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 data set 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 (Hi, Hei-ii, Ni-iii, [Nii], [Neiii-iv], Mgi-ii, Siii, [Sii-iii], [Ariii-v], [Cav-vii], [Feii-vii], [Niii-iii]). [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/L_solar)<=6.5. We determine the physical parameters and abundances from the nebular forbidden lines. For T_e=13000K and N_e=3x10^6cm^-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 with 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-line 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-infrared.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- September 1999
- DOI:
- 10.1046/j.1365-8711.1999.02707.x
- arXiv:
- arXiv:astro-ph/9903410
- Bibcode:
- 1999MNRAS.308...82C
- Keywords:
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- Astrophysics
- E-Print:
- 16 pages, 12 figures, accepted for MNRAS, uses mn.sty