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

TL;DR

This paper presents initial spectroscopic findings of six faint planetary nebula nuclei with high temperatures, including five hydrogen-deficient PG 1159 stars and one Wolf-Rayet-like star, revealing their spectral features, estimated temperatures, and variability.

Contribution

It reports the discovery and detailed spectral analysis of six new high-temperature planetary nebula nuclei, expanding the known sample of such objects with unique spectral characteristics.

Findings

Six new planetary nebula nuclei with strong O VI emission identified.

Five are hydrogen-deficient PG 1159 stars with ~170,000 K temperatures.

One star shows Wolf-Rayet-like spectrum with variable emission lines.

Abstract

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 A 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 A. 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 A 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.