The cometary knots found in the Helix and other planetary nebulae, together with the proplyds found in the Orion nebula and other H II regions, are both examples of photoevaporation flows in which a neutral condensation is being eroded by the heating and ionizing effects of ultraviolet radiation from an external source. However, there are important differences between the cometary knots and the proplyds, both in terms of the internal structure of their neutral components, and in the structure of the ionized flow. I compare ionization-kinematic models with subarcsecond-resolution HST imaging of the Helix knots and the Orion proplyds. In the case of the Helix knots, the ionization front is found to be of a thickness comparable to the size of the knots and is clearly resolved in the HST images. In the proplyds, on the other hand the ionization front is much thinner and is not resolved. This leads to differences in the relative spatial distribution of the Hα and [N II] 6583Å emission between the two cases.
Galaxies and their Constituents at the Highest Angular Resolutions
- Pub Date:
- January 2001