Small-scale structures in planetary nebulae

Two planetary nebulae (PNe) were studied in order to investigate small scale molecular structures in planetary nebulae, the survival of which affects the structure and composition of the interstellar medium (ISM) from which the next generation of stars is born. Molecular hydrogen (H[subscript 2]) was used a tracer for molecular emission and its excitation properties used as a probe of shock physics. Previous studies of several planetary nebulae have indicated a relationship between molecular hydrogen and cometary knots as a shielding mechanism allowing for continued survival and formation of the molecule. Our first PN studied, the Dumbbell nebula (NGC 6853), revealed not only H[subscript 2] bearing knots (similar to the Helix and Ring nebulae), but also regions of H[subscript 2] emission which were poorly sculpted or unstructured diffuse regions, unlike previously studied nebulae. The second PN, NGC 2392 has well documented cometary knots, however we find no detection of molecular hydrogen emission from the knots or otherwise making it an outlier among cometary knot bearing planetary nebulae. Both nebula studied in this research challenge current models of H[subscript 2] bearing small scale structures in PNe and indicate a need to expand our rather small sample of such objects so that we may begin to understand the processes behind H[subscript 2] formation, excitation, and its role in cometary knots of the PN as a whole.