Photoevaporating flows from the cometary knots in the Helix Nebula (NGC 7293)

We explain the Ha emission of the cometary knots in the Helix Nebula (NGC 7 293) with an analytical model that describes the emission of the head of th e globules as a photoevaporated flow produced by the incident ionizing radi ation of the central star. We compare these models with the H alpha emissio n obtained from the Hubble Space Telescope (HST) archival images of the Hel ix Nebula. From a comparison of the Ha emission with the predictions of the analytical model we obtain a rate of ionizing photons from the central sta r of about 5 x 10(45) s(-1), which is consistent with estimates based on th e total H beta flux of the nebula. We also model the tails of the cometary knots as a photoevaporated wind from a neutral shadow region produced by th e diffuse ionizing photon field of the nebula. A comparison with the HST im ages 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 H II region with those obtained from the obse rvational data through the analytical tail and head wind model. The agreeme nt of this model with the values determined from the observations of the kn ots is excellent.