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 the nebulae around the two LMC luminous blue variables (LBVs) R127 (=HDE 269858) and R143 (=HDE 269929) and the Ofpe/WN9 star S119 (=HDE 269687). We have used the ground-based long-slit spectra to investigate the kinematics of the nebulae around R127 and R143. We find that the circumstellar environment of R127 is complex and suggestive of two discrete ejections in the mass loss history of the central star. There is an inner expanding shell, with a radius of 0.6 pc, an expansion velocity of 29 km s-1, and a dynamical age of 2 × 104 yr. There is also material beyond the bright inner nebula that may represent an outer expanding shell with a radius of 1.9 pc and a dynamical age of 7 × 104 yr. For R143, we find that the velocity field in the northern and southern circumstellar regions (1''-20'') is predominantly constant. Therefore, the nebular emission previously believed to be associated with R143 (the fingers) is most likely part of the 30 Doradus complex, with the exception of a small emission-line region located 2'' north of the star. This compact region displays a blueshifted motion with a differential velocity of 130 km s-1 with respect to the central star. The spectrum of this clump shows a high [N II]/Hα ratio, suggestive of nitrogen-enriched material that has been ejected from the star some 3.7 × 103 yr ago. We have used the optical HST/FOS spectra to determine reddenings, electron temperatures and densities, and N and O abundances for the three nebulae. For R143 our derived abundances indicate that the region 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 find that N is enriched by a factor of 10.7 ± 2.2, and O is depleted by a factor of 2.0 ± 1.0, with N/O = 0.89 ± 0.40. For the S119 nebula we do 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 abundances to the expected surface abundances of LBVs and discuss the likely stellar evolutionary state at the time of nebular ejection. If the atmospheres of LBVs consist of CNO-processed material, this implies that the event that formed the nebula took place before or at the very start of the LBV phase. Comparison of the observed nebular abundances with other objects shows that the abundance pattern for the R127 nebula is remarkably 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 envelope of an RSG. We consider whether the R127 nebular parameters can be reproduced with the evolutionary models of Meynet et al., and we find that the pre-LBV mass loss has to be low enough for redward evolution such that a surface N/O ~ 1 is achieved while the star is a cool supergiant inside the Humphreys-Davidson limit. Our findings are broadly in accord with the model of Stothers & Chin that incorporates a brief RSG phase. We also consider the possibility of a pseudo-RSG phase by discussing the suggestion that a star in a super-Eddington condition may develop a very extended outer convective envelope that may become detached. We also present a coarse analysis of the FOS stellar spectrophotometry using the Q-method to estimate reddenings and spectral types, and we compare our results to more sophisticated methods.