Abstract We do know that planetary nebulae (PNe) are ionized gaseous clouds of material ejected by evolved dying stars. The intense ultraviolet radiation field of these stars leads to the dissociation of the cold molecular gas and then to the ionization/excitation of the resultant atomic gas. The chemical composition, ionization structure, physical conditions and formation process of nebular shells, rims, and halos are well comprehended. On the contrary, the origin of low-ionization structures (LISs) frequently found in PNe break the overall picture, and it still remains poorly understood. The latest discoveries of molecular hydrogen (H 2 ) in LISs have changed how we think about their origin. Besides the detection of H 2 emission, the Fe ii 1.644 μ m and C i 8727Å lines have also been detected in LISs. These results add new pieces to the puzzling problem of LISs opening a new window to enrich our knowledge and understanding on these microstructures.
S. Akras (Fri,) studied this question.
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