Spectroscopic survey of faint planetary-nebula nuclei – I. Six new ‘O VI’ central stars

We report initial results from an ongoing spectroscopic survey of central stars of faint planetary nebulae (PNe), obtained with the Low-Resolution Spectrograph on the Hobby–Eberly Telescope. The six PN nuclei (PNNi) discussed here all have strong emission at the O VI 3811–3834 Å doublet, indicative of very high temperatures. Five of them – the nuclei of Ou 2, Kn 61, Kn 15, Abell 72, and Kn 130 – belong to the hydrogen-deficient PG 1159 class, showing a strong absorption feature of He II and C IV at 4650–4690 Å. Based on exploratory comparisons with synthetic model-atmosphere spectra, and the presence of Ne VIII emission lines, we estimate them to have effective temperatures of order 170 000 K. The central star of Kn 15 has a Wolf–Rayet-like spectrum, with strong and broad emission lines of He II, C IV, N V, and O V–VI. We classify it [WO2], but we note that the N V 4604–4620 Å emission doublet is extremely strong, indicating a relatively high nitrogen abundance. Several of the emission lines in Kn 15 vary in equivalent width by factors as large as 1.5 among our four observations from 2019 to 2022, implying significant variations in the stellar mass-loss rate. We encourage spectroscopic monitoring. Follow-up high-time-resolution photometry of these stars would be of interest, given the large fraction of pulsating variables seen among PG 1159 and [WO] PNNi.