Ejected Nebulae as Probes of the Evolution of Massive Stars in the Large Magellanic Cloud

Abstract

We present new Hubble Space Telescope (HST) Faint Object Spectrograph(FOS) and ground-based long-slit spectroscopic observations of thenebulae around the two LMC luminous blue variables (LBVs) R127 (=HDE269858) and R143 (=HDE 269929) and the Ofpe/WN9 star S119 (=HDE 269687).We have used the ground-based long-slit spectra to investigate thekinematics of the nebulae around R127 and R143. We find that thecircumstellar environment of R127 is complex and suggestive of twodiscrete ejections in the mass loss history of the central star. Thereis an inner expanding shell, with a radius of 0.6 pc, an expansionvelocity of 29 km s-1, and a dynamical age of 2 x 104 yr. There is alsomaterial beyond the bright inner nebula that may represent an outerexpanding shell with a radius of 1.9 pc and a dynamical age of 7 x 104yr. For R143, we find that the velocity field in the northern andsouthern circumstellar regions (1''-20'') is predominantly constant.Therefore, the nebular emission previously believed to be associatedwith R143 (the fingers) is most likely part of the 30 Doradus complex,with the exception of a small emission-line region located ~=2'' north ofthe star. This compact region displays a blueshifted motion with adifferential velocity of ~=130 km s-1 with respect to the central star.The spectrum of this clump shows a high [N II]/H alpha ratio, suggestiveof nitrogen-enriched material that has been ejected from the star some3.7 x 103 yr ago. We have used the optical HST/FOS spectra to determinereddenings, electron temperatures and densities, and N and O abundancesfor the three nebulae. For R143 our derived abundances indicate that theregion we observed to the south of the star is 30 Doradus H II gas,since it shows a typical LMC H II region N/O ratio. For R127 we findthat N is enriched by a factor of 10.7 +/- 2.2, and O is depleted by afactor of 2.0 +/- 1.0, with N/O = 0.89 +/- 0.40. For the S119 nebula wedo not have a secure electron temperature, but find N/O = 1.41-2.45,similar to the value found for R127. We compare the nebular abundancesto the expected surface abundances of LBVs and discuss the likelystellar evolutionary state at the time of nebular ejection. If theatmospheres of LBVs consist of CNO-processed material, this implies thatthe event that formed the nebula took place before or at the very startof the LBV phase. Comparison of the observed nebular abundances withother objects shows that the abundance pattern for the R127 nebula isremarkably similar to that determined for the inner ring of SN 1987A,which is thought to be composed of red supergiant (RSG) wind material.This coincidence, together with the observed low expansion velocity,suggests that the nebula was once the CN-processed convective envelopeof an RSG. We consider whether the R127 nebular parameters can bereproduced with the evolutionary models of Meynet et al., and we findthat the pre-LBV mass loss has to be low enough for redward evolutionsuch that a surface N/O ~ 1 is achieved while the star is a coolsupergiant inside the Humphreys-Davidson limit. Our findings are broadlyin accord with the model of Stothers {\amp} Chin that incorporates abrief RSG phase. We also consider the possibility of a pseudo-RSG phaseby discussing the suggestion that a star in a super-Eddington conditionmay develop a very extended outer convective envelope that may becomedetached. We also present a coarse analysis of the FOS stellarspectrophotometry using the Q-method to estimate reddenings and spectraltypes, and we compare our results to more sophisticated methods.