Journal Article

The nature of the cometary knots in the Helix planetary nebula (NGC 7293)

J. Meaburn, C. A. Clayton, M. Bryce, J. R. Walsh, A. J. Holloway and W. Steffen

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 294, issue 2, pages 201-223
Published in print February 1998 | ISSN: 0035-8711
Published online February 1998 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-8711.1998.01152.x
The nature of the cometary knots in the Helix planetary nebula (NGC 7293)

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Abstract

The system of cometary knots in the Helix planetary nebula (NGC 7293) has been systematically observed using ground-based images and long-slit, high-resolution spectroscopy. CCD images in the [N ii] 6584-Å line, taken with the ESO NTT, are used to determine the spatial distribution of the knots; images in the [O iii] 5007-Å line distinguish their position within the nebula on the basis of the absorption of the central [O iii] 5007-Å-emitting zone of the Helix nebula. The kinematics of 50 of the brighter knots and their tails were studied primarily with the Manchester Echelle Spectrograph (MES) as well as the ESO NTT using the EMMI spectrograph. Three regions were covered by multiple exposures of a 10-element multislit with MES to obtain [N ii] 6584-Å line profiles along 300 individual slit positions. In addition, many long-slit MES spectra, placed diametrically across the nebula, were obtained to compare the kinematics of the knots with the large-scale kinematical structure of the nebula.

The global expansion of the system of knots is around 14 km s-1, some 17 km s-1 less than for the overrunning gas, and they are distributed in a thick central disc. The velocity field of the system of knots is similar to that of molecular (CO) emission but with a lower expansion velocity. Most knots are external to the central [O iii] 5007-Å-emitting region, but some still show localized [O iii] 5007-Å emission from their heads.

The kinematical structure of two of the latter knots as revealed by profiles of the [O i] 6300-Å, [N ii] 6584-Å, [O iii] 5007-Å and Hα emission lines is considered in detail and compared with their CO emission-line profiles. The interstellar absorption by the core of a further knot is derived from an [O iii] 5007-ÅHST image combined with MES [O iii] 5007-Å profiles. Accurate masses, densities and ages are determined as a consequence. Furthermore, a kinematical model shows that the [N ii] 6584-Å-emitting flow around a dusty, molecular globule (the core of an ionized knot) arises mainly from the expanding ionized gas of the Helix nebula engulfing the knot. The flow, parallel to the globule's photoionized surface, is mildly supersonic. The relationship between the system of knots and the large-scale structure is discussed.

Keywords: planetary nebulae: individual: NGC 7293

Journal Article.  0 words. 

Subjects: Astronomy and Astrophysics

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