Paper
BUBBLES AND KNOTS IN THE KINEMATICAL STRUCTURE OF THE BIPOLAR PLANETARY NEBULA NGC 2818
Published Mar 27, 2012 · R. V'azquez
The Astrophysical Journal
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Abstract
High-resolution Hubble Space Telescope archive imaging and high-dispersion spectroscopy are used to study the complex morphological and kinematical structure of the planetary nebula, NGC 2818. We analyze narrowband Hα, [O iii], [N ii], [S ii], and He ii images, addressing important morphological features. Ground-based long-slit echelle spectra were obtained crossing NGC 2818 at five different positions to precisely determine kinematical features in the structure of the nebula. A distance of 2.5 kpc was used to determine physical scales. Constructing models to fit the data with modern computational tools, we find NGC 2818 is composed of (1) a non-uniform bipolar structure with a semimajor axis of 0.92 pc (75″), possibly deformed by the stellar wind, (2) a 0.17 pc (14″) diameter central region, which is potentially the remnant of an equatorial enhancement, and (3) a great number of cometary knots. These knots are preferentially located inside a radius of 0.24 pc (20″) around the central star. The major axis of the main structure is oriented at i ≃ 60° with respect to the line of sight and at P.A. = +89° on the plane of the sky. Expansion velocities of this nebula are Vpol = 105 km s−1 and Veq = 20 km s−1, which lead to our estimate of the kinematical age of τk ≃ 8400 ± 3400 yr (assuming homologous expansion). Our observations do not support the idea that high-velocity collimated ejections are responsible for the formation of microstructures inside the nebula. We determine the systemic velocity of NGC 2818 to be VHEL = +26 ± 2 km s−1.
NGC 2818 is a bipolar planetary nebula with a non-uniform structure and a large number of cometary knots, suggesting a kinematical age of 8400 3400 years.
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