|
Документ взят из кэша поисковой машины. Адрес
оригинального документа
: http://www.stsci.edu/science/preprints/prep1219/prep1219.html
Дата изменения: Wed May 24 00:03:20 2000 Дата индексирования: Sat Dec 22 15:27:55 2007 Кодировка: Поисковые слова: п п п п п п п п п п п п п п п п п п п п п п п п п п п п п п п п п р п р п р п р п р п |
STScI Preprint #1219
Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA for NASA under contract NAS5-26555, and observations obtained at the European Southern Observatory, La Silla.
We present new HST/FOS and ground-based long slit spectroscopic observations of the nebulae around the two LMC Luminous Blue Variables R127 (= HDE269858) and R143 (= HDE269929), and the Ofpe/WN9 star S119 (= HDE269687). 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 which 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
ratio, suggestive of nitrogen-enriched material that has been ejected from the star some 3.7×103 yrs 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 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 with 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 (Fransson et al. 1989; Panagia et al. 1997) which is thought to be composed of 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.\ (1994), and find that the pre-LBV mass loss has to be low enough for redward evolution such that a surface N/O ratio of ~ 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 (1996) which incorporates a brief RSG phase. We also consider the possibility of a pseudo-RSG phase by discussing the suggestion of Owocki & Gayley (1997) that a star in a super-Eddington condition may develop a very extended outer convective envelope which 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 compare our results with more sophisticated methods.