Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...286..543r&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 286, no. 2, p. 543-554
Astronomy and Astrophysics
Astrophysics
33
Abundance, Helium, Local Thermodynamic Equilibrium, Planetary Nebulae, Stellar Atmospheres, Stellar Evolution, Nitrogen, Photoionization, Spectrum Analysis, Stellar Composition, Stellar Models, Stellar Structure
Scientific paper
We present an analysis of the hydrogen-deficient central star of K 1-27 based on non-local thermodynamic equilibrium (NLTE) model atmospheres and an analysis of the planetary nebula based on photoionization models. From line profile fits we derive an effective temperature of Teff = 100 +/- 15kK and a surface gravity of log g = 6.5 +/- 0.5 for the central star. The atmosphere is dominated by helium while hydrogen cannot be detected (nH/nHe less than 0.2 by number). We find a nitrogen enrichment (nH/nHe = 0.005) but carbon lines are not detected (nC/nHe less than 0.005). This abundance pattern indicates that the star is a late helium flash object which has lost its hydrogen-rich envelope. The observed abundances may be also affected by element diffusion and radiative levitation. The stellar mass is relatively low (M = 0.52-0.02+0.06 solar mass and the spectroscopic distance is 1.3kpc. The high excitation, low density PN has a mass of M/solar mass = 0.018 +/- 0.5dex and a radius of 0.15pc. Its helium abundance is definitely solar, while the trace elements could also have lower abundances. A stratification of the He/H abundance ratio in the nebula can be excluded. The kinematical age is about an order of magnitude smaller than the evolutionary age of the central star. We discuss if the PN ejection took place at the final He flash. In that case He-enrichment in the nebula should be expected. However, in contrast to the central star no He enrichment is found in the PN. If a former PN was ejected at all at the first departure from the asymptotic giant branch (AGB), it has probably been dispersed.
Koeppen Joachim
Rauch Thomas
Werner Kirstin
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