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

We present a study of the peculiar Galactic emission-line object NaSt1 (WR1 22, IRAS 18497 + 0056), which has previously been classified as a Wolf-Raye t (WR) star. Our spectroscopic data set comprises Keck I-HIRES, WHT-ISIS an d UKIRT-CGS4 observations, which shaw that NaSt1 has a highly reddened nebu lar spectrum with extremely strong permitted and forbidden lines covering a wide range in excitation (H I, He I-II, N I-III, [N II], [Ne III-IV], Mg I -II, Si II, [S II-III], [Ar III-V], [Cav-VII], [Fe II-VII], [Ni II-III]). [ O II-III] is unusually weak, with He I-II and [N II] very strong, and carbo n absent, suggestive of chemical peculiarities. Narrow-band WHT imaging rev eals an elliptical nebula with an average diameter of 6.8 arcsec.We measure an interstellar extinction of E(B - V) similar to 2.1 mag and estimate a d istance of 1-3 kpc, suggesting that NaSt1 is a luminous object, with 4 less than or equal to log(L/L.) less than or equal to 6.5. We determine the phy sical parameters and abundances from the nebular forbidden lines. For T-e= 13000K and N-e = 3 X 10(6) cm(-3), we obtain He/H > 0.64, N enhanced by a f actor of 20, O deficient by a factor of 140, while Ne, Ar and S are normal compared with average HII region abundances. This unusual abundance pattern suggests that the nebula consists of fully CNO-processed material. We comp are the spectral appearance of NaSt1 with other luminous emission-line obje cts, 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 heavil y 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 MIR star. The only object that shares some o f the peculiarities of NaSt1 is eta Carinae. Whatever its true nature, NaSt 1 should no longer be considered as a late-WN classification standard in th e near-infrared.