Photoevaporating Flows from the Cometary Knots in the Helix Nebula (NGC 7293)

Abstract

We explain the H« emission of the cometary knots in the Helix Nebula (NGC 7293) with an analytical model that describes the emission of the head of the globules as a photoevaporated flow produced by the incident ionizing radiation of the central star. We compare these models with the H« emission obtained from the HST (Hubble Space Telescope) archival images of the Helix Nebula. From a comparison of the H« emission with the predictions of the analytical model we obtain a rate of ionizing photons from the central star of about 5 ¢10 45 s −1 , which is consistent with estimates based on the total H¬ flux of the nebula. We also model the tails of the cometary knots as a photoevaporated wind from a neutral shadow region produced by the diffuse ionizing photon field of the nebula. A comparison with the HST images allows us to obtain a direct determination of the value of the diffuse ionizing flux. We compare the ratio of diffuse to direct stellar flux as a function of radius inside an HII region with those obtained from the observational data through the analytical tail and head wind model. The agreement of this model with the values determined from the observations of the knots is excellent.

A Multi-Instrument Study of the Helix Nebula Knots with theHubble Space Telescope

“…Once this is taken into account, the predictions are in very good agreement with the observed values (Fig. 5) and in particular fall well below the line derived from equating recombinations in the flow to the incident ionizing flux, which is an illustration of the importance of the advection of neutrals through the front, as discussed in López-Martín et al (2001). It is heartening that our simple radiation hydrodynamic models are qualitatively consistent with the trends seen in the observations, although there are some quantitative discrepancies.…”

“…The initial conditions for the simulation were a cylindrical 4 concentration of dense neutral gas, with core radius 2 ; 10 15 cm and density 3 ; 10 4 cm À3 , which was illuminated by an ionizing spectrum similar to that of the Helix central star (T eA ¼ 120; 000 K; ionizing luminosity Q H ¼ 7:8 ; 10 45 s À1 ). The local diffuse field is treated in the on-the-spot approximation, and the global nebular diffuse field, with an expected strength of only 1%-4% of the direct radiation (López-Martín et al 2001), is neglected. The only source of opacity considered in the simulations is 4 Although the initial shape of the globule may affect its subsequent evolution and eventual destruction, the properties of the ionized flow from the globule head should be insensitive to this.…”

The diffuse nature of Strömgren spheres

“…It is well known (Hong & Sung 1989;López-Martín et al 2001) that diffuse photons dominate near the Strömgren radius, but for practical reasons this is often ignored, under the assumption that it is a small effect. In this Letter, we quantitatively show that it is a non-negligible effect, especially in the presence of density gradients.…”

X-ray Shaping of Planetary Nebulae

“…In this particular cases, soft thermal emission from H-poor knots ablated by the stellar wind, which is mass loaded to raise its density and damp its velocity, produce the extremaly soft spectrum mostly consistent of a single C VI line emission [13,41]. The evaporation of clumps of cold material, whcih may survive inside the hot bubble and photo-evaporate [22,24], may produce similar effects.…”