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

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.