Friday, January 18, 2013

1301.3994 (David J. Frew et al.)

The planetary nebula Abell 48 and its [WN4] nucleus    [PDF]

David J. Frew, Ivan S. Bojicic, Q. A. Parker, M. Stupar, S. Wachter, K. DePew, A. Danehkar, M. F. Fitzgerald
We have conducted a detailed multi-wavelength study of the peculiar planetary nebula Abell 48 and its central star. We present evidence for a new classification of the nucleus as a helium-rich, hydrogen-deficient star of type [WN4], one of only two currently known examples of its type. The evidence for either a massive WN or a low-mass [WN] interpretation is critically examined, and we firmly conclude that Abell 48 is a planetary nebula around an evolved low-mass star, rather than a Population I ejecta nebula. Importantly, the surrounding nebula has a morphology typical of planetary nebulae, and is not enriched in nitrogen, and thus not the 'peeled atmosphere' of a massive star. We estimate a distance of 1.6 kpc and a reddening, E(B-V) = 1.90 mag, the latter value clearly showing the nebula lies on the near side of the Galactic bar, and therefore cannot be a massive WN star. The planetary nebula has an ionized mass (0.3 M_sun) and electron density (740 cm^-3) typical of somewhat evolved objects. The observed stellar spectrum was compared to a grid of models from the Potsdam Wolf-Rayet (PoWR) grid. The best fit temperature is 71 kK, and the atmospheric composition is dominated by helium (>86 per cent by mass), with about 3-4 per cent nitrogen present. An upper limit on the hydrogen abundance is 10 per cent. The unusually high nitrogen fraction is higher than any other low-mass H-deficient star, and is not explained by current post-AGB models. We give a discussion of the implications of this discovery for the late-stage evolution of intermediate-mass stars.
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