UV spectral analysis of very hot H-deficient [WCE]-type central stars of planetary nebulae: NGC 2867, NGC 5189, NGC 6905, Pb 6 and Sand 3

Abstract

We analysed ultraviolet Far Ultraviolet Spectroscopic Explorer, International Ultraviolet Explorer, and Hubble Space Telescope/Space Telescope Imaging Spectrograph spectra of five of the hottest [WCE]-type central stars of planetary nebulae: NGC 2867, NGC 5189, NGC 6905, Pb 6, and Sand 3. The analysis leveraged on our grid of CMFGEN synthetic spectra, which covers the parameter regime of hydrogen-deficient central stars of planetary nebulae and allows a uniform and systematic study of the stellar spectra. The stellar atmospheremodels calculated by us include many elements and ionic species neglected in previous analyses, which allowed us to improve the fits to the observed spectra considerably and provided an additional diagnostic line: the Ne VII λ973 Å, which had not been modelled in [WCE] spectra and which presents, in these stars, a strong P-Cygni profile. We report newly derived photospheric and wind parameters and elemental abundances. The central stars of NGC 2867, NGC 5189, and Pb 6 had their temperatures revised upward in comparison with previous investigations and we found the carbon to helium mass ratio of the sample objects to span a wide range of values, 0.42 < C : He < 1.96. Modelling of the Ne VII λ973 Å P-Cygni profile indicated strong neon overabundances for the central stars of NGC 2867, NGC 5189, NGC 6905, and Pb 6, with Ne mass fractions between 0.01 and 0.04. Nitrogen abundances derived by us for the central stars of NGC 5189, Pb 6, and Sand 3 are higher than previous determinations by factors of 3, 10, and 14, respectively.