Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000aj....119.2910o&link_type=abstract
The Astronomical Journal, Volume 119, Issue 6, pp. 2910-2918.
Astronomy and Astrophysics
Astronomy
19
Planetary Nebulae: Individual (Ngc 7293)
Scientific paper
We compare the morphology of the cometary knots in the Helix Nebula in different emission lines using Hubble Space Telescope Wide Field Planetary Camera 2 and Space Telescope Imaging Spectrograph observations. We find that the [N II] 6658 Å line emission from the cometary heads is displaced with respect to Hα, peaking at a position that is closer to the central ionizing star. This result seems at first sight to be in conflict with simple photoionization models, which predict that the [N II] emission is closer to the ionization front (IF) because it is confined to a thin H+-He0 layer, a prediction confirmed by calculations with both our own and the CLOUDY programs. However, the ratio of [N II] to Hα is very temperature sensitive, and the observations can be explained if the knots are modeled as photoevaporating globules. In this case, there is a strong temperature gradient across the IF, resulting in the [N II] emission being depressed in the partially neutral zones. We also find a strong correlation between the strength of the [N II] and [O III] emission in individual knots, with both being higher in the knots that are closer to the central star. On the current evidence, it is unclear whether this is due to interknot variations in metal abundances or in gas temperature. Based on observations with the NASAESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
Burkert Andreas
Henney William J.
O'Dell Charles Robert
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