SwSt 1: An O-rich planetary nebula around a C-rich central star
The hydrogen-deficient [WCL] type central star HD 167362 and its planetary nebula (PN) SwSt 1 are investigated. The central star has a carbon-rich emission-line spectrum, and yet the nebula exhibits a 10-μm emission feature from warm silicate dust, perhaps indicating a recent origin for the carbon-rich stellar spectrum. Its stellar and nebular properties might therefore provide further understanding as to the origin of the [WCL] central star class. The central star optical and UV spectra are modelled with state-of-the art non-LTE codes for expanding atmospheres, from which the stellar parameters are determined. Using the Sobolev approximation code ISA-Wind, we find Teff = 40000 K, log(M/M⊙ yr-1) = -6.72, L = 8900 L⊙ (for a distance of 2.0 kpc), and v∞ ≃ 900 km s-1. The abundance mass fractions for helium, carbon and oxygen are determined to be 37, 51 and 12 per cent, respectively. From this we derive C/O = 4.3 (by mass), confirming that the star suffered efficient third dredge-up. The nitrogen abundance is close to zero, while an upper limit of <10 per cent by mass is established for H. The model uses a composite beta velocity law which allows us to reproduce the optical line profiles. The overall shape of the dereddened spectrum agrees with the V-scaled [mv = 11.48 mag, E(B - V) = 0.46 mag] model atmosphere, showing the nebular-derived reddening to be consistent with the reddening indicated by the stellar analysis. We confirm our model results by using the comoving frame code CMFGEN, although a few differences remain. The PN has a high electron density [log(Ne/cm-3) = 4.5] and a small ionized radius (0.65 arcsec - measured from the HST-WF/PC Hβ images), indicating a young object. Its nebular abundances are not peculiar. The nebular C/O ratio is close to solar, confirming the PN as an O-rich nebula. The nebular N/O ratio of 0.08 is not indicative of a Type-I PN, although the high stellar luminosity points to a relatively high stellar mass. Near-IR spectroscopy is presented and fitted together with IRAS fluxes by using two blackbody curves with temperatures of 1200 and 230 K, indicating the presence of hot dust. We also report the first detection of H2 in this young and compact PN. All of the published spectroscopy since the discovery of SwSt 1 in 1895 has been re-examined, and it is concluded that no clear spectral variability is seen, in contrast to claims in some previously published studies. If an event occurred that has turned it into a hydrogen-deficient central star, it did not happen in the last 100 years.
Publication Source (Journal or Book title)
Monthly Notices of the Royal Astronomical Society
De Marco, O., Crowther, P., Barlow, M., Clayton, G., & De Koter, A. (2001). SwSt 1: An O-rich planetary nebula around a C-rich central star. Monthly Notices of the Royal Astronomical Society, 328 (2), 527-554. https://doi.org/10.1046/j.1365-8711.2001.04887.x