lead to changes in the radiative acceleration due to spectral lines, are most likely responsible for such effect on the velocity law. We have also determined the geometry of the wind and found that a spherically symmetric wind model can well reproduce the observed continuum energy distributions of the five LBVs. Based on these results we suggest that the wind of LBVs be basically quasi-spherical, maybe with some clumpy structure in the spherical wind to produce some observed aspherical features.