The central star of the planetary nebula PB 8: a Wolf-Rayet-type wind of an unusual WN/WC chemical composition

H. Todt1, M. Peña2, W.-R. Hamann1, G. Gräfener1, 3

1Institut für Physik und Astronomie, Universität Potsdam, Germany
2Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 70264, México D.F. 04510, México
3Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland

A considerable fraction of the central stars of planetary nebulæ (CSPNe) are hydrogen-deficient. As a rule, these CSPNe exhibit a chemical composition of helium, carbon, and oxygen with the majority showing Wolf-Rayet-like emission line spectra. These stars are classified as CSPNe of a spectral type [WC]. We perform a spectral analysis of CSPN PB 8 with the Potsdam Wolf-Rayet (PoWR) models for expanding atmospheres. The source PB 8 displays wind-broadened emission lines from strong mass loss. Most strikingly, we find that its surface composition is hydrogen-deficient, but not carbon-rich. With mass fractions of 55% helium, 40% hydrogen, 1.3% carbon, 2% nitrogen, and 1.3% oxygen, it differs greatly from the 30-50% of carbon which are typically seen in [WC]-type central stars. The atmospheric mixture in PB 8 has an analogy in the WN/WC transition type among the massive Wolf-Rayet stars. Therefore we suggest to introduce a new spectral type [WN/WC] for CSPNe, with PB 8 as its first member. The central star of PB 8 has a relatively low temperature of T* = 52kK, as expected for central stars in their early evolutionary stages. Its surrounding nebula is less than 3000 years old, i.e. relatively young. Existing calculations for the post-AGB evolution can produce hydrogen-deficient stars of the [WC] type, but do not predict the composition found in PB 8. We discuss various scenarios that might explain the origin of this unique object.

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