SIMBAD references

Linear features of up to 0.25 pc length are seen in both the Helix Nebula and the Orion Nebula. These features are most evident in emission-line ratio images. It is demonstrated that in both objects the features are along projections of lines between the dominant star and an optically thick compact feature (the cometary knots in the Helix and the proplyds in Orion). In general, these features are caused by shadowing of Lyman continuum radiation from the photoionizing star, although in the case of the Helix features, extinction by entrained dust plays a role. Some of the rays have neutral hydrogen cores. It is shown that the characteristics of these linear features can be used to analyze the three-dimensional structure of the nebulae. If the most recent model for these rays is correct and the electron temperature of the shadowed gas is only two-thirds that of the ambient material, then these rays play a small but measurable role in solving the t² problem.