Analyses of PNNi with (WC) spectral type

Details Analyses of PNNi with (WC) spectral type Analyses of PNNi with (WC) spectral type

Oct 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aca....43..329l&link_type=abstract

Acta Astronomica, vol. 43, no. 4, p. 329-335

Astronomy and Astrophysics

Astronomy

24

Astronomical Spectroscopy, Carbon Stars, Hot Stars, Planetary Nebulae, Stellar Atmospheres, Stellar Composition, Stellar Evolution, Stellar Models, Stellar Spectra, Stellar Winds, White Dwarf Stars, Wolf-Rayet Stars, Emission Spectra, Line Spectra, Stellar Mass

Scientific paper

A number of Planetary Nebular Nuclei (PNNi) exhibit emission-line spectra resembling those of Population I Wolf-Rayet stars of the carbon sequence (WC). As these kinds of emission-line spectra are formed by expanding stellar atmospheres (stellar winds), their analysis requires special non-Local Thermodynamic Equilibrium model calculations which we have developed in Kiel. The WC-type PNNi form two clearly separated groups. One group termed as 'early' subtypes (WCE) consists of very hot stars. So far we have analyzed four representatives (Abell 30, Abell 78, Longmore4, NGC 6751) obtaining effective temperatures above 100 kK. Their chemical composition (predominantly helium, carbon and oxygen) reveals their close relationship to the very hot white-dwarf progenitors called PG 1159 stars, the only difference to the latter being the higher mass-loss rate of the WCE stars. In one star (Longmore4) a transient change of the spectral appearance between these two types was observed fortuitously. The second group is of 'late' spectral subtypes (WCL) and comprises relatively cool stars. The representative which we have analyzed so far (V 348 Sgr) shows a very strange composition, as hydrogen is present simultaneously with enriched carbon. The low effective temperature of that star (20 kK) cannot sustain the ionization of its nebula, leading to the hypothesis of a very rapid phase of evolution.

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