Chemical composition and origin of nebulae around Luminous Blue Variables

We use the analysis of the heavy element abundances (C, N, O, S) in circumstellar nebulae around Luminous Blue Variables to infer the evolutionary phase in which the material has been ejected. We concentrate on four aspects. (1) We discuss the different effects that may have changed the gas composition of the nebula since it was ejected: mixing with the swept up gas from the windblown bubble, mixing with the gas from the faster wind of the central star, and depletion by CO and dust. (2) We calculate the expected abundance changes at the stellar surface due to envelope convection in the red supergiant phase. We show that this depends strongly on the total amount of mass that was lost prior to the onset of the envelope convection. If the observed LBV nebulae are ejected during the red supergiant phase, the abundances of the LBV nebulae require a significantly smaller amount of mass to be lost than assumed in the evolutionary calculations of Meynet et al. (1994). (3) We calculate the changes in the surface composition during the main sequence phase by rotation induced mixing. If the nebulae are ejected at the end of the main sequence phase, the abundances in LBV nebulae are compatible with mixing times between 5 × 10 and 1 × 10 years. These values are reasonable, considering the high rotational velocities of main sequence O-stars. The existence Astronomical Institute, Princetonplein 5, NL-3584 CC Utrecht, The Netherlands; [email protected], [email protected] SRON Laboratory for Space Research, Sorbonnelaan 2, NL-3584 CA, Utrecht, The Netherlands Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA; [email protected], [email protected] Affiliated with the Astrophysics Division, Space Science Department of the European Space Agency University College London, Gower Street, London, UK; [email protected]