Giant emission features at large distances from PNe: A preliminary investigation

Extract

“Senile” PNe are very old nebulae of extremely low surface brightness and large linear size; they offer e.g. the possibility of probing the conditions of the ISM also non-locally. We give three examples of huge nebulae in a preliminary inquire.

NGC 3242: An ESO Schmidt plate obtained in April 1996 shows the ‘wrapped’ morphology of the faint arc discovered by Deeming (1966, Astrophys. J. 146, 287) 11′ south-west of this PN. Probably this arc is part of a giant halo of very low surface brightness around NGC 3242, but up to now the detection of the halo is questionable. A spectrum of the filament obtained at Las Campanas shows [O iii]/Hα ∼ 5 compared with a value of ∼ 3 in the nebula (Perinotto et al. 1994, Astron. Astrophys. Supp., 107, 481). This high ratio is larger than that expected for the excitation class of NGC 3242, even larger than the maximum allowed for a photo-ionized nebula of more or less normal chemical abundance. Thus, due to the spectral characteristic and to the filamentary structure of the arc we suspect that shocks appear to play a role.