Abundances of Evolved Massive Stars in the LMC from Studies of their Circumstellar Nebulae

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Hst Proposal Id #6110 Hot Stars

Scientific paper

We propose to obtain FOS spectra of circumstellar ejecta around extremely luminous, evolved LMC stars in order to determine heavy element abundances. The targets are the three Ofpe/WN9/LBV stars R127, S119, and R143, which represent objects in transition between types O and Wolf-Rayet. They are the key to understanding the relation between the initial and final stages in the evolution of a massive star. The Ofpe/WN9/LBV phase is characterized by a sudden increase of the mass loss rate and the appearance of chemically processed material on the stellar surface. Atmospheric analyses of the heavy element abundances are currently very uncertain, if not infeasible. On the other hand, the objects are surrounded by spatially resolved ejecta which allow a `classical' nebular analysis. The brightness contrast between the stars and the ejecta precludes any ground-based study of this type. We demonstrate that HST observations --- although challenging -- - will allow us to reach this scientific goal. The observations will be used to derive electron-temperatures and -densities and heavy element abundances. Since the nebular abundances reflect the current stellar surface abundances, the proposed analysis is a critical test to reveal deficiencies in atmospheric non-LTE modeling. The derived abundances will be compared to current evolutionary models in an attempt to clarify the astrophysical significance of Ofpe/WN9/LBV stars.

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