NaSt1: A Wolf-Rayet star cloaked by an eta Car--like nebula?

Abstract

We present a study of the peculiar Galactic emission line object NaSt1 (WR122, IRAS 18497+0056) which has previously been classified as a Wolf-Rayet (WR) star. Our spectroscopic dataset comprises Keck I-HIRES, WHT-ISIS and UKIRT-CGS4 observations which show that NaSt1 has a highly reddened nebular spectrum with extremely strong permitted and forbidden lines covering a wide range in excitation. [OII-III] is unusually weak, with HeI-II and [NII] very strong, and carbon absent, suggestive of chemical peculiarities. Narrow-band WHT imaging reveals an elliptical nebula with an average diameter of 6.8arcsec. We measure an interstellar extinction of E(B-V)=2.1mag and estimate a distance of 1-3kpc, suggesting that NaSt1 is a luminous object, with 4<log(L/Lo)<6.5. We determine the physical parameters and abundances from the nebular forbidden lines. For Te=13,000K and Ne=3x10^6 cm^3, we obtain He/H>0.64, N enhanced by a factor of 20, O deficient by a factor of 140, while Ne, Ar and S are normal compared to average HII region abundances. This unusual abundance pattern suggests that the nebula consists of fully CNO-processed material. We compare the spectral appearance of NaSt1 with other luminous emission objects, and conclude that it is not an Ofpe/WN9, B[e] star or symbiotic nova although it does share several characteristics of these systems. We suggest instead that NaSt1 contains a massive evolved star that ejected its heavily CNO-processed outer layers a few thousand years ago. Although the stellar remnant is completely hidden from view by the dense nebula, we argue that the star must be an early-type WR star. The only object that shares some of the peculiarities of NaSt1 is eta Carinae. Whatever its true nature, NaSt1 should no longer be considered as a late-WN classification standard in the near-IR.