Bubbles and Knots in the Kinematical Structure of the Bipolar Planetary Nebula NGC 2818

Abstract

High-resolution Hubble Space Telescope (HST) archive imaging and high-dispersion spectroscopy are used to study the complex morphological and kinematical structure of the planetary nebula, NGC 2818. We analyze narrow band Hα, [O III], [N II], [S II] and He II images, addressing important morphological features. Ground-based longslit 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 by: (1) a non-uniform bipolar structure with a semi-major axis of 0.92 pc (75 arcsec), possibly deformed by the stellar wind, (2) a 0.17 pc (14 arcsec) 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 arcsec) around the central star. The ma jor axis of the main structure is oriented at i \simeq 60\degree with respect to the line-of-sight and at PA = +89\degree on the plane of the sky. Expansion velocities of this nebula are Vpol = 105 km/s and Veq = 20 km/s, which lead to our estimate of the kinematical age of τk \simeq 8,400 \pm 3, 400 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 \pm 2 km/s.

20 pages, 6 figures

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